Detecting contamination of cryogenic refrigerants in cryogenic refrigeration systems

Abstract

A sensor, method and refrigeration system for detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system is disclosed. The sensor comprises an inlet (10) for connection to a cryogenic refrigerant flow path of the cryogenic refrigeration system and a thermal conductivity detector (5) in fluid communication with the inlet. The thermal conductivity detector (5) is configured to generate a signal indicative of a detected thermal conductivity of the cryogenic refrigerant received from the cryogenic refrigeration system when the sensor is connected. The sensor also comprises circuitry configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant, and an output configured to output the indication of contamination of the cryogenic refrigerant.

Description

[Technical Field]

[0001] The field of the invention relates to detecting contamination of cryogenic refrigerants in cryogenic refrigeration systems. [Background technology]

[0002] Cryogenic cooling systems use a refrigerant, such as helium. Refrigerant purity is important because impurities can freeze at the system's low temperatures, causing damage or reduced performance. Traditional systems monitor the refrigerant periodically to ensure that refrigerant contaminant levels are below acceptable levels. This can be done by taking samples and measuring the contaminants in the samples using an RGA (residual gas analyzer). RGAs are complex devices, typically installed in remote locations, and require significant expertise to operate. Summary of the Invention [Problem to be solved by the invention]

[0003] Analytical techniques are desirable that allow analyses to be performed regularly and inexpensively in a manner that does not unduly interfere with the normal operation of the system. [Means for solving the problem]

[0004] One aspect provides a sensor for detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system, the sensor comprising: an inlet for connecting to a cryogenic refrigerant flow path of the cryogenic refrigeration system; a thermal conductivity detector in fluid communication with the inlet, the thermal conductivity detector configured to generate a signal indicative of a detected thermal conductivity of the cryogenic refrigerant received from the cryogenic refrigeration system when the sensor is connected; circuitry configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant; and an output configured to output the indication of contamination of the cryogenic refrigerant.

[0005] The inventors of the present invention have recognized that refrigerants, such as helium, used in cryogenic systems often have high electrical conductivity, which is quite different from the conductivity of other impurities that may leak into the refrigerant within the system. They also recognized that being able to identify the presence of impurities or contamination in a cryogenic refrigerant on-site would allow the refrigerant to be tested more regularly, resulting in earlier detection of any contamination. Accordingly, embodiments provide a sensor having a thermal conductivity detector for detecting the conductivity of a refrigerant within a cryogenic refrigeration system. This allows for periodic testing with immediate results. This provides for early detection of contamination using a relatively low-cost, easy-to-use sensor. While such a sensor may not be able to identify the nature of the contamination as accurately as a traditional RGA analyzer, it can detect the presence or absence of refrigerant contamination, thereby protecting the cryogenic system and allowing for more accurate analysis if it is determined that such analysis would be useful. In this way, when used, more cumbersome remote measurement of the refrigerant can be precisely targeted to situations where results are needed, and results are required much less frequently than in the past.

[0006] In some embodiments, the sensor has only one port for connection to the cryogenic refrigerant flow path, which acts as an inlet to allow flow to enter the sensor and also as an outlet when detection is taking place, at which point the refrigerant can be discharged from the sensor. However, in other embodiments, there is both an inlet and an outlet, with a thermal conductivity detector positioned between the two, allowing flow to pass through the sensor, and the refrigerant in the sensor is continuously replenished to be representative of the refrigerant in the refrigeration system.

[0007] While the refrigerant can take a variety of forms, provided they are configured to operate at cryogenic temperatures, generally below 100 K, in some embodiments the refrigerant is helium. Helium is often used as a refrigerant in cryogenic systems, and in embodiments, helium is particularly effective for detecting contaminants in helium because it has a very high electrical conductivity in its gaseous state compared to other gases, particularly compared to the gaseous impurities likely to be found in contaminated refrigerants. As a result, any contamination in the helium stream causes a large change in thermal conductivity and can therefore be detected with a great deal of accuracy.

[0008] In some embodiments, the sensor further comprises a control circuit having an input for receiving at least one signal indicative of a current state of the refrigeration system, the control circuit being configured to control operation of the thermal conductivity detector based on the at least one received signal.

