DEVICE FOR REGULATING THE PRESSURE IN A HELIUM TANK OF AN NMR MAGNET, WITH TWO PRESSURE SENSORS

Description

[0001] The invention relates to a device and a method for regulating the pressure in a helium tank of an NMR magnet.

[0002] Superconducting magnets for NMR instruments, such as NMR spectrometers or NMR tomographs, are often cooled with liquid helium. A cryostat comprises a vacuum-insulated tank containing boiling liquid helium and the superconducting magnet.

[0003] The pressure in this helium tank should be above the surrounding atmospheric pressure to prevent air from being drawn into the tank. Air that is drawn in can freeze, forming ice due to the freezing of atmospheric moisture or other components such as nitrogen. This ice can block pipes or valves, thus compromising operational safety.

[0004] Furthermore, the pressure in the helium tank should be as constant as possible, since pressure fluctuations in the helium tank can lead to artifacts in the NMR measurements, for example through minimal deformations of the helium tank and resulting movements of the superconducting magnet.

[0005] In many applications, the helium tank is equipped with a spring-loaded pressure relief valve, which mechanically establishes a fixed pressure differential between the helium tank pressure and atmospheric pressure. Consequently, the pressure in the helium tank is dependent on atmospheric pressure and fluctuates with the weather-dependent atmospheric pressure. This can lead to significant artifacts in NMR measurements, depending on the weather conditions.

[0006] According to the Bruker Corporation, Billerica, MA, USA, dated January 20, 2020, the company publication "EAPD II - Electronic Atmospheric Pressure Device II User Manual Version 002," published on January 20, 2020, a pressure sensor regulates the pressure in the helium tank of an NMR magnet to a predetermined, fixed setpoint. An electronic control unit actuates a control valve through which helium gas can escape from the helium tank. A pressure approximately 15 mbar above the highest atmospheric pressure expected at the location is recommended as the setpoint. This ensures a relatively constant helium tank pressure in most cases and prevents air from being drawn in.

[0007] However, atmospheric pressure can fluctuate significantly at many locations. For example, during hurricane season in the southern and eastern United States, weather-related pressure drops of up to approximately 100 mbar can occur. In such cases, the pressure difference between the helium tank pressure and atmospheric pressure can become so large that safety pressure relief valves open to release helium gas, and NMR measurements are no longer possible under stable conditions. With very large pressure differences, even the helium tank's rupture discs can shatter.

[0008] Many cryostats also require occasional refilling with liquid helium. To insert a transfer line, the helium tank must be opened to the atmosphere. If the pressure in the helium tank is suddenly reduced when opening it to atmospheric pressure, a large quantity of cold helium gas is released abruptly. The enthalpy of this gas cannot be used to absorb heat, which is energetically inefficient.

[0009] From US patent 3,412,568 A, a pressure regulator is known with which a constant pressure can be set in a cryostat which has a bath of coolant.

[0010] It has become known from JP 2015 060973 A to adjust the pressure in a helium tank of a cryostat using an electric heater.

[0011] From DE 10 2005 058 650 B3 it has become known to monitor the continuity of a tower tube of a cryomagnet in an MRI device using optical means.

[0012] US Patent 2009 / 0280989 A1 describes a control apparatus and an associated method for regulating gas pressure and gas flow in a cryogenic container for superconducting magnetic coils. Sensors measure the pressure inside the container and in the surrounding environment. The pressure inside the container can be controlled as a function of the ambient pressure. Furthermore, a gradual pressure reduction within the container is described.

[0013] US Patent 2009 / 0261830 A1 describes an imaging magnetic resonance scanner. The pressure in a cryogenic chamber and in the surrounding environment are measured and fed into a processor, which controls a pressure regulator to maintain the largest possible buffer before pressure relief valves open. This prevents unnecessary loss of refrigerant. In one example, the pressure in the cryogenic chamber is maintained 0.1 psi (approximately 689 Pa) above the ambient pressure.

[0014] EP 1 587 114 A2 describes a superconducting magnetic apparatus with a coil container in which a cooling medium, in particular liquid helium, is stored. A drain line leads from the coil container to the environment. The pressure in the coil container can be measured by a pressure detector located near the end of the drain line on the coil container. Another pressure detector is located near the end of the line close to the outlet to the environment. A gravity check valve is integrated into the drain line. The pressure detectors are connected to the control unit. The control unit controls a cooling device for the medium and a heating device for the drain line. Object of the invention

[0015] The object of the invention is to present a device and a method for regulating the pressure in a helium tank, with which the availability of the NMR magnet is improved, and improved operational safety and greater user-friendliness can be achieved. Description of the invention

[0016] This problem is solved according to the invention by a device for regulating the pressure in a helium tank of an NMR magnet, comprising a first pressure sensor for measuring a first pressure in the helium tank, at least a second pressure sensor for measuring a second pressure outside the helium tank, a control valve for adjusting the outflow of helium gas from the helium tank, an electronic control device for controlling the control valve, wherein the electronic control device is configured to obtain first pressure values ​​D1 measured by the first pressure sensor, and to adjust the position of the control valve (5) depending on the measured first pressure values ​​D1, wherein the first pressure values ​​D1 are adjusted to a predetermined setpoint SW, to obtain second pressure values ​​D2 measured by the second pressure sensor, and wherein the electronic control device is further configured to determine the setpoint SW in normal operation for NMR measurements depending on the first pressure values ​​D1 and the second pressure values ​​D2, namely to change the setpoint SW for the pressure in the helium tank in stages as soon as a difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds predetermined threshold values, wherein the setpoint SW is increased by one stage when the difference DIF becomes less than or equal to an increase threshold HSW,and the setpoint SW is lowered by one step if the difference DIF becomes greater than or equal to a lowering threshold SSW, with HSW <SSW, und wobei eine jeweilige Stufe, um die der Sollwert SW verändert wird, sobald die Differenz DIF=D1-D2 zwischen dem gemessenen ersten Druckwert D1 und dem gemessenen zweiten Druckwert D2 die vorgegebenen Schwellwerte erreicht oder überschreitet, im Bereich 5-25 mbar gewählt ist. ,

[0017] The invention provides for the provision of a second pressure sensor, in addition to the first pressure sensor which monitors the first pressure in the helium tank ("helium tank pressure"), which monitors a second pressure outside the helium tank. The second pressure is typically atmospheric pressure or another pressure that depends on atmospheric pressure, for example, the pressure in a helium recovery system (which is usually a small pressure difference above atmospheric pressure). The second pressure, or rather the corresponding second pressure values ​​D2 measured by the second pressure sensor, are incorporated into the control of the helium tank pressure, at least via the setpoint for the helium tank pressure (i.e., the setpoint for the first measured pressure values ​​D1).This allows suitable operating conditions with (at least temporarily) stable helium tank pressure and thus stable measurement conditions with good operational reliability to be achieved even under fluctuating environmental conditions (recognizable by fluctuating secondary pressure values ​​D2). Furthermore, it is possible to adjust the helium tank pressure relative to the secondary pressure in a desired manner to achieve high energy efficiency in the use of liquid helium within the system. Finally, hazardous operating conditions (for example, with a high pressure differential between D1 and D2) can be more easily detected, enabling increased operational reliability through user alerts or automatic countermeasures.

[0018] The initial pressure in the helium tank is set by specifying a setpoint SW for the pressure in the helium tank in the control unit. The control unit then adjusts the measured initial pressure value D1 to the setpoint SW by actuating the control valve. To decrease the pressure in the helium tank, the control valve can be opened further and / or held in an open position. Conversely, to increase the pressure in the helium tank, the control valve can be closed further and / or held in a closed position. Commonly known control methods such as P, I, PI, PD, or PID control can be used.

[0019] According to the invention, the setpoint SW of the pressure in the helium tank is determined in the control unit during normal operation, taking into account the (instantaneous) measured second pressure value D2, the (instantaneous) measured first pressure value D1, and programmed settings. During the adjustment of the first pressure values ​​D1 to the setpoint SW, the control valve is therefore adjusted depending on both the first pressure values ​​D1 and the second pressure values ​​D2, as well as the programmed settings.

[0020] The electronic control device keeps (if there is no operational fault, with a constant setpoint or only slowly changing setpoint, for example with a setpoint change of 10 mbar / h or less, usually 5 mbar / h or less) the measured first pressure values ​​D1 always close to the setpoint SW, typically with a deviation of a maximum of 1 mbar, preferably a maximum of 0.5 mbar, particularly preferably a maximum of 0.25 mbar.

[0021] During normal operation for NMR measurements, the setpoint SW is typically kept constant (at least for the duration of an NMR measurement), and the pressure in the helium tank is accordingly maintained close to the setpoint by the control system. In certain operating situations, such as refilling with liquid helium, specific helium tank pressure profiles can be programmed. For example, a gradual reduction of the helium tank pressure to atmospheric pressure to prepare for opening the tank to insert a filling nozzle, or a gradual increase of the helium tank pressure from atmospheric pressure back to normal operating pressure. For this, the setpoint is adjusted accordingly by the control system. The atmospheric pressure (or a pressure in the helium recovery system) can be determined via the second pressure sensor.

