Method for operating a Pirani pressure sensor and arrangement for operating the Pirani pressure sensor

Abstract

Method for operating a Pirani pressure sensor (1) with a measuring resistor (2) and a measuring volume (3) surrounding the measuring resistor, wherein the measuring resistor is supplied with an electrical power (P) and an equilibrium temperature of the measuring resistor (2) is established in the medium of the measuring volume, wherein a control unit (4) performs a switching between a first measuring mode (M1) and a second measuring mode (M2) during pressure measurement, wherein in the first measuring mode (M1) the measuring resistor is supplied with a defined maximum power (Pmax), wherein the temperature (T) of the measuring resistor is determined as a measured quantity of the pressure to be determined, and in the second measuring mode (M2) the temperature (T) of the measuring resistor is fixed at a maximum temperature (Tmax), wherein the power (P) required to maintain the maximum temperature (Tmax) is determined as a measured quantity of the pressure to be determined.

Description

[0001] The invention relates to a method for operating a Pirani pressure sensor according to claim 1 and an arrangement for operating a Pirani pressure sensor according to claim 7.

[0002] Measuring pressure in a given vacuum is an essential component of many industrial and scientific applications. The vacuum sensors used differ in their suitability for various pressure ranges. Generally, the use of a specific type of vacuum sensor is limited by constraints regarding accuracy, sensitivity, and the optimal measuring range of the pressure to be measured. If a particularly large measuring range needs to be captured, for example, between rough vacuum, fine vacuum, high vacuum, and even extra-high vacuum, this can practically only be achieved through the combined use of different sensor technologies. So-called wide-range sensors are typically used to measure pressure across many orders of magnitude. These sensors combine various measuring principles.This allows the pressure to be read as a continuous quantity on a device, which, however, must combine various sensor technologies or be compatible with them.

[0003] Pirani-type pressure sensors are well-known and widely used for measuring vacuum pressure. A Pirani pressure sensor utilizes the property that the thermal conductivity of gases depends on the gas pressure within certain pressure ranges. The higher the surrounding gas pressure, the greater the thermal conductivity of the medium; conversely, the lower the gas pressure, the less efficiently heat is dissipated from a hot surface.

[0004] On a given hot surface, an equilibrium temperature is established at a specific electrical power input (i.e., a specific heat input) and through pressure-dependent heat dissipation to the surrounding medium. This forms the basis of pressure measurement using a Pirani pressure sensor.

[0005] In a Pirani pressure sensor, the hot surface is created either by a filament or by an electrical resistor integrated into a so-called MEMS chip. Under these conditions, the equilibrium temperature of the filament or the integrated resistor on the MEMS chip, when subjected to a constant electrical power, depends on the thermal conductivity of the surrounding medium. The pressure of the surrounding medium can therefore be determined from the temperature of the filament or resistor. The temperature of the filament or resistor is thus determined by its electrical resistance. The filament or resistor therefore functions as a resistance thermometer.

[0006] Electrical resistance is therefore a measure of the temperature of the filament or resistive component, and temperature, in turn, is a measure of the pressure of the medium. Thus, with a constant electrical power supply, the pressure of the surrounding medium can be determined from the resistance.

[0007] Examples of Pirani pressure sensors are disclosed in publications GB 2105472 A, US 6,945,119 B2, DE 4 324 119 C2 and EP 1 552 265 B1.

[0008] The measuring principle of a Pirani pressure sensor is therefore easy to control; the technology used in its basic design is simple and requires no mechanically moving parts. Furthermore, Pirani pressure sensors offer a wide measuring range from high vacuum to atmospheric pressure. However, due to their operating principle, Pirani pressure sensors are better suited for low pressures. At higher pressures, the proportion of heat exchange with the surrounding medium via convective processes increases compared to pure heat conduction.

[0009] Convective effects generally do not occur in Pirani sensors based on MEMS pressure sensor technology due to their small size. This limits the measuring range of MEMS Pirani sensors compared to classic wire-based Pirani sensors.

[0010] Unlike heat conduction, convection is generally dependent on the specific geometry and installation situation of the Pirani pressure sensor. This means that the pressure-temperature characteristic of a particular Pirani pressure sensor depends on its installation position, making precise pressure measurement across the entire measuring range more difficult. Consequently, pressure measurements are less reliable and more prone to error at higher pressures.

[0011] The pressure measuring range of known Pirani sensors is therefore relatively limited in practice, and the accuracy of the Pirani sensors as well as the reproducibility of the measurement results are in need of improvement in some cases. Pirani sensors are also relatively sensitive to changing environmental influences.

[0012] To implement Pirani sensors, so-called MEMS chips are primarily used. MEMS stands for "micro electrical mechanical systems." These chips are typically housed in TO packages, meaning they are bonded, glassed, and laminated. TO refers to a package type borrowed from transistor technology. MEMS chips for Pirani sensors generally contain a measuring resistor whose temperature is determined according to the Pirani measurement principle. This resistor is surrounded on the chip by a cavity that connects to the vacuum medium being measured. The heat conduction between the measuring resistor and the chip body via the medium in the cavity (i.e., the gas) then serves as a measure of the measured pressure. A reference resistor may also be present on the MEMS chip. Such MEMS chips are primarily suitable for use in a fine vacuum.