[0009] The sensor is configured to be mounted in a refrigeration system to enable in-situ measurement of the thermal conductivity, and therefore contamination, of the cryogenic refrigerant. The sensor may be controlled by a control circuit that receives signals indicative of the current operating state or condition of the refrigeration system, and these signals are used to control the sensor to perform thermal conductivity detection. In this regard, the signals may be indicative of the current operating state, such as whether or not a regeneration state is being entered, and / or may be indicative of properties of the cryogenic refrigerant in the refrigeration system, such as temperature, pressure, flow rate, etc., which are indirect indicators of the current operating state of the refrigeration system and affect the thermal conductivity measurement.

[0010] In some embodiments, the sensor further comprises at least one valve positioned to control flow of the cryogenic refrigerant to and from the sensor.

[0011] In some embodiments, the sensor can be mounted within the refrigeration system within the cryogenic refrigerant flow, and there may be no valve associated with the sensor. However, in other embodiments, there may be one or more valves associated with the sensor's inlet and / or outlet, the valves configured to control flow to and from the sensor. A valve may be used, for example, when the sensor is in a bypass line, such that the cryogenic refrigerant is diverted from its normal flow path to flow through the bypass line and thus the sensor when a measurement is being taken. When no measurement is being taken, the valve may be closed, allowing the refrigerant to pass through the bypass line. In this regard, it may be preferable to perform thermal sensing measurements within a stagnant flow, and therefore it may be advantageous to be able to control the flow with a valve, such that when a measurement is taken, the refrigerant indicative of the system is trapped but stagnant.

[0012] Additionally and / or alternatively, the control circuitry may control the measurements in response to operation of the refrigeration system, particularly when the flow of cryogenic refrigerant is stopped.

[0013] In other embodiments, the sensor may be within the flowing coolant, and the circuitry configured to convert the thermal conductivity measurement to a contamination indicator may receive a signal indicative of the flow rate such that the conversion can take into account flow effects, such as turbulence, on the thermal conductivity measurement.

[0014] In some embodiments, the control circuit is configured to control operation of the at least one valve.

[0015] Operation of the control circuitry of the thermal conductivity detector may also control operation of one or more valves associated with the sensor, if such valves are present.

[0016] In some embodiments, in response to determining that the received signal indicates that the cryogenic refrigeration system is in a regeneration phase, the control circuit is configured to activate the thermal conductivity detector to perform thermal conductivity detection.

[0017] As mentioned above, a problem with contamination in cryogenic refrigeration systems is that contaminants freeze at cryogenic temperatures, which can cause damage and / or restrict flow. Therefore, it is more accurate and effective to test for contamination at temperatures above the cryogenic temperatures at which contaminants do not freeze and are present in the refrigerant within the sensor. For example, when used as a cryogenic vacuum pump, a cryogenic refrigeration system will periodically undergo a regeneration phase during which the cryopump's cryocooler reaches or exceeds the cryogenic temperature and releases trapped molecules. In such systems, it may be advantageous to perform contamination detection measurements during such periods. Accordingly, in some embodiments, the control circuitry is configured to activate the sensor to perform a thermal conductivity measurement when it determines that the system is in a regeneration phase and has reached approximately a certain temperature.

[0018] A further advantage of making thermal conductivity measurements during the regeneration phase is that during much of the regeneration phase the compressor and cryopump are not running and the flow is stagnant, which means that the measurements do not need to be corrected for flow effects such as turbulence, making for more accurate or at least simpler measurements.

[0019] In some embodiments, the control circuit is configured to, in response to determining that the cryogenic refrigeration system is below 200 K, preferably below 100 K, and that the contaminant is frozen within the cryogenic refrigeration system, activate the thermal conductivity detector to perform the thermal conductivity detection as a baseline thermal conductivity detection.