[0022] The second pressure sensor or the second setpoint pressure values ​​can detect when a change in the setpoint SW for the helium tank pressure is necessary for safe continued operation, for example, due to severe weather changes. Likewise, the second pressure sensor or the second setpoint pressure values ​​D2 can be used to establish a desired, energy-efficient profile for pressure changes and / or helium gas flows.

[0023] Note that, within the scope of the invention, a pressure or pressure value of interest can be measured directly, or indirectly via a difference between the pressure or pressure value of interest and another known pressure or pressure value. For example, second pressure values ​​D2 of atmospheric pressure can be measured via differential pressure values ​​between the atmosphere and the helium tank if the helium tank pressure is known via the first pressure values ​​D1.

[0024] The invention provides for, that the control device is configured to change the setpoint SW for the pressure in the helium tank in increments during normal operation as soon as a difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds predetermined thresholds, whereby the setpoint SW is raised by one step when the difference DIF becomes less than or equal to an increase threshold HSW, and the setpoint SW is lowered by one step when the difference DIF becomes greater than or equal to a decrease threshold SSW, with HSW <SSW. Dies verbessert die Verfügbarkeit des NMR-Magneten. Durch die stufenweise Veränderung des Sollwerts kann die Häufigkeit von Änderungen des Sollwerts (und damit eine entsprechende Änderungen des Drucks im Heliumtank) gering gehalten werden, und entsprechend sind dann Unterbrechungen des Messbetriebs nur selten erforderlich.

[0025] Weather changes cause atmospheric pressure to change, sometimes quite drastically.

[0026] If atmospheric pressure were to rise sharply and exceed the currently setpoint for the initial pressure values ​​or the helium tank pressure, the control system would no longer be able to maintain a constant helium tank pressure. This is because the control system relies on a pressure gradient between the helium tank and the helium sink (atmosphere or helium recovery system). To ensure a stable pressure in the helium tank, the existing setpoint is increased by one step to a new, higher setpoint. During the pressure adjustment to the new setpoint, an NMR measurement would be disrupted. However, a stable pressure in the helium tank can then be maintained.

[0027] If atmospheric pressure drops too far below the current setpoint for the helium tank pressure, helium would escape from the tank through safety devices (pressure relief valves or even rupture discs). In this case, the pressure in the helium tank would no longer be stable, valuable helium would be lost, and rupture discs would require repair. To prevent this, the setpoint is then adjusted by one step to a new, lower setpoint. While the pressure in the helium tank is being adjusted to the new setpoint, an NMR measurement would be disrupted. Afterward, however, a stable pressure can be maintained in the helium tank.

[0028] Note that a setpoint change from a previous setpoint to the new setpoint can be achieved by making several partial setpoint changes until the new setpoint is reached. Alternatively, a continuous setpoint change (e.g., using a linear ramp) can be implemented until the new setpoint is reached. This allows the pressure change per unit of time in the helium tank to be limited.

[0029] A step here refers to the difference between the old setpoint and the new setpoint, where continuous operation / measuring operation of the helium tank or the NMR magnets was or is provided for at each setpoint. According to the invention, a step is selected in the range of 5-25 mbar, preferably between 8 and 20 mbar, and particularly preferably between 10 and 15 mbar. Note that the steps by which the setpoint is lowered can differ from the steps by which the setpoint is raised. Furthermore, note that the steps should not be chosen too small, and the rise and fall thresholds should not be too close together, in order to limit the frequency of setpoint changes and maintain a high availability of the NMR measuring setup.The new threshold is usually changed by the specified step relative to the previous setpoint; however, it is also possible to set the new threshold relative to a current pressure value (e.g., the current atmospheric pressure).

[0030] The lift threshold HSW and the lower threshold SSW effectively establish an interval for the difference DIF, and the setpoint SW is adjusted when the difference DIF leaves this interval. As long as DIF remains within the interval, the setpoint remains constant. Designs relating to sensors

[0031] In a preferred embodiment of the device according to the invention, the second pressure is a pressure in the surrounding atmosphere. The pressure of the surrounding atmosphere (also referred to simply as atmospheric pressure) is particularly relevant for operational safety, especially since the safety devices of the helium tank (pressure relief valves and rupture discs) react to the pressure difference between the helium tank pressure and atmospheric pressure. Furthermore, the helium tank pressure can be reduced to atmospheric pressure with good accuracy (via a programmed profile of the target pressure) when the helium tank is to be opened (for example, to prepare for a helium transfer).

[0032] A preferred embodiment also uses a pressure in a helium recovery system. A helium recovery system can capture valuable helium that has evaporated from the helium tank and collect it, for example, for reliquefaction. The pressure in the helium recovery system is typically slightly (usually up to 5 mbar) above atmospheric pressure. If a helium recovery system is connected to the helium line leading from the helium tank, the pressure in the helium tank can only be reduced to the pressure in the helium recovery system, which can then be easily regulated using the second pressure sensor.

[0033] A particularly preferred embodiment is one in which the device further comprises a third pressure sensor to measure a third pressure outside the helium tank, The second and third pressures represent the pressure in the surrounding atmosphere and the pressure within the helium recovery system, respectively. For example, the second pressure sensor measures the pressure of the surrounding atmosphere, and the third pressure sensor measures the pressure within the helium recovery system. By measuring the second and third pressures, malfunctions in the helium recovery system (e.g., a valve that has been accidentally closed or a failed compressor) can be easily identified. A typical malfunction (and thus a typical alarm situation) is a pressure in the helium recovery system that is significantly higher (e.g., more than 5 mbar) than atmospheric pressure.

[0034] A particularly preferred embodiment is one in which the device further comprises A flow sensor is used to measure the helium flow rate of the outgoing helium gas stream from the helium tank. The flow sensor is connected in series with the control valve (typically in the outgoing helium line). The flow sensor enables enhanced monitoring and analysis capabilities. In particular, it allows for the easy detection of dangerous icing that blocks the flow of helium gas from the helium tank.

[0035] An advantageous embodiment includes a storage device or a connection for a storage device, which records sensor values ​​received by the control device. Storing the sensor data makes it available for later analysis. In particular, artifacts in NMR measurements can be correlated with pressure fluctuations in and / or outside the helium tank, or the helium consumption of the laboratory or the NMR magnet can be analyzed. Typically, the storage device also records generated control information, such as the current setpoint for the helium tank pressure or control commands to the control valve. The storage device usually logs the operation of a predefined past period, for example, the last 30 days or the last 180 days. Embodiments relating to the regulation of tank pressure

[0036] Particularly preferred is an embodiment in which the control device is configured to convert the setpoint SW for the pressure in the helium tank to a target value ZW over a predetermined period of time in a special operation for refilling liquid helium, wherein the target value ZW depends on the second pressure value D2, in particular wherein the period of time can be selected by a user.

[0037] This allows the helium tank pressure to be transferred to the target value in an energy-efficient manner as part of the continuous adjustment of the initial pressure values ​​D1 to the time-dependent (slowly) changing setpoint.

[0038] This function is particularly useful for preparing a helium transfer (refilling the helium tank with liquid helium). Note that helium transfers are usually planned several days in advance. For the helium transfer, the helium tank must be opened (at a designated access point) against atmospheric pressure.

[0039] Rapid pressure changes in a helium tank are energetically inefficient. This is especially true for a rapid pressure drop, such as occurs when a helium tank whose pressure is above atmospheric pressure is suddenly opened. The slightly pressurized gas in the tank before opening escapes abruptly. The enthalpy of this gas cannot then be used for cooling purposes (e.g., at the tank supports). Furthermore, a large quantity of cold gas escapes due to a sudden increase in the evaporation rate of the boiling helium (which can also trigger a quench), and its enthalpy cannot be utilized either.

[0040] If the setpoint is changed depending on the second pressure value, a slow and precise transition of the helium tank pressure to the desired target value (usually atmospheric pressure itself), which depends on the second pressure, can be achieved, for example, with setpoint changes of 10 mbar / h or less, or preferably 5 mbar / h or less. A user can schedule the transition time so that the desired target value is reached at a specific time, e.g., when the helium transfer is scheduled to begin.

[0041] In a preferred further development of this embodiment, the target value ZW corresponds to the second pressure value D2 with ZW=D2, or the target value ZW is slightly higher than the second pressure value D2 by a small pressure increase DA with ZW=D2+DA and DA≤3 mbar. With ZW=D2 (where the second pressure is atmospheric pressure), an opening to the atmosphere can occur without a pressure drop. When ZW=D2+DA (where the second pressure is atmospheric pressure), a small overpressure can be maintained in the helium tank for a comparatively long time, even at the end of the transfer, preventing the ingress of contaminants.

[0042] Further training is advantageous, whereby the control system is configured to linearly adjust the setpoint SW to the target value ZW over time during special operation for refilling liquid helium. This is particularly easy to implement, especially via software programming.

[0043] In an alternative further training, the control device is set up to change the setpoint SW non-linearly over time in the special operation for refilling liquid helium, whereby a helium flow through the control valve is kept approximately constant during the duration of the transition from the setpoint SW to the target value ZW.

[0044] Either a transition curve for the setpoint can be calculated / determined in advance as a function of time so that an approximately constant helium flow rate is achieved. Alternatively, a flow meter can be provided with which a helium flow rate of the outflowing helium gas stream from the helium tank through the control valve can be measured, and the control device uses the measured helium flow rate as a (further) controlled variable.