[0013] The stability of the measurement in Pirani pressure sensors depends particularly on ambient temperature fluctuations. Various solutions to compensate for these influences are known in the prior art.

[0014] In GB 2105472 A, US 5,608,168 A1, or WO 2000 / 054018 A1, it is proposed to operate the Pirani sensor at a constant temperature, i.e., to regulate it to a constant temperature. For this purpose, the power supplied to the sensor is actively adjusted so that the temperature in the filament remains constant. In this operating mode, the power supplied during this regulation is a measure of the heat dissipated into the medium, and thus of the thermal conductivity of the medium and ultimately the pressure prevailing in the medium. Ultimately, this approach also determines the temperature of the filament, whereby the applied power is an indirect measure of how much a fixed temperature difference between the filament and the surrounding medium needs to be adjusted.

[0015] German patent DE 11 2005 002 501 B4 discloses a Pirani vacuum measuring device. The patent describes two operating modes for the Pirani sensor: one with a constant current or constant voltage, and the other with a constant temperature. In these modes, either a constant current or a constant voltage is supplied to the bridge circuit. According to the patent, in the constant-temperature measuring mode, when an unbalanced voltage is detected, a current is fed back to the bridge circuit in such a way that the resistance or temperature of the Pirani sensor's heating element is kept constant, thus maintaining the equilibrium of the bridge circuit.

[0016] Another known measurement method involves measuring the change in electrical resistance in the filament when power is applied to it in pulses. The resulting decay curve, i.e., the relaxation of the filament to a specific temperature, then provides information about the thermal conductivity of the surrounding medium and thus about the pressure to be measured.

[0017] The objective is to specify a Pirani measuring arrangement and a method for pressure measurement using a Pirani sensor, in which the measuring range of the Pirani pressure sensor is extended, fluctuations in ambient temperature are compensated for more reliably, and existing measuring sensors can be used over a pressure range that significantly expands the original pressure range. In particular, for Pirani pressure sensors based on the MEMS principle, the aim is to eliminate the inherent limitation of convection effects, thus extending the available pressure range for measurement, even for MEMS sensors.

[0018] The problem is solved by a method for operating a Pirani pressure sensor with the features of claim 1 and an arrangement for operating a Pirani pressure sensor with the features of claim 7. The respective dependent claims contain expedient and / or advantageous embodiments of the method and arrangement.

[0019] The method for operating a Pirani pressure sensor with a measuring resistor and a measuring volume surrounding the measuring resistor, wherein the measuring resistor is subjected to electrical power and an equilibrium temperature of the measuring resistor is established in the medium of the measuring volume, is carried out according to the invention such that a control unit switches between a first measuring mode and a second measuring mode during pressure measurement. In the first measuring mode, the measuring resistor is subjected to a defined maximum power Pmax, which results from the material limits, and the temperature of the measuring resistor is determined as a measure of the pressure to be determined. In the second measuring mode, the temperature of the measuring resistor is fixed at a maximum temperature, and the power required to maintain the maximum temperature is determined as a measure of the pressure to be determined.

[0020] The basic idea of ​​the method according to the invention is to operate a given Pirani pressure sensor in such a way as to fully utilize its technical design. This is achieved, firstly, by heating the measuring resistor of the Pirani pressure sensor in the first measuring mode up to a maximum temperature, which results from the material limits. This maximum temperature results in particular from the material limits of the given Pirani pressure sensor. This maximum temperature is reached primarily when the pressure in the measuring volume decreases at a certain minimum pressure. As soon as this maximum temperature is reached, the temperature of the measuring resistor is fixed, and the measuring resistor is operated in a second measuring mode. Now, the temperature of the measuring resistor is kept constant, and the power supply to the measuring resistor is regulated so that the temperature remains stable even with a further decrease in pressure.In the first measurement mode, the temperature of the measuring resistor serves as the measured quantity for determining the pressure; in the second measurement mode, the power supplied to maintain the temperature of the measuring resistor is evaluated as the measured quantity for the pressure. Switching between the first and second measurement modes makes it possible to use a given Pirani pressure sensor for pressure measurements even below its originally specified pressure ranges and to obtain a stable and usable measurement signal with sufficient signal strength even at very low pressures.

[0021] The method thus aims to generate the maximum possible pressure signal and make it available for metrological processing by utilizing a maximum permissible device-related threshold for a given Pirani pressure sensor.

[0022] In a suitable embodiment, the method is carried out in the following steps: When the pressure in the receiver, i.e., within the measuring volume, changes, the electrical power supply is increased and the temperature in the measuring resistor is measured. This ensures a sufficiently strong measurement signal in the first measurement mode.