[0020] As mentioned above, when a refrigeration system is at a cryogenic temperature, it can capture contaminants. Specifically, when a cryogenic refrigeration system is below 200 K, preferably below 100 K, most contaminants will freeze within the coldest portion of the system, and detecting the thermal conductivity of the cryogenic refrigerant away from the coldest portion will provide a baseline indication of a substantially pure refrigerant. Therefore, it may be advantageous to periodically perform such thermal conductivity measurements. These are useful for accurately comparing differences in the thermal conductivity of cryogenic refrigerants with and without contaminants, measured by the same sensor at the same location, and enabling these measurements to be converted to contaminant quantities. If the refrigeration system periodically performs regeneration phases, this baseline detection can be performed at or near the beginning of the regeneration phase, when the cryogenic refrigerant is stagnant, there is no flow, and the coldest portion of the refrigeration system is still at cryogenic temperatures. This baseline measurement can be performed for each regeneration cycle, or for a subset, and after a predetermined time or number of regeneration cycles, the baseline measurement is repeated and the thermal conductivity measurement used as comparison is updated.

[0021] In some embodiments, the control circuit is configured to, in response to determining that the refrigeration system is above 220 K, preferably above 270 K, control the thermal conductivity detector to activate the thermal conductivity detector to perform the thermal conductivity detection as a contamination thermal conductivity detection.

[0022] If contaminants are present in the cryogenic refrigerant, it is advantageous to perform the measurement when the refrigeration system is determined to be above the cryogenic temperature, e.g., above 220 K, preferably above 270 K, in order to obtain an accurate measurement of the contaminants. If the refrigeration system includes a regeneration phase, this high temperature may occur sometime during the regeneration phase, and the system may perform a mixing cycle before performing thermal conductivity detection. The mixing cycle may involve the refrigeration system's compressor and pump operating for a very short time to mix the refrigerant around the system, so that any contaminants held within the cooling section of the system are dispersed within the refrigerant. The mixing cycle is not performed for a long time, as no cooling is desired during this cycle. It is typically no longer than 30 seconds and typically no longer than 5 minutes.

[0023] In some embodiments, the circuitry is configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant based on both the baseline thermal conductivity detection and the contaminant thermal conductivity detection.

[0024] The circuitry that converts the determined thermal conductivity into an indication of contamination can take into account the baseline thermal conductivity detection, if there is one, so that the difference between the two signals can give an accurate indication of contamination.

[0025] The thermal conductivity detector can take many forms, and in some embodiments the thermal conductivity detector comprises a filament thermal conductivity detector.

[0026] Filament thermal conductivity detectors are relatively inexpensive and easy to operate. In some embodiments, there can be two filament thermal conductivity detectors: a filament thermal conductivity detector and a further reference filament thermal conductivity detector, the further reference filament thermal conductivity detector being isolated from the refrigeration system and containing a refrigerant of a predetermined purity, and the circuitry being configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant based on a comparison of the thermal conductivity detection of the further reference filament thermal conductivity detector and the filament thermal conductivity detector.

[0027] To enable accurate detection of contamination, the filament thermal conductivity detector may require a reference thermal conductivity detector that detects the conductivity of a pure or substantially pure refrigerant, and the difference in thermal conductivity between that and the thermal conductivity of the actual refrigerant is used as an indicator of contamination.

[0028] In another embodiment, the thermal conductivity detector comprises a micro-electromechanical systems (MEMS) device.

[0029] MEMS devices may be preferred over filament detectors because they are more accurate and independent of turbulence and sensor orientation. Also, MEMS devices do not require a second device to act as a reference device, but can themselves take baseline measurements at different times and use this comparison to improve accuracy.

[0030] In some embodiments, the circuitry is configured to receive a signal indicative of at least one of a temperature and a pressure of the refrigerant and convert the thermal conductivity signal into an indication of the contamination based on at least one of the temperature and pressure.

[0031] The thermal conductivity measured by the thermal conductivity detector depends on the temperature and pressure of the refrigerant, and therefore in some embodiments the circuitry can receive signals indicative of these values ​​and use them in determining contamination. In some embodiments, the sensor itself can include at least one of a temperature sensor and / or a pressure sensor, or can have inputs for receiving signals indicative of the temperature and pressure of the refrigerant from the refrigeration system. In this regard, it may be advantageous to include a temperature sensor, as the sensor itself has a significant effect on the thermal conductivity, and it may be desirable to measure the temperature as close as possible to the location where the thermal conductivity is measured.

[0032] A further aspect provides a cryogenic refrigeration system that includes a cryogenic refrigerant and a sensor for determining contamination of the cryogenic refrigerant, according to one aspect.