[0045] The pressure and temperature of the boiling helium in the helium tank are related via the vapor pressure curve. However, this only applies to the liquid at the surface, as helium has a relatively poor thermal conductivity. Within the liquid helium, a temperature gradient temporarily forms when the pressure changes.

[0046] When the pressure in the helium tank is rapidly increased, the temperature of the liquid helium at the surface rises. The liquid helium below the surface remains cooler and warms up to the surface temperature only slowly. During this time, less liquid helium evaporates than would be expected given the heat load on the helium tank, because the helium below the surface absorbs heat due to its considerable heat capacity.

[0047] When the pressure in the helium tank is rapidly reduced, the temperature of the liquid helium at the surface drops. The liquid helium below the surface remains warmer and cools only slowly to the surface temperature. During this time, more liquid helium evaporates than would be expected based on the heat load on the helium tank, because energy must be extracted from the helium below the surface to cool it. The cooling effect is provided by this increased evaporation.

[0048] If the helium flow rate is kept approximately constant, the enthalpy of the cold gas can be optimally utilized. Typically, the helium flow rate is kept constant with an accuracy around a target helium flow rate of ±20% (or even more precise), preferably ±15% (or even more precise), and most preferably ±10% (or even more precise).

[0049] In a preferred sub-variant of this further development, the control device is configured to lower the setpoint SW more quickly at the beginning of the transition than towards the end of the transition during the special operation for refilling liquid helium when the setpoint SW is reduced to the target value ZW. When reducing the pressure in the helium tank, it is advantageous to reduce the pressure more quickly at the beginning (to artificially accelerate the initial evaporation) and to reduce the pressure more slowly towards the end of the pressure reduction phase (to keep the evaporation rate constant). Designs for the alarm system and alarm situations

[0050] A particularly advantageous embodiment includes an alarm device that automatically triggers an alarm message in one or more predefined alarm situations, in particular wherein the alarm device comprises an acoustic signal generator and / or a visual signal generator and / or a radio signal generator and / or a data signal generator. The alarm message can alert a user to hazardous conditions or prompt them to perform a more thorough check of the cryostat system. The alarm message can initiate manual troubleshooting or safety measures, or it can automatically trigger and execute such measures.

[0051] A preferred embodiment includes an alarm situation in which the pressure value D1 of the helium tank falls below the pressure value D2 of atmospheric pressure. If the pressure in the helium tank falls below atmospheric pressure, a potentially hazardous situation arises because contaminants can be drawn into the helium tank through small leaks. This is particularly true for drawn-in air, whose components (e.g., nitrogen) or moisture could freeze in the helium tank and cause it to ice up. This hazardous situation can arise especially in systems with active cooling if the cooling is "too strong," i.e., if the active cooling overcompensates for the heat load on the helium tank.

[0052] Further training is also advantageous in which an alarm situation includes the pressure value D1 of the helium tank pressure exceeding a predetermined helium tank pressure maximum value, in particular wherein a first alarm situation for D1 includes the pressure value D1 of the helium tank pressure exceeding a predetermined first helium tank pressure maximum value EHM, a second alarm situation for D1 includes the pressure value D1 of the helium tank pressure exceeding a predetermined second helium tank pressure maximum value ZHM, and ZHM>EHM, and the alarm messages for the first alarm situation for D1 and for the second alarm situation for D1 are different.

[0053] A helium tank pressure above a predetermined maximum pressure can indicate various types of malfunctions, such as a blocked outgoing helium line or a fault causing an increased helium flow rate (see below). If the cryostat and the device (including the control valve, and optionally the helium recovery system and active cooling) are functioning correctly, an elevated helium pressure indicates a malfunction in the superconducting NMR magnet, leading to an increased evaporation rate and potentially even triggering a quench. A maximum pressure can be set so that it is reached shortly before, or alternatively, at the opening pressure of a safety device (pressure relief valve or rupture disc).If two thresholds (EHM and ZHM) are set up, the first helium tank pressure maximum value (EHM) is typically assigned to triggering the pressure relief valves (which can be caused, for example, by an accidentally closed valve at the magnet output), and the second helium tank pressure maximum value (ZHM) is assigned to triggering / destroying the rupture discs (typically in the case of a quench, i.e., a sudden loss of superconductivity in the NMR magnet, "quench alarm").

[0054] Preferred training includes an alarm scenario where a measured helium flow rate exceeds a predefined maximum value. An increased helium flow rate usually indicates a problem in the NMR magnet, such as a defective active cooling system, a thermal insulation fault, an open switch, or a quenched joint.

[0055] A preferred variant of this improved system is one in which the predefined maximum helium flow rate depends on current or recently controlled changes in the helium tank pressure, particularly where the maximum helium flow rate is higher during and / or shortly after reductions in the helium tank pressure than when the helium tank pressure is kept constant. When the pressure in the helium tank is reduced, there is a (planned) brief increase in the helium flow rate above the normal value. No alarm should be triggered during this time. Due to the poor thermal conductivity of liquid helium, mentioned above, the helium flow rate remains elevated for a certain period after a pressure reduction. This brief period can generally be estimated at 3 hours or less, usually 2 hours or less. A change in the helium tank pressure is typically initiated by modifying the predefined setpoint SW of the helium tank pressure.

[0056] A further advantage is a development in which an alarm situation includes a measured helium flow rate falling below a predetermined minimum value. If the helium flow rate is too low, this could indicate a leak in the line between the helium tank and the flow sensor, a leak in the helium tank itself, or icing in the helium tank or in the line leading from the helium tank that contains the flow sensor. It is also possible that the NMR magnet is simply not connected to the monitoring system (i.e., the device according to the invention).

[0057] An advantageous variant of this advanced design is one in which the predefined minimum helium flow rate depends on current or recently implemented changes in the helium tank pressure. Specifically, the minimum helium flow rate is lower during and / or shortly after increases in the helium tank pressure than when the pressure is constant. When the pressure in the helium tank is increased, the helium flow rate drops briefly below the normal value. In this case, no alarm should be triggered. Due to the poor thermal conductivity of liquid helium, as mentioned above, the helium flow rate is also reduced for a certain period after a pressure increase. Note that the predefined minimum helium flow rate can even be set to "zero" during and / or shortly after increases in the helium tank pressure, so that even a helium flow rate of "zero" will not trigger an alarm during this time.The short time mentioned above can generally be estimated at 3 hours or less, usually 2 hours or less.

[0058] An advantage of further training is a system where an alarm situation is triggered when the measured helium flow rate is zero and, simultaneously, the control valve is not in its current position or is currently controlled by the system. A drop in flow rate to zero indicates a dangerous, complete freezing of the cryostat or at least the outgoing helium line. However, the control state should be considered: If the control valve is (as intended) completely closed, for example, to (deliberately) increase the pressure in the helium tank, no alarm should be triggered. Freezing of the cryostat (or the outgoing helium line) is a dangerous situation. The suspension tubes through which the helium normally flows are then blocked, and the helium can no longer escape from the helium tank.Because the heat input causes helium to constantly evaporate in the helium tank, pressure builds up until the tank bursts. With NMR magnets, it is recommended to regularly check whether helium is escaping from the helium tank outlet. According to the invention, this monitoring can be carried out continuously and automatically.

[0059] A further preferred method is one in which an alarm situation is triggered when the measured helium flow rate is zero and, simultaneously, the pressure value D2 in the helium recovery system has risen to the pressure value D1 in the helium tank. This function allows differentiation between icing and a closed valve in the helium recovery system. In both cases, the helium flow rate drops to zero; in the case of icing, the measured recovery system pressure does not rise, while in the case of a closed valve, it does. The alarm situation thus detects a closed valve in the helium recovery system.

[0060] A further preferred method is one in which an alarm situation is triggered when the difference DHA=DHR-DAT between a measured pressure value DHR in the helium recovery system and a measured pressure value DAT in the surrounding atmosphere exceeds a predefined threshold SWW, in particular where SWW is chosen in the range of 2.5 mbar to 20 mbar. SWW is usually also chosen to be greater than 5 mbar. This function detects a malfunction in the helium recovery system, for example, a defective compressor.

[0061] In an advantageous further development, it is stipulated that at least some of the alarm situations include the current position of the control valve or a position of the control valve currently being controlled by the control device. This allows hazardous situations or malfunctions to be detected in a targeted manner in many cases and more easily distinguished from intended operating states. For example, a vanishing measured helium flow is not critical if a closed control valve is being controlled as intended. Pressure control methods

[0062] The present invention also includes a

[0063] Method for controlling the pressure in a helium tank of an NMR magnet, wherein a first pressure in the helium tank (is measured with a first pressure sensor, wherein a control valve regulates the outflow of helium gas from the helium tank, wherein an electronic control device actuates the control valve, wherein at least one second pressure sensor measures a second pressure outside the helium tank, wherein the electronic control device receives first pressure values ​​D1 measured by the first pressure sensor, and adjusts the position of the control valve depending on the measured first pressure values ​​D1, such that the first pressure values ​​D1 are regulated to a predetermined setpoint SW, and receives second pressure values ​​D2 measured by the second pressure sensor, and wherein the electronic control device further determines the setpoint SW as a function of the measured first pressure values ​​D1 and second pressure values ​​D2 in normal operation for NMR measurements, namely by stepping the setpoint SW for the pressure in the helium tank (2),As soon as a difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds predefined thresholds, the setpoint SW is raised by one step if the difference DIF becomes less than or equal to an increase threshold HSW, and the setpoint is lowered by one step if the difference DIF becomes greater than or equal to a decrease threshold SSW, with HSW <SSW, und wobei eine jeweilige Stufe, um die der Sollwert SW verändert wird, sobald die Differenz DIF=D1-D2 zwischen dem gemessenen ersten Druckwert D1 und dem gemessenen zweiten Druckwert D2 die vorgegebenen Schwellwerte erreicht oder überschreitet, im Bereich 5-25 mbar gewählt wird, insbesondere wobei das Verfahren unter Verwendung einer erfindungsgemäßen,The above-mentioned device is used. The inventive method for regulating the pressure in the helium tank allows for improved operational reliability and greater user-friendliness.