[0033] In some embodiments, the cryogenic refrigeration system further comprises at least one compressor for compressing the cryogenic refrigerant, at least one pump, and a controller for controlling operation of the cryogenic refrigeration system, the controller being configured to control a mixing cycle by causing operation of the compressor and the at least one pump for a predetermined time before sending a signal to the sensor to initiate detection of the contamination of the cryogenic refrigerant.

[0034] In some embodiments, the at least one pump comprises a cryogenic pump.

[0035] As mentioned above, when determining contamination of a cryogenic refrigerant by detecting thermal conductivity, it may be desirable to perform a mixing cycle to disperse contaminants within the refrigerant. This is particularly advantageous when the system is warming up and stagnating, near the regeneration stage. Thus, in some embodiments, there may be control circuitry associated with the refrigeration system configured to control the mixing cycle before sending a signal to the sensor to initiate the thermal conductivity measurement.

[0036] A further aspect provides a method for detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system, the method comprising: connecting a sensor including a thermal conductivity detector to a cryogenic refrigerant flow path of the cryogenic refrigeration system such that the cryogenic refrigerant flows into the thermal conductivity detector; measuring the thermal conductivity of the cryogenic refrigerant using the thermal conductivity detector; converting the measured thermal conductivity into an indication of the level of contamination of the cryogenic refrigerant; outputting the indicator of contamination of the cryogenic refrigerant; Includes:

[0037] In some embodiments, the refrigeration system comprises a cryogenic pump system.

[0038] In some embodiments, the method includes an initial step of determining that the refrigeration system is entering a regeneration phase, and the step of measuring the thermal conductivity is performed during the regeneration phase.

[0039] In some embodiments, the method comprises determining that the cryogenic refrigeration system is at a cryogenic temperature, preferably below 100K, and performing a baseline thermal conductivity measurement.

[0040] In some embodiments, the method includes determining that the refrigeration system is above a cryogenic temperature, preferably above 200K, before performing the step of measuring the thermal conductivity.

[0041] In some embodiments, converting the measured thermal conductivity into an indication of the amount of contamination of the cryogenic refrigerant comprises comparing the measured thermal conductivity to the measured baseline thermal conductivity.

[0042] Further particular and preferred aspects are set out in the independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims as appropriate or in combinations other than those explicitly set out in the claims.

[0043] Where features of a device are described as operable to provide a certain functionality, this should be understood to include features of a device that provide that functionality or that are adapted or configured to provide that functionality.

[0044] Embodiments of the present invention are further described below with reference to the accompanying drawings. [Brief explanation of the drawings]

[0045] [Figure 1] 1 illustrates a cross section of a sensor according to one embodiment. [Figure 2] 1 illustrates a sensor according to one embodiment. [Figure 3] 1 illustrates a MEMS-based TCD according to one embodiment. [Figure 4] 1 illustrates a refrigeration system depicting possible locations of sensors according to one embodiment. [Figure 5] FIG. 1 illustrates a refrigeration system depicting a sensor according to one embodiment in a bypass line. [Figure 6] 1 shows the difference in thermal conductivity measurements of pure helium as opposed to helium contaminated with 100 pmm of CO2. [Figure 7] FIG. 2 is a flow diagram illustrating steps of a method according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0046] Before describing the embodiments in detail, an overview will be given first.

[0047] Embodiments provide methods and means for monitoring refrigerants, particularly helium stream purity, using thermal conductivity measurements to determine the presence of gaseous contamination in cryogenic refrigeration systems, such as cryopump systems. This development uses an in situ method to monitor refrigerants, and in some cases helium stream purity, using gas thermal conductivity measurements to determine the presence of contaminants before they adversely affect the system, thereby alleviating the need for ex-situ RGA analysis.

[0048] Various thermodynamic cycles, such as the Gifford-McMahon (GM) cycle or Stirling cycle, are used to generate cryogenic temperatures and use helium as the working fluid or refrigerant. Applications of this technology include cryopumps used to generate high vacuums; other applications include MRI scanners and high-temperature superconductor refrigeration. For proper operation of these systems, it is important to maintain a well-defined purity level of the helium within the system. A decrease in helium purity leads to reduced system performance and ultimately to failure, resulting in the need for service and maintenance. Embodiments provide a method for measuring the purity of the refrigerant, e.g., helium, within a cryogenic refrigeration system (during operation) so that impurities can be detected before they reach levels that could cause pump failure.

[0049] The detection system uses a thermal conductivity detector (TCD) used in gas chromatography. A TCD contains an electrical resistor placed within a gas flow path / volume. As heat is carried away from the resistor due to gas flow across the resistor, the resistor changes temperature (resistivity changes). Different gases have different thermal conductivities, so gases can be detected based on the rate of heat loss (change in resistivity) of the resistor.

[0050] The embodiments broadly describe two methods of using TCD to determine refrigerant purity. The applicability of one method over another is based on many factors, including but not limited to, the system type, layout, and required detection sensitivity. The term helium or refrigerant environment refers to a contained volume or flow of helium or refrigerant gas at some pressure and flow rate.

[0051] Use of a two sensor system - Filament TCD This method uses two TCD sensors, one placed in a high-purity refrigerant environment and the other in a potentially contaminated refrigerant environment. In this example, the refrigerant is helium, and the TCD sensors have significantly different resistivities due to the different conductivities of pure and impure helium environments. A Wheatstone bridge electrical circuit is used to convert the resistances of the two sensors into an output voltage that can be read by the system and indicates how different the two flows are from each other. A large difference in voltage between the pure helium system and the system being tested indicates a more contaminated system.

[0052] The thermal conductivity of all gases, except hydrogen, is lower than that of helium. For this reason, helium is frequently used as a reference gas for comparing the thermal conductivity of gases, and it is also a common refrigerant used in cryogenic systems. The standard or reference environment for pure helium is 200 PSIG, and the reference voltage observed in this environment is 5.2 mV. This voltage is collected for a set period of time and used in zero-point calibration. The reference voltage is then subtracted from the voltage obtained when the impurity gas of interest passes through the TCD for detection. This allows the user to calculate the change in voltage due to contamination of the helium stream compared to pure helium. Figure 6 shows an example illustrating these voltage differences.

[0053] Use of one sensor system - MEMS-based TCD Using a MEMS (microelectromechanical system) TCD, accurate contaminant concentration predictions can be made with a single sensor. MEMS have a much higher signal-to-noise ratio compared to their filament counterparts. This allows a single MEMS sensor assembly to detect changes in gas purity of less than 100 ppm.

[0054] The MEMS solution is more elegant than a filament-based design because a single sensor can be zeroed with pure gas refrigerant. Any variation from the zero point at constant pressure and temperature can be attributed to changes in gas purity. Using a TCD to monitor the purity of a cryogenic helium loop can detect contamination before it adversely affects the system. This monitoring can be done in the field on a warm system. The TCD can be integrated directly into the refrigerant loop.

[0055] FIG. 1 shows a cross section of a filament-type thermal conductivity detector (TCD) 5 according to one embodiment. The TCD 5 has an inlet connection 10 and an outlet connection 20 for connection to a refrigeration system, such as a cryopump. When connected to the system, refrigerant passes through the TCD 5 from the inlet 10 to the outlet 20. The TCD has a heated filament 30 whose resistance depends on temperature, which in turn depends on the thermal conductivity of the refrigerant. There is a passage 25 for accepting electrical wires that pass a current through the filament 30 and allow detection of changes in resistance. Circuitry (not shown) determines the resistance of the filament and, in some embodiments, compares it to the resistance of a corresponding filament in pure refrigerant and outputs a measure of refrigerant contamination from the difference in values.

[0056] FIG. 2 shows a TCD 5 (not in cross section) with inlet connection 10 and outlet connection 20 TCD 5 of FIG.

[0057] 3 shows a schematic of a MEMS-based TCD 5 with inlet and outlet connections 10 and 20, and also with an integrated temperature sensor 40 and pressure transducer 50. Because the thermal conductivity of refrigerant gas will vary with temperature and pressure, some TCDs incorporate these sensors, and values ​​from these sensors are used in converting the detected thermal conductivity measurements into contamination levels.