[0064] According to the invention, it is possible to change a previous setpoint for the helium tank pressure (with which a substantially constant helium tank pressure was established) to a modified, new setpoint as needed (with which a substantially constant helium tank pressure can then be re-established after a brief disturbance). The need for the change can be identified via the measured second pressure values ​​D2 (and the first pressure values ​​D1).The need for adjustment typically arises when the second pressure values ​​D2 (representing atmospheric pressure or another pressure dependent on atmospheric pressure, for example, in a helium recovery system) indicate that the pressure differential required for control between the helium tank and the subsequent helium sink (further installation downstream of the control valve) is becoming too small, or that a pressure differential between the helium tank and the surrounding atmosphere is becoming so large that safety devices (pressure relief valves or rupture discs) could be triggered. According to the invention, this setpoint change is performed in stages. Between the staged changes of the setpoint, a stable helium tank pressure can be established in this way, allowing for highly accurate NMR measurements. Simultaneously, the NMR magnet or...whose cryostat, including the helium tank, can be operated safely, in particular without control failure or intake of air and without triggering safety devices due to excessive pressure differences.

[0065] It is also possible to slowly adjust the helium tank pressure relative to the second pressure to a target value in a defined manner by means of a correspondingly programmed setpoint change, in order to utilize the cooling effect of the gas escaping from the helium tank particularly efficiently.

[0066] Finally, some hazardous situations can also be detected with the second pressure sensor.

[0067] A variant that provides for is also preferred. that the control device in a special operation for refilling liquid helium transitions the setpoint SW for the pressure in the helium tank to a target value ZW over a predetermined period of time, wherein the target value ZW depends on the second pressure value D2, in particular wherein the period can be selected by a user, and in particular wherein the method is carried out using a device according to claim 9. The target value ZW, which depends on the second pressure values ​​D2, allows a defined final state with desired properties relative to the second pressure to be reliably achieved, in particular so that subsequent measures, such as opening the helium tank to the atmosphere, can be reliably carried out under the desired conditions (desired pressure conditions). In particular, a sharp, sudden pressure drop can be avoided when opening the helium tank to the atmosphere, for example, before a helium transfer.Furthermore, a desired, energetically favorable helium pressure profile can be established via the programmed transition of the setpoint, thus enabling high energy efficiency; the transition (change in the setpoint) is typically continuous and monotonic, preferably strictly monotonic. The transition is carried out in such a way that the initial pressure values ​​D1 always remain close to the setpoint SW (which can be achieved by a suitable control speed, taking into account the interaction of the control system and the NMR magnet, see above).

[0068] A preferred further development of this variant is one in which the control device in the special operation for refilling liquid helium changes the setpoint SW non-linearly over time, wherein a helium flow through the control valve is kept approximately constant during the transition from the setpoint SW to the target value ZW, particularly wherein the method is carried out using a device according to claim 10. The non-linear change of the setpoint allows for the consideration of temperature gradients in the liquid helium that arise during controlled pressure changes in the helium tank, which affect the evaporation rate of helium gas and only dissipate over a certain period of time. The non-linear change of the setpoint allows for the consideration of temperature gradients in the liquid helium that occur during controlled pressure changes in the helium tank.By controlling the pressure change in the helium tank, the approximately constant evaporation rate of helium gas (corresponding to a constant helium gas flow) can then be set, which is particularly energy-efficient.

[0069] A preferred variant of the method according to the invention provides that that an alarm device automatically triggers an alarm message in one or more predefined alarm situations, in particular wherein the alarm device comprises an acoustic signal generator and / or an optical signal generator and / or a radio signal generator and / or a data signal generator, and that an alarm situation comprises that a measured helium flow rate of the outflowing helium gas stream from the helium tank through the control valve is zero and at the same time a current position of the control valve or a position of the control valve currently controlled by the control device is not closed, in particular wherein the method is carried out using a device according to claim 19.A helium flow rate of zero, despite the control valve being open, indicates dangerous icing of the helium tank or the helium line leading from the tank that contains the control valve. This condition is detected and reported by the system's function, particularly to initiate countermeasures or safety measures. If the control unit intends to increase the pressure in the helium tank (as planned), for example, because the ambient pressure has risen sharply due to weather conditions, the control unit closes the control valve (as planned), and the helium flow rate drops to zero (as planned) for a period of time, which then does not trigger an alarm. Cryostat setups and NMR measurement setups

[0070] The present invention also includes a cryostat arrangement comprising a vacuum-insulated helium tank, and a device according to the invention as described above, wherein the first pressure sensor is connected to the helium tank, and the second pressure sensor is connected to a location outside the helium tank, in particular wherein the second pressure sensor is connected to the surrounding atmosphere or to a helium recovery system, and wherein the control valve is arranged in a helium line leading from the helium tank. The cryostat arrangement enables pressure control in the helium tank with improved operational reliability and ease of use.

[0071] A preferred embodiment of the cryostat arrangement according to the invention comprises a helium recovery system connected to the outgoing helium line. This allows the valuable evaporating helium gas to be captured and stored, and in particular, reused after reliquefaction for further cooling of the NMR magnet.

[0072] Furthermore, the present invention also includes an NMR measurement arrangement comprising The invention comprises a cryostat arrangement as described above, a superconducting NMR magnet in the helium tank of the cryostat arrangement, an NMR probe head that projects into a room-temperature bore of the vacuum-insulated helium tank, and an NMR spectrometer control unit for controlling NMR measurements with the NMR probe head. The NMR measurement arrangement enables high-resolution NMR measurements with high operational reliability and ease of use, particularly while avoiding artifacts caused by pressure fluctuations in the helium tank.

[0073] The use of an NMR measurement arrangement according to the invention, described above, for carrying out NMR measurements also falls within the scope of the present invention. The electronic control unit communicates the status of the helium tank pressure control to the NMR spectrometer control unit, and the NMR spectrometer control unit pauses the NMR measurements during periods of pressure instability in the helium tank. This procedure avoids artifacts in the NMR measurements. Typical periods of pressure instability during which the NMR measurements are paused are times when the control unit initiates a change in the setpoint, and the pressure in the helium tank changes accordingly.Furthermore, NMR measurements can also be paused during periods following a change in the setpoint, while temperature gradients in the liquid helium are still equalizing (i.e., the helium tank, and therefore the pressure within it, is not yet in thermal equilibrium). These periods typically last up to three hours and can often be identified by a still-fluctuating helium flow rate (or a corresponding change in the controlled or actual position of the regulating valve). While the helium flow rate is still changing, the temperature distribution within the cryostat can also change, which in turn can cause length changes, for example, in the helium tank's suspensions; such length changes can lead to artifacts in NMR measurements.

[0074] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those described in more detail below can each be used individually or in any combination according to the invention. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for illustrating the invention. Detailed description of the invention and drawing

[0075] Fig. 1 schematically illustrates a first embodiment of a device according to the invention for regulating the pressure in a helium tank, wherein a second pressure sensor determines a second pressure in the surrounding atmosphere; Fig. 2 schematically illustrates a second embodiment of a device according to the invention, wherein a second pressure sensor determines a second pressure in the surrounding atmosphere, and a flow sensor measures an outflowing helium gas flow; Fig. 3 schematically illustrates a third embodiment of a device according to the invention, wherein a second pressure sensor determines a second pressure in a helium recovery system; Fig. 4 schematically illustrates a fourth embodiment of a device according to the invention, wherein a second pressure sensor determines a second pressure in the surrounding atmosphere and a third pressure sensor determines a third pressure in a helium recovery system; Fig.Figure 5 schematically illustrates a fifth embodiment of a device according to the invention, similar to the embodiment of . Fig. 4 , wherein an additional conduit is provided for the outflow of helium in the event of a power failure; Fig. 6 schematically illustrates an embodiment of an NMR measuring arrangement according to the invention, comprising an embodiment of a cryostat arrangement according to the invention, wherein the cryostat arrangement comprises a device according to the invention, which here is configured according to the embodiment of Fig. 5is formed; Fig. 7 illustrates in a schematic diagram a first variant of a method according to the invention for controlling the pressure in a helium tank, wherein in a special operating mode the setpoint for the pressure in the helium tank is reduced along a linear ramp to a target value corresponding to the measured atmospheric pressure; Fig. 8 illustrates in a schematic diagram a second variant of a method according to the invention for controlling the pressure in a helium tank, wherein in a special operating mode the setpoint for the pressure in the helium tank is reduced along a non-linear ramp to a target value corresponding to the measured atmospheric pressure; Fig. 9 illustrates in a schematic diagram a third variant of a method for controlling the pressure in a helium tank, wherein in normal operating mode the setpoint for the pressure in the helium tank is changed in steps; Fig.Figure 10 schematically illustrates a test procedure for detecting icing of the cryostat and a corresponding output of an alarm message, wherein the test procedure can be applied in a method according to the invention for regulating the pressure in a helium tank.

[0076] The Figure 1 Figure 1 schematically shows a first embodiment of a device 1 according to the invention for regulating the pressure in a helium tank 2 of an NMR magnet.

[0077] A helium line 3, leading from the helium tank 2, connects the helium tank 2 to a helium recovery system 4. An automatically actuated control valve 5 is integrated into the helium line 3. The control valve 5 is actuated, for example, by an electric motor (not shown). A first pressure sensor 6 is also located in the helium line 3 upstream of the control valve 5, which measures the pressure in the helium tank 2.

[0078] The first pressure values ​​D1 determined by the first pressure sensor 6 are processed by an electronic control device (not shown in detail here, but see below). Fig. 6 The measured values ​​(for this purpose) are read out and compared with a predefined setpoint SW for the helium tank pressure. The control unit actuates the control valve 5 so that the helium tank pressure, or rather the measured initial pressure values ​​D1, are adjusted to the setpoint SW. If the adjustment works correctly (i.e., there is no fault / defect), the initial pressure values ​​D1 always remain close to the setpoint SW, typically with a deviation of 1 mbar or less.

[0079] Furthermore, a second pressure sensor 7 is provided, which measures the pressure difference between the pressure in the helium tank and the pressure of the surrounding atmosphere (atm). This (and knowledge of the first pressure values ​​D1) indirectly measures a second pressure outside the helium tank 2, in this case, atmospheric pressure. The corresponding second pressure values ​​D2 of the atmospheric pressure are evaluated by the control unit, which uses these second pressure values ​​D2 to determine the predetermined setpoint SW, which is then used to adjust the control valve 5.

[0080] Note that atmospheric pressure depends on the weather and changes over time, and may change drastically under certain weather conditions.

[0081] Under normal operating conditions, the setpoint SW is kept constant as long as the measured second pressure values ​​D2 (and possibly also the first pressure values ​​D1) do not indicate a need to change the setpoint SW. The criteria for when a change to the setpoint SW is necessary and how this change should then be implemented are defined in the control unit's programming. According to the invention, the setpoint SW is kept constant as long as the second pressure values ​​D2 remain within a predetermined interval relative to the previous setpoint SW (or relative to the respective first pressure value D1). If the second pressure values ​​D2 deviate from the predetermined interval, the setpoint SW is changed incrementally (more on this in [reference]). Fig. 9 ).

[0082] Furthermore, it is possible to program a gradual transition of the helium tank pressure or the setpoint SW to a target value ZW for special operation within the control system, whereby the target value ZW is determined using the second pressure values ​​D2. For example, the respective second pressure value D2, here atmospheric pressure, can be selected as the target value to prepare for opening the helium tank to the atmosphere in order to refill liquid helium in helium tank 2 (more on this in [reference]). Fig. 7 and Fig. 8 ).

[0083] In the embodiment of Fig. 1 The control valve 5 is designed so that in the event of a power failure, it assumes a fully open position. This ensures that no dangerous pressure can build up in the helium tank 2. The flow of helium gas when the control valve 5 is fully open is limited by a throttle 8 in the helium line 3.

[0084] The Figure 2Figure 1 schematically shows a second embodiment of a device 1 according to the invention for regulating the pressure in a helium tank 2. The embodiment of Fig. 2 largely corresponds to the embodiment of Fig. 1 , so that only the essential differences are explained.

[0085] In device 1 of Fig. 2 The helium line 3 also contains a flow sensor 14, which measures the current flow of helium gas through the helium line 3 or the control valve 5. The corresponding helium flow values ​​are read by the control unit. The flow sensor 14 is located downstream of the control valve 5 in the helium line 3, in the direction of flow.

[0086] The control device can use the helium flow values ​​to monitor the correct operating state of device 1 or the entire associated cryostat (see Fig. 6(for this purpose) perform various checks. Typically, the control unit checks for different alarm situations. In particular, it can check whether there is dangerous icing of the cryostat (see also). Fig. 10 ).

[0087] The flow sensor 14 typically determines the instantaneous volume of helium gas flowing through per unit time (dV / dt); a turbine flow meter can be used for this purpose. However, it is also possible to determine the instantaneous mass of helium gas flowing through per unit time (dm / dt) with a flow sensor, for example with a Coriolis mass flow meter or a thermal mass flow meter.

[0088] The Figure 3 shows a third embodiment of a device according to the invention, which is largely the embodiment of Fig. 2 This corresponds to the above. Therefore, only the essential differences will be explained.

[0089] In the embodiment of Fig. 3 A second pressure sensor 7a is provided, which measures the pressure difference between the pressure in the helium tank 2 and the pressure in the helium recovery system 4 (here at its inlet). This (and knowledge of the first pressure values ​​D1) indirectly measures a second pressure outside the helium tank 2, in this case the pressure in the helium recovery system 4. The corresponding second pressure values ​​D2 of the pressure in the helium recovery system 4 are evaluated by the control unit, whereby the predetermined setpoint SW is determined using the second pressure values ​​D2, which is used for adjusting the control valve 5.

[0090] Note that the pressure in the helium recovery system 4 is typically slightly above atmospheric pressure, usually by up to 5 mbar.

[0091] The Figure 4 shows a fourth embodiment of a device according to the invention, which largely corresponds to the embodiment of Fig. 2This corresponds to the above. Therefore, only the essential differences will be explained.

[0092] In this embodiment, a second pressure sensor 7 is provided, which measures the differential pressure between the helium tank 2 and the surrounding atmosphere (atm), thereby indirectly measuring the atmospheric pressure (using the first pressure values ​​D1). The corresponding second pressure values ​​D2 (also referred to as DAT) of the measured atmospheric pressure are transmitted to the control unit. A third pressure sensor 9 is also provided, which measures the pressure difference between the helium tank 2 and the helium recovery system 4 (here at its inlet), thereby indirectly measuring the pressure in the helium recovery system 4 (using the first pressure values ​​D1). The corresponding third pressure values ​​D3 (also referred to as DHR) of the pressure in the helium recovery system 4 are also transmitted to the control unit.

[0093] The second pressure value, D2, is used to determine the specified setpoint. Various alarm conditions are checked using the first pressure value, D1, the second pressure value, D2, the third pressure value, D3, and the helium flow rate values. These alarm conditions allow the electronic control unit to monitor the proper operation of device 1 and the entire cryostat. In particular, malfunctions in the helium recovery system 4 can be detected by comparing the second pressure value, D2 (=DAT), and the third pressure value, D3 (=DHR). For example, a failed compressor in the helium recovery system 4 will cause the pressure, DHR, to exceed the pressure, DAT, by more than a predefined threshold value, SWW, where SWW is, for example, set to 10 mbar.

[0094] The Figure 5 shows a fifth embodiment of a device according to the invention, which largely corresponds to the embodiment of Fig. 4This corresponds to the above. Therefore, only the essential differences will be explained.

[0095] In the embodiment of Fig. 5 An additional line 10 is provided, running parallel to the helium line 3 from the helium tank 2 to the helium recovery system 4. The additional line 10 includes a shut-off valve 11 and a throttle 12. During normal operation, the shut-off valve 11 is held closed by an electric actuator (not shown), thus blocking the additional line 10, and allowing helium gas flow only through the helium line 3.

[0096] The control valve 5 in the helium line 3 is designed to close in the event of a power failure; the helium line 3 is then blocked. In contrast, the shut-off valve 11 is designed to open in the event of a power failure (the electric actuator can no longer keep the shut-off valve 11 closed during a power failure; instead, the shut-off valve 11 opens, typically by spring force). This allows helium gas to flow from the helium tank 2 to the helium recovery system 4 through the additional line 10 during a power failure; the flow of helium gas is limited by the throttle 12 in the additional line 10. This prevents a dangerous pressure build-up in the helium tank 2 during a power failure.

[0097] The Figure 6Figure 20 illustrates an exemplary embodiment of an NMR measuring device 20 according to the invention, in which an exemplary embodiment of a cryostat arrangement 21 according to the invention is included.

[0098] The cryostat arrangement 21 comprises, firstly, an evacuated container 22 (also called a vacuum tank) in which a helium tank 2 is arranged; the helium tank 2 is thus vacuum-insulated. The helium tank 2 is partially filled with liquid helium and partially filled with gaseous helium (helium not shown in detail), and also contains a superconducting NMR magnet 23, which is cooled by the liquid helium.

[0099] Secondly, the cryostat arrangement 21 includes a device 1 for regulating the pressure in the helium tank 2, which is essentially the same as in Fig. 5The electronic control unit 13 is shown in the diagram. Also shown is the electronic control unit 13, which receives measured values ​​from the pressure sensors 6, 7, 9 and the flow meter 14, and can also control and read the position of the control valve 5. The control unit also includes a storage device 17 in which received measurement data and generated control data from the last 30 days are stored. The electronic control unit 13 is also connected to an alarm device 15, which includes an acoustic and visual signal generator 16 that indicates detected alarm situations.