[0058] 4 is a schematic diagram of a refrigeration system according to embodiments and potential locations for a TCD 5. The refrigeration system includes a compressor 60 and multiple refrigeration units 72. In this system, sensors according to embodiments can be used to monitor impurities. These TCD sensors 5 can be located within the refrigerant lines themselves, and four exemplary locations for the TCD are shown: a compressor refrigerant supply line 5A, a compressor refrigerant return line 5B, a refrigerator unit supply line 5C, or a refrigerator unit return line 5D.

[0059] Operation of the TCD can be initiated by a control circuit (not shown) to take measurements at the appropriate times. The thermal conductivity measurements can be converted into an indicator of refrigerant contamination, which can be used to determine service to prevent contaminants from rising above critical levels. In some embodiments, the TCD is a filament-type TCD and is operated with a reference filament TCD containing pure refrigerant, and the difference in thermal resistance between the reference TCD and the other TCD is used to determine the level of contaminants.

[0060] In other embodiments, the TCD can be a MEMS-type TCD, and a reference baseline measurement can be made by taking measurements at cryogenic temperatures where contaminants are trapped in the coldest part of the system and the refrigerant is therefore pure; these measurements can be compared to measurements taken at warmer temperatures where contaminants are present in the refrigerant, possibly after a mixing cycle. The difference in thermal conductivity between the baseline and warmer temperature measurements is used to determine the contamination level. These two measurements can be made during a period when refrigerant is not flowing, with the baseline measurement taken at the beginning of that period when the temperature is low and the other measurement taken once the system has warmed up.

[0061] In some embodiments, measurements from pressure, temperature, and possibly flow sensors, either associated with the TCD itself or as separate components within the refrigeration system, can be used in converting the thermal conductivity measurements into a fouling indicator.

[0062] FIG. 5 shows an alternative system in which the refrigeration unit of FIG. 4 is replaced with a cryopump 70. In this embodiment, the TCD 5 is in the bypass line. Flow in the bypass line is controlled by valves 75 and 76, which are controlled by a control circuit 80. The control circuit 80 also controls the operation of the TCD 5 and receives a signal indicative of conductivity from the TCD 5 along with pressure and temperature measurements from other sensors (not shown). The control circuit 80 also receives and sends signals from a refrigeration system controller 90, which controls the operation of the refrigeration system. Thus, in some embodiments, the control circuit 80 can receive a signal from the refrigeration system controller 90 indicating that a regeneration cycle is about to begin and, in response, control the bypass line valve 75 to open and the bypass line valve 76 to close. Refrigerant then flows into the TCD 5, and a baseline thermal conductivity measurement can be obtained along with pressure and temperature measurements at the start of the regeneration cycle. The control circuit 80 can then open valve 76 and close valve 75 and request a mix cycle from the refrigeration controller 90 after a predetermined time or when the refrigerant reaches a predetermined temperature. The refrigeration controller 90 can initiate a mixing cycle by turning on the compressor 60 and cryopump 70 for approximately one minute and then turning them off. The control circuit 80 can then control the bypass line valve 75 to open and the valve 76 to close. The warm mixed refrigerant then flows into the TCD 5, and thermal conductivity measurements can be taken along with pressure and temperature measurements. Note that during the regeneration cycle, the compressor and pump are typically not running and the refrigerant is stagnant, which can improve measurement accuracy by eliminating flow effects. The processing circuit 82 within the control circuit 80 can then determine the amount of refrigerant contamination from the respective thermal conductivity measurements and the temperature and pressure measurements.

[0063] Figure 6 shows the difference in corrected voltage measured by the sensor at different sample points for pure refrigerant and refrigerant contaminated with 100 ppm CO. These measurements were taken at different times, and the system was cleaned between measurements.

[0064] FIG. 7 is a flow diagram illustrating steps of a method according to an embodiment. Initially, in step S10, a TCD is connected to the cryogenic refrigerant flow path of a cryogenic refrigeration system. This may involve opening several valves or may involve initially installing the TCD in the system. Once connected, in step S20, the cryogenic refrigerant flows into a thermal conductivity detector, and the thermal conductivity of the cryogenic refrigerant is measured in step S30. In step S40, the measured thermal conductivity is converted into an indication of the amount of contamination in the cryogenic refrigerant. This may involve comparison with a thermal conductivity measurement of an uncontaminated refrigerant and / or adjustment of the refrigerant temperature, pressure, and potentially flow rate measured during the thermal conductivity measurement. In step S50, the calculated indication of contamination is output directly to a user via a display (not shown) and / or as a signal to the refrigeration system control circuitry.