[0100] The helium tank 2 is connected to the device 1, more precisely to a middle section 26 of the helium line 3 which runs inside the device 1, via neck tubes 24 and a front section 25 of the outgoing helium line 3. Accordingly, the first pressure sensor 6 can measure the pressure in the helium tank 2.

[0101] The cryostat arrangement 21 also includes the helium recovery system 4, which is connected to a rear section 27 of the helium line 3. The helium recovery system 4 comprises a balloon reservoir 28 in which helium gas is initially collected. The balloon reservoir 28 is exposed to atmospheric pressure and, during operation, inflates slightly against atmospheric pressure due to the inflow of helium, so that the pressure inside the balloon reservoir 28 is slightly above atmospheric pressure. A compressor 29 is connected to an outlet of the balloon reservoir 28. This compressor can compress helium from the balloon reservoir (for example, to a pressure of up to 200 bar) and store the compressed helium gas in pressurized gas cylinders 30.

[0102] The NMR measurement system 20 comprises, in addition to the cryostat assembly 21, the superconducting NMR magnet 23 in the helium tank 2, and an NMR probe 31 that protrudes into a room-temperature bore 22a of the vacuum tank 22 or the vacuum-insulated helium tank 2. The NMR measurement system 20 also includes an NMR spectrometer control unit 32, which can be used to control NMR measurements with the NMR probe 31. NMR measurements can be performed on samples that are placed in a sample volume 33 in the region of the outer end of the NMR probe 31, typically with the samples being introduced from above into the room-temperature bore 22 via a sample feeder system (sample feeder system not shown). The NMR spectrometer control unit 32 also receives information about the status of the helium tank pressure control from the electronic control unit 13.

[0103] The Figure 7Illustrates in an example the course of a first variant of an inventive method for controlling the pressure in a helium tank for special operation by means of a diagram (for normal operation according to the invention see Fig. 9 The graph to the right shows time based on example times. The graph above shows the measured pressure (for the dashed curve 71, the pressure in the helium tank, i.e., the first pressure values ​​D1, and for the solid curve 72, the measured atmospheric pressure, i.e., the second pressure values ​​D2) and the measured helium flow rate (helium flow rates, dotted curve 73). In this scenario, the helium tank is to be opened against atmospheric pressure for refilling with liquid helium, and the helium tank pressure is to be adjusted / lowered accordingly. Note that the Fig. 7 was obtained with experimental measurements. Brief overview

[0104] Before the temperature reduction begins, between 7:00 a.m. and approximately 11:00 a.m., normal operation of the helium tank takes place, during which, for example, NMR measurements can be performed. The target value for the helium tank pressure is set to 985 mbar, and the measured helium tank pressure values ​​in curve 71 are very close to 985 mbar. The measured atmospheric pressure in curve 72 fluctuates slightly around a value of approximately 965 mbar. The slight overpressure in the helium tank prevents the intake of contaminants (especially humid air). During this normal operation, a relatively small, approximately constant flow of helium gas of about 10 ml / h liquid equivalent flows through the control valve; the cooling effect associated with the evaporation of this amount of helium balances the heat load on the cryostat.

[0105] At 11:00 AM, the pressure in the helium tank begins to decrease. The decrease is programmed so that the target pressure of the helium tank is reduced at a constant rate (i.e., with a linear ramp) from the previous target value of 985 mbar to atmospheric pressure. The rate of decrease and the control loop as a whole are configured so that the helium tank pressure, or rather the initial pressure values ​​D1 of curve 71, can follow the linearly decreasing target value quite accurately (promptly). To reduce the helium tank pressure according to the programmed target value curve, the electronic control unit opens the control valve significantly wider than during normal operation, and the helium flow rate increases considerably, here to up to 80 ml / h liquid equivalent. Note that during the ongoing decrease in atmospheric pressure, as can be seen from curve 72, it decreases slightly, here to approximately 962 mbar at 17:00 h.

[0106] Shortly after 5:00 PM, the programmed setpoint reaches atmospheric pressure, and the control unit moves the control valve to a less open position. However, due to temperature gradients within the liquid helium in the helium tank, the evaporation rate remains elevated for some time. To maintain the pressure in the helium tank approximately at the setpoint (which, from shortly after 5:00 PM, essentially corresponds to atmospheric pressure), a corresponding outflow of helium gas must be allowed via the control valve by adjusting its position. Around 8:00 PM, the temperature gradients in the helium tank have equalized, and the helium flow rate returns to a constant, low value of just under 10 ml / h liquid equivalent.

[0107] In this state, the helium tank can be opened to the atmosphere without a sudden, energetically unfavorable evaporation of a large quantity of liquid helium. Detailed explanation

[0108] A significant advantage of the invention's use of two pressure sensors is that the pressure in the helium tank (first pressure) can be automatically adjusted to the second pressure (e.g., atmospheric pressure or the pressure in the helium recovery system) at a user-defined time interval. This is desirable, for example, before a helium transfer. For a helium transfer, an opening providing access to the helium tank must be opened to insert the transfer line. During this process, helium gas escapes from the tank—typically so rapidly that the tank expands to atmospheric pressure. This rapid pressure change poses a risk to the magnet (there is a possibility of magnet quenching) and is also thermodynamically inefficient, as the enthalpy of the cold helium gas remains unused during the rapid outflow.

[0109] Within the scope of the invention, the pressure in the helium tank can be reduced slowly and precisely; in particular, the pressure reduction can begin the day before the planned helium transfer. This is more energy-efficient than a "sudden" pressure reduction. An example of a pressure reduction over a predetermined period of 6 hours is shown in the Fig. 7 The solid line (curve 72) shows the atmospheric pressure, the dashed line (curve 71) the pressure in the helium tank, and the dotted line (curve 73) the helium flow rate.

[0110] It can be clearly seen from curve 73 that the helium flow rate initially increases only slowly after the pressure reduction begins around 11:00 AM, then continues to increase during the further pressure reduction (although the pressure reduction is linear), and then remains elevated for several hours after the pressure reduction is completed around 5:00 PM.

[0111] The pressure and temperature of the boiling helium in the helium tank are related via the vapor pressure curve. However, this only applies to the liquid at the surface, as helium has a relatively poor thermal conductivity. Within the liquid helium, a temperature gradient temporarily forms when the pressure changes.

[0112] If the pressure in the helium tank is rapidly reduced, as in the illustrated example above, the temperature of the liquid helium at the surface drops, while the liquid helium below the surface remains warmer and cools only slowly to the surface temperature. During this time, more liquid helium evaporates than would be expected given the heat load on the helium tank, because energy must be extracted from the helium below the surface to cool it. This cooling effect is provided by increased evaporation. After the pressure reduction is complete, the helium below the surface is usually not yet in thermal equilibrium with the surface, and the evaporation rate (or the measured helium flow rate) remains elevated for some time even after the pressure reduction has finished.

[0113] This effect is strongly time-dependent - the faster the pressure reduction is carried out, the more pronounced the effect, as there is less time to approach thermal equilibrium within the liquid volume.

[0114] It is particularly advantageous to keep the helium flow rate as constant as possible during pressure reduction, as this allows for optimal utilization of the enthalpy of the cold gas. Therefore, when reducing the pressure in the helium tank, it is advantageous to initially reduce the pressure more rapidly (to artificially accelerate the initial evaporation) and to reduce the pressure more slowly towards the end of the pressure reduction phase (to keep the evaporation rate constant). This can be achieved with the device according to the invention. Figure 8 This illustrates a second variant of a method according to the invention for pressure control in a helium tank for a special operation.

[0115] The diagram of Fig. 8plots time (in any units) on the right, and pressure (in curve 81 the first pressure / first pressure values ​​D1 in the helium tank, and in curve 82 the atmospheric pressure / second pressure values ​​D2) and the helium flow through the control valve (curve 83, helium flow values) on the top.

[0116] Before time t1, normal operation prevails, and the target value for the helium tank pressure is 1000 mbar. The first pressure values ​​D1 of curve 81 are therefore constant at this target value. The atmospheric pressure of curve 82 is 970 mbar; this atmospheric pressure remains constant throughout the entire observation period. The helium flow rate is initially a low 15 ml / h liquid equivalent.

[0117] Starting at time t1 and continuing through the reduction period until time t2, the setpoint of the helium tank pressure is reduced to atmospheric pressure according to a non-linear ramp; the first pressure values ​​D1 of curve 81 promptly follow the setpoint according to the initial adjustment. At the beginning of the reduction period, the pressure reduction per unit of time is still relatively large and then decreases continuously until the end of the reduction period. By appropriately selecting the profile of the setpoint change, it can be ensured that the helium flow rate remains approximately constant over the entire reduction period from t1 to t2, in the example shown at approximately 30 ml / h liquid equivalent, as can be seen in curve 83. At time t2, the control valve can then be closed again to such an extent that a low helium flow rate of 15 ml / h liquid equivalent is once again established.

[0118] The procedure according to Fig. 7A gradual linear reduction of the setpoint already achieves a significant energy improvement compared to a sudden pressure drop when opening the helium tank, and with the procedure according to Fig. 8 A further increase in energy efficiency can be achieved with non-linear setpoint reduction.

[0119] The Figure 9 Figure 1 shows a third variant of the inventive method for regulating the pressure in a helium tank for normal operation according to the invention. The diagram plots time (in arbitrary units) on the right and pressure on the top (with curve 91 representing the first pressure / helium tank pressure with first pressure values ​​D1, which corresponds with good accuracy to curve 93 of the setpoint pressure in the helium tank, and further with curve 92 representing the second pressure / atmospheric pressure with second pressure values ​​D2).

[0120] In the third variant, the helium tank pressure is stabilized for normal operation of the NMR magnet to minimize measurement artifacts in NMR measurements and simultaneously account for weather conditions, thus achieving a high level of operational reliability. The electronic control unit sets the target value for the helium tank pressure and, in particular, also makes changes to this target value, taking into account the second pressure, in this case, atmospheric pressure.

[0121] Under normal operating conditions, the helium tank pressure should remain constant during NMR measurements, as pressure fluctuations can cause dimensional changes in the measuring system, which can, for example, alter the sample position relative to the magnet or even distort the magnet itself. Accordingly, the target value of the helium tank pressure (see curve 93) is maintained at a constant value for most of the time, specifically during the periods t1 to t2, t3 to t4, t5 to t6, and from t7 onwards; the helium tank pressure (see curve 91) follows the target value quite precisely / promptly. During these periods, NMR measurements can be performed under stable conditions.

[0122] Furthermore, the helium tank pressure (see curve 91) should always be significantly higher than atmospheric pressure (see curve 92) to prevent the intake of contaminants (such as humid air) into the helium tank. As atmospheric pressure rises, it approaches the helium tank pressure or the target value. In the illustrated example, the current difference DIF between the helium tank pressure (with pressure values ​​D1) and atmospheric pressure (with pressure values ​​D2) is checked and compared with a boost threshold HSW. Using DIF = D1 - D2, it is checked whether DIF ≤ HSW. If so, the target value (see curve 93) is raised by one step, where one step here is 20 mbar. In this case, DIF becomes less than HSW at time t2, and the previous target value of 980 mbar before t2 is raised to a new target value of 1000 mbar from t3 onwards.Between t2 and t3, the target value (curve 93) is increased with a linear ramp-up curve, allowing the helium tank pressure (curve 91) to follow this change quickly. From time t3 onwards, the target value and the helium tank pressure remain constant until t4, despite various fluctuations in curve 92.

[0123] Furthermore, it should be ensured that the overpressure in the helium tank does not become too high compared to the surrounding atmosphere. Under no circumstances should the helium tank explode due to overpressure. To ensure this, the helium tank has pressure relief valves that open and release helium once a certain pressure difference is reached. As an additional safety measure, there are rupture discs through which (after the discs rupture) large quantities of helium gas can escape from the helium tank, particularly in the event of a quench. However, the activation of the safety devices should be reserved for unforeseen emergencies. Therefore, in the illustrated example, the current pressure difference DIF between the helium tank pressure (with pressure values ​​D1) and the atmospheric pressure (with pressure values ​​D2) is also monitored and compared with a setback threshold SSW. Using DIF = D1 - D2, it is checked whether DIF ≥ SSW. If so, the setpoint (see...) is adjusted.Curve 93) is reduced by one step, with each step also being 20 mbar. In this case, DIF at t4 is greater than SSW, and the previous target value of 1000 mbar before t4 is reduced to a new target value of 980 mbar from t5 onwards. Between t4 and t5, the target value (curve 93) is reduced with a linear reduction curve, so that the helium tank pressure (curve 91) can follow this change well / promptly. From time t5 onwards, the target value and the helium tank pressure then remain constant again, here until t6. Furthermore, DIF at t6 is again greater than SSW, and the previous target value of 980 mbar before t4 is reduced to a new target value of 960 mbar from t7 onwards. Between t6 and t7, the setpoint (curve 93) is reduced with a linear decrease curve, so that the helium tank pressure (curve 91) can follow this change quickly. From time t7 onwards, the setpoint and the helium tank pressure then remain constant again.

[0124] During the respective periods of setpoint change, i.e., from t2 to t3, then from t4 to t5, and then from t6 to t7, no NMR measurements are performed due to unstable pressure conditions (fluctuating pressure) in the helium tank. The electronic control unit reports the ongoing setpoint change or pressure instability to the NMR spectrometer control unit, which then schedules measurement pauses for these periods.

[0125] The Figure 10 Figure 1 illustrates, in a schematic flowchart, an exemplary monitoring of the correct operation of a device according to the invention for regulating the pressure in a helium tank or an associated NMR measuring arrangement, using an alarm situation for icing monitoring of the cryostat. Other alarm situations can be monitored in an analogous manner. The device is, for example, as shown in Figure 1. Fig. 5 depicted, or the NMR measurement setup as shown in Fig. 6 depicted.

[0126] After the monitoring starts (100), the current helium flow rate is measured using the flow meter (200). The measured helium flow rate is then compared to a helium flow rate of "zero" (300). If the measured helium flow rate is greater than zero, the helium flow rate is measured again (as part of continuous monitoring) (200), and so on.

[0127] If the measured helium flow rate is zero, the position of the control valve 400, as controlled by the control unit, is determined. This position is then compared to a (fully) closed position 500. If the controlled position of the control valve is a (fully) closed position, it can be assumed that, according to the control unit's programming, the pressure in the helium tank is currently being increased, and therefore the control valve is closed as planned, thus blocking the helium flow. Monitoring is then continued (as part of continuous monitoring) with the next measurement of the helium flow rate 200.

[0128] If the controlled position of the valve is not a (fully) closed position, it must be assumed that an undesirable and dangerous blockage of the helium flow has occurred due to icing of the cryostat or the helium line leading from the helium tank. Accordingly, an alarm message 600 is then issued. Based on alarm message 600, manual or automatic countermeasures or safety measures can then be taken. Reference symbol list

[0129] 1 Device 2 Helium tank 3 Helium line leading from the helium tank 4 Helium recovery system 5 Control valve 6 First pressure sensor (for helium tank pressure) 7 Second pressure sensor (here: differential pressure gauge, for atmospheric pressure) 7 Second pressure sensor (here: differential pressure gauge, for pressure in the helium regeneration system) 8 Throttle (in helium line 3) 9 Third pressure sensor (here: differential pressure gauge, for pressure in the helium regeneration system) 10 Further line 11 Shut-off valve 12 Throttle (in further line 10) 13 Electronic control device 14 Flow sensor 15 Alarm device 16 Acoustic and optical signal generator 17 Storage device 20 NMR measuring arrangement 21 Cryostat arrangement 22 Evacuated container / vacuum tank 22a Room temperature bore 23 Superconducting NMR magnet 24 Neck tubes 25 Front section of the helium line 26 Middle section of the helium line in the device 27 Rear section of the helium line 28 Balloon storage 29 Compressor 30 Compressed gas cylinders 31 NMR probe head32 NMR spectrometer control unit 33 Sample volume 71 Curve first pressure values ​​(helium tank pressure) 72 Curve second pressure values ​​(atmospheric pressure) 73 Curve helium flow values ​​81 Curve first pressure values ​​(helium tank pressure) 82 Curve second pressure values ​​(atmospheric pressure) 83 Curve helium flow values ​​91 Curve first pressure values ​​(helium tank pressure) 92 Curve second pressure values ​​(atmospheric pressure) 93 Curve setpoint for helium tank pressure 100 Start 200 Measurement of helium flow value 300 Comparison of helium flow value with zero 400 Determination of actuated position of the control valve 500 Comparison of actuated position of the control valve with (fully) closed position 600 Alarm message ambient atmosphere D1 First pressure values (Helium tank pressure) D2 Second pressure values ​​(usually atmospheric pressure here) D3 Third pressure values ​​DIF Difference between first and second pressure values, DIF=D1-D2 HSW Lifting threshold SSW Lowering threshold t1-t7 Time points

Claims

1. A device (1) for regulating the pressure in a helium tank (2) of an NMR magnet (23), comprising - a first pressure sensor (6) for measuring a first pressure in the helium tank (2), - at least one second pressure sensor (7; 7a) for measuring a second pressure outside the helium tank (2), - a control valve (5) for adjusting an outflowing helium gas flow from the helium tank (2), - an electronic control device (13) for controlling the control valve (5), wherein the electronic control device (13) is configured to - obtain first pressure values D1 measured by the first pressure sensor (6), - and to adjust a position of the control valve (5) depending on the measured first pressure values D1, wherein the first pressure values D1 are adjusted to a predetermined setpoint value SW, - obtain second pressure values D2 measured by the second pressure sensor (7; 7a), and wherein the electronic control device (13) is further configured in a normal operating mode for NMR measurements - to determine the setpoint value SW depending on the first pressure values D1 and the second pressure values D2, - namely to change the setpoint value SW for the pressure in the helium tank (2) in steps as soon as a difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds predetermined threshold values, wherein the setpoint value SW is raised by one step when the difference DIF becomes less than or equal to an increase threshold value HSW, and the setpoint value SW is lowered by one step when the difference DIF becomes greater than or equal to a decrease threshold value SSW, with HSW<SSW, and wherein a respective step by which the setpoint value SW is changed as soon as the difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds the predetermined threshold values is selected in the range 5-25 mbar.

2. The device (1) according to claim 1, characterized in that the second pressure is a pressure in the surrounding atmosphere (atm).

3. The device (1) according to claim 1, characterized in that the second pressure is a pressure in a helium recovery system (4).