[0065] Although exemplary embodiments of the present invention are disclosed in detail herein with reference to the accompanying drawings, it should be understood that the invention is not limited to the precise embodiments, and that various changes and modifications may result by those skilled in the art without departing from the scope of the present invention as defined by the appended claims and their equivalents. [Explanation of symbols]

[0066] 5, 5A-D TCD 10 Inlet connection 20 Outlet Connection 25 Cable conduit 30 filaments 40 Temperature Sensor 50 Pressure Sensor 60 Compressor 70 Pump 72 Refrigeration unit 75 Bypass line valve 76 valves 80 Control circuit 82 Processing circuit 90 Refrigeration system control device

Claims

1. A sensor for detecting contamination of a cryogenic refrigerant in a cryogenic refrigeration system used in a cryogenic vacuum pump, comprising: an inlet for connecting to a cryogenic refrigerant flow path of the cryogenic refrigeration system; a thermal conductivity detector in fluid communication with the inlet, the thermal conductivity detector configured to generate a signal indicative of a detected thermal conductivity of the cryogenic refrigerant received from the cryogenic refrigeration system when the sensor is connected; and a circuit configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant; an output configured to output the indication of contamination of the cryogenic refrigerant; Equipped with the sensor further includes a control circuit, the control circuit having an input for receiving at least one signal indicative of a current state of the refrigeration system, the control circuit being configured to control operation of the thermal conductivity detector based on the at least one received signal; In response to determining that the received signal indicates that the cryogenic vacuum pump is in a regeneration phase, the control circuit is configured to activate the thermal conductivity detector to perform thermal conductivity detection.

2. The sensor of claim 1 , further comprising at least one valve positioned to control the flow of the cryogenic refrigerant to and from the sensor.

3. The sensor of claim 2 , wherein the control circuit is configured to control operation of the at least one valve.

4. 2. The sensor of claim 1, wherein the control circuit is configured to, in response to determining that the cryogenic refrigeration system is below 200 K and the contaminants are frozen within the cryogenic refrigeration system, activate the thermal conductivity detector to cause the thermal conductivity detection to provide a baseline thermal conductivity detection.

5. 5. The sensor of claim 4, wherein the control circuit is configured to, in response to determining that the refrigeration system exceeds 220 K, control the thermal conductivity detector to activate the thermal conductivity detector to perform the thermal conductivity detection to provide contamination thermal conductivity detection.

6. The sensor of claim 5 , wherein the circuitry is configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant dependent on both the baseline thermal conductivity detection and the contaminant thermal conductivity detection.

7. The sensor of claim 1 , wherein the thermal conductivity detector comprises a filament thermal conductivity detector.

8. a further reference filament thermal conductivity detector; 8. The sensor of claim 7, wherein the further reference filament thermal conductivity detector is isolated from the refrigeration system and contains a refrigerant of a predetermined purity, and the circuitry is configured to convert the thermal conductivity signal into an indication of contamination of the cryogenic refrigerant based on a comparison of the thermal conductivity detection of the further reference filament thermal conductivity detector and the filament thermal conductivity detector.

9. The sensor of claim 1 , wherein the thermal conductivity detector comprises a micro-electro-mechanical system (MEMS) device.

10. 2. The sensor of claim 1, wherein the circuitry is configured to receive a signal indicative of at least one of a temperature and a pressure of the refrigerant and convert the thermal conductivity signal into the indicator of contamination based on the at least one of the temperature and pressure.

11. 10. A cryogenic vacuum pump comprising a cryogenic refrigeration system comprising a cryogenic refrigerant and a sensor according to claim 1 for determining contamination of said cryogenic refrigerant.

12. 12. The cryogenic vacuum pump of claim 11, further comprising at least one compressor for compressing the cryogenic refrigerant, at least one pump, and a controller for controlling operation of the cryogenic refrigeration system, the controller being configured to control a mixing cycle by causing operation of the compressor and the at least one pump for a predetermined time period before sending a signal to the sensor to initiate detection of the contamination of the cryogenic refrigerant.

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