4. The device (1) according to claim 1, characterized in that the device (1) further comprises - a third pressure sensor (9) for measuring a third pressure outside the helium tank (2), wherein the second pressure and the third pressure comprise a pressure in the ambient atmosphere (atm) and a pressure in a helium recovery system (4).

5. The device (1) according to any of the preceding claims, characterized in that the device (1) further comprises - a flow sensor (14) for measuring a helium flow value of the outflowing helium gas flow from the helium tank (2).

6. The device (1) according to any of the preceding claims, characterized in that the control device (13) comprises a memory device (17) or a connection for a memory device (17) with which sensor values obtained by the control device (13) are recorded.

7. The device (1) according to any of the preceding claims, characterized in that the control device (13) is configured in a special operating mode for refilling liquid helium - to transfer the setpoint value SW for the pressure in the helium tank (2) to a target value ZW over a predetermined period of time, wherein the target value ZW is dependent on the second pressure value D2, in particular wherein the period of time can be selected by a user.

8. The device (1) according to claim 7, characterized in that the target value ZW corresponds to the second pressure value D2 with ZW=D2, or the target value ZW is a small pressure increase DA above the second pressure value D2, with ZW=D2+DA and with DA≤3 mbar.

9. The device (1) according to claim 7 or 8, characterized in that the control device (13) is configured in the special operating mode for refilling liquid helium to transition the setpoint value SW linearly over time to the target value ZW.

10. The device (1) according to claim 7 or 8, characterized in that the control device (13) is configured in the special operating mode for refilling liquid helium to change the setpoint value SW non-linearly over time, wherein a helium flow through the control valve (5) is kept approximately constant during the duration of the transition of the setpoint value SW to the target value ZW.

11. The device (1) according to claim 10, characterized in that the control device (13) is configured in the special operating mode for refilling liquid helium to lower the setpoint value SW more quickly at the beginning of the transition than towards the end of the transition when transitioning the setpoint value SW to the target value ZW.

12. The device (1) according to any of the preceding claims, characterized in that the device (1) further comprises an alarm device (15) with which an alarm message is automatically triggered (600) in one or more predetermined alarm situations, in particular wherein the alarm device (15) comprises an acoustic signal generator (16) and / or an optical signal generator (16) and / or a radio signal generator and / or a data signal generator.

13. The device (1) according to claim 12 and according to any of claims 2 or 4, characterized in that an alarm situation comprises that the pressure value D1 of the helium tank pressure (2) falls below the pressure value D2 of the atmospheric pressure.

14. The device (1) according to claim 12 or 13, characterized in that an alarm situation comprises that the pressure value D1 of the helium tank pressure exceeds a predetermined helium tank pressure maximum value, in particular wherein a first alarm situation for D1 comprises that the pressure value D1 of the helium tank pressure exceeds a predetermined first helium tank pressure maximum value EHM, a second alarm situation for D1 comprises that the pressure value D1 of the helium tank pressure exceeds a predetermined second helium tank pressure maximum value ZHM, and ZHM>EHM, and the alarm messages for the first alarm situation for D1 and the second alarm situation for D1 are different.

15. The device (1) according to any of claims 12 to 14 and according to claim 5, characterized in that an alarm situation comprises that a measured helium flow value exceeds a predetermined maximum helium flow value.

16. The device (1) according to claim 15, characterized in that the predetermined maximum helium flow value depends on changes in the helium tank pressure that are currently or recently controlled, in particular wherein the maximum helium flow value is higher during and / or shortly after reductions made in the helium tank pressure than when the helium tank pressure is controlled to be constant.

17. The device (1) according to any of claims 12 to 16 and according to claim 5, characterized in that an alarm situation comprises that a measured helium flow value falls below a predetermined minimum helium flow value.

18. The device (1) according to claim 17, characterized in that the predetermined minimum helium flow value depends on current or recently controlled changes in the helium tank pressure, in particular wherein the minimum helium flow value during and / or shortly after increases made in the helium tank pressure is lower than when the helium tank pressure is controlled to be constant.

19. The device (1) according to any of claims 12 to 18 and according to claim 5, characterized in that an alarm situation comprises that a measured helium flow value is zero and at the same time a current position of the control valve (5) or a position of the control valve (5) currently controlled by the control device (13) is not closed.

20. The device (1) according to any of claims 12 to 19 and according to claim 5 and according to any of claims 3 or 4, characterized in that an alarm situation comprises that a measured helium flow value is zero and at the same time a pressure value D2 in the helium recovery system (4) has risen to the pressure value D1 in the helium tank (2).

21. The device (1) according to any of claims 12 to 20 and according to claim 4, characterized in that an alarm situation comprises that a difference DHA=DHR-DAT of a measured pressure value DHR in the helium recovery system (4) and a measured pressure value DAT in the ambient atmosphere (atm) exceeds a predetermined threshold value SWW, in particular wherein SWW is selected in a range from 2.5 mbar to 20 mbar.

22. The device (1) according to any of claims 12 to 21, characterized in that at least some of the alarm situations take into account a current position of the control valve (5) or a position of the control valve (5) currently controlled by the control device (13).

23. A method for regulating the pressure in a helium tank (2) of an NMR magnet (23), wherein a first pressure in the helium tank (2) is measured by a first pressure sensor (6), wherein a control valve (5) is used to adjust an outflowing helium gas flow from the helium tank (2), wherein an electronic control device (13) controls the control valve (5), wherein a second pressure outside the helium tank (2) is measured with at least one second pressure sensor (7; 7a), wherein the electronic control device (13) - obtains the first pressure values D1 measured by the first pressure sensor (6), - and adjusts a position of the control valve (5) depending on the measured first pressure values D1, so that the first pressure values D1 are adjusted to a predetermined setpoint value SW, - and obtains second pressure values D2 measured by the second pressure sensor (7; 7a), and wherein the control device (13) in a normal operating mode for NMR measurements further - determines the setpoint value SW in dependence on the measured first pressure values D1 and second pressure values D2, - namely changes the setpoint value SW for the pressure in the helium tank (2) in steps as soon as a difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds predetermined threshold values, wherein the setpoint value SW is raised by one step when the difference DIF becomes less than or equal to an increase threshold value HSW, and the setpoint value SW is lowered by one step when the difference DIF becomes greater than or equal to a decrease threshold value SSW, with HSW<SSW, and wherein a respective step by which the setpoint value SW is changed as soon as the difference DIF=D1-D2 between the measured first pressure value D1 and the measured second pressure value D2 reaches or exceeds the predetermined threshold values is selected in the range 5-25 mbar, in particular wherein the method is carried out using a device (1) according to any of the preceding claims.

24. The method according to claim 23, characterized in that in a special operating mode for refilling liquid helium the control device (13) transitions the setpoint value SW for the pressure in the helium tank (2) to a target value ZW over a predetermined period of time, wherein the target value ZW is dependent on the second pressure value D2, in particular wherein the period of time can be selected by a user, in particular wherein the method is carried out using a device (1) according to claim 9.

25. The method according to claim 24, characterized in that in the special operating mode for refilling liquid helium the control device (13) changes the setpoint value SW non-linearly over time, wherein a helium flow through the control valve (5) is kept approximately constant during the duration of the transfer of the setpoint value SW to the target value ZW, in particular wherein the method is carried out using a device (1) according to claim 10.

26. The method according to any of claims 23 to 25, characterized in that an alarm message is automatically triggered (600) by an alarm device (15) in any or more predetermined alarm situations, in particular wherein the alarm device (15) comprises an acoustic signal generator (16) and / or an optical signal generator (16) and / or a radio signal generator and / or a data signal generator, and that an alarm situation comprises that a measured helium flow value of the outflowing helium gas flow from the helium tank (2) through the control valve (59) is zero and at the same time a current position of the control valve (5) or a position of the control valve (5) currently controlled by the control device (13) is not closed, in particular wherein the method is carried out using a device (1) according to claim 19.

27. A cryostat arrangement (21), comprising - a vacuum-insulated helium tank (2), - a device (1) according to any of claims 1 to 22, wherein the first pressure sensor (6) is connected to the helium tank (2) and the second pressure sensor (7; 7a) is connected to a location outside the helium tank (2), in particular wherein the second pressure sensor (7; 7a) is connected to the surrounding atmosphere (atm) or to a helium recovery system (4), and wherein the control valve (5) is arranged in a helium line (3) leading away from the helium tank (2).

28. The cryostat arrangement (21) according to claim 27, characterized in that the cryostat arrangement (21) further comprises a helium recovery system (4) which is connected to the outgoing helium line (3).

29. An NMR measuring arrangement (20), comprising - a cryostat arrangement (21) according to any of claims 27 or 28, - a superconducting NMR magnet (23) in the helium tank (2) of the cryostat arrangement (21), - an NMR probe head (31) which projects into a room-temperature bore (22a) of the vacuum-insulated helium tank (2), and - an NMR spectrometer control device (32) for controlling NMR measurements with the NMR probe head (31).

30. A use of an NMR measuring arrangement (20) according to claim 29 for carrying out NMR measurements, wherein the electronic control device (13) communicates a status of the regulation of the helium tank pressure to the NMR spectrometer control device (32), and wherein during times of pressure instability in the helium tank (2), the NMR spectrometer control device (32) pauses the NMR measurements.