[0023] The measuring resistor is then controlled by a control unit until a threshold state is reached with a defined maximum temperature and / or a defined maximum supplied power using the first measuring mode or the second measuring mode.

[0024] In the first measurement mode, when the maximum power is reached in a first pressure range, the power input is fixed at the maximum value and the pressure in the measuring volume is determined by measuring the temperature in the measuring resistor.

[0025] When a maximum temperature is reached in the measuring resistor, the system switches to the second measurement mode. In the second measurement mode, the temperature is fixed at the maximum temperature, and the power supplied to the measuring resistor at this maximum temperature is determined.

[0026] The maximum temperature can be reached in two ways, depending on the measurement conditions. Firstly, the maximum temperature is reached when the pressure in the measuring volume decreases due to the reduction in the thermal conductivity of the medium. Switching to the second measurement mode then allows pressure measurements to be taken at even lower pressures. Secondly, the maximum temperature can also be reached due to changes in the ambient temperature of the Pirani pressure sensor. In such a case, switching to the second measurement mode compensates for the fluctuating ambient temperature.

[0027] In one embodiment, the first measurement mode is performed at a higher first pressure range and the second measurement mode at a lower second pressure range, with the control unit automatically switching between the first measurement mode and the second measurement mode, whereby the pressure ranges can be traversed in both directions.

[0028] Due to the circuit design, the two pressure ranges may overlap, or a switching hysteresis may occur depending on the direction traversed. In one embodiment, during this transition, the control unit executes either the first or second measurement mode as the pressure ranges are passed through. This can be used for pressure-dependent switching hysteresis between the first and second measurement modes.

[0029] In one embodiment, the Pirani pressure sensor uses an arrangement consisting of a MEMS chip with a cavity arranged on the MEMS chip as the measuring volume and a first conductor structure as the measuring resistor. A MEMS chip proves to be particularly advantageous for carrying out the method, especially because a reference resistor arranged on the chip allows for independent temperature measurement.

[0030] An arrangement for operating a Pirani pressure sensor comprises a measuring resistor, a measuring volume that at least partially surrounds the measuring resistor, and a control unit for controlling the operation of the measuring resistor. The control unit includes a controller for supplying electrical power to the measuring resistor, a measuring unit for acquiring the resistance value of the measuring resistor with a first calibration memory for a stored temperature calibration function for converting the resistance value into a temperature value and the temperature value into a value for a measured pressure at a predefined power supply.This arrangement is designed such that the control unit includes a power measurement unit for electrical power applied to the measuring resistor and a second calibration memory with a stored power calibration function for converting electrical power supplied at a fixed temperature into a measured pressure. Furthermore, the control unit has a switching mechanism between pressure measurement via the temperature calibration function and pressure measurement via the power calibration function.

[0031] The control unit is designed to allow two measurement modes: firstly, determining the temperature of the measuring resistor by measuring its resistance at a fixed electrical power; and secondly, determining the temperature under varying electrical power while ensuring a constant temperature. The control unit includes a switching device for this purpose.

[0032] The switching mechanism and the calibration functions can be implemented either through circuitry or in the form of program code. In one embodiment, the control unit includes program code for implementing the temperature calibration function, the power calibration function, and / or the switching mechanism.

[0033] In one embodiment, the Pirani pressure sensor is designed as a MEMS chip, in which the measuring resistor is provided as a first conductor track, a cavity adjacent to the first conductor track as a measuring volume and a second conductor track as a reference thermometer.

[0034] The method according to the invention and the arrangement for carrying out the method will be explained in more detail below using exemplary embodiments. The attached figures serve to illustrate this. Fig. 1 to 8. The same reference symbols are used for identical and / or equivalent components.

[0035] It shows: Fig. 1 a schematic sequence of the method according to the invention, Fig. 2 a sequence of the individual measurement modes depending on the surrounding pressure of the vacuum, Fig. 3. A cycle through the measurement modes of the measurement procedure at decreasing pressure, including the passage through a transition range. Fig. 4. A cycle through the measurement modes of the measurement procedure with increasing pressure, including the passage through the transition range. Fig. 5 exemplary curves of the measuring resistance and the associated electrical power as a function of pressure in the first and second measurement modes, Fig. 6 exemplary curves of the measuring resistance and the associated electrical power as a function of pressure in the first and second measurement modes with a transition range, Fig. 7 an exemplary control unit coupled with a Pirani pressure sensor, Fig. 8 an exemplary MEMS chip for use as a Pirani pressure sensor.

[0036] The method, in its embodiments described below, pursues various objectives and approaches. A primary objective is to empirically compensate for temperature fluctuations occurring in the area of ​​the Pirani pressure sensor without additional physical measurement and control loops. Classical Pirani pressure sensors known from the prior art, in the form of a filament, cannot measure ambient temperatures. Therefore, one embodiment utilizes a Pirani pressure sensor in the form of a MEMS chip, which features a second meandering conductor structure. This structure allows for the determination of ambient temperatures.

[0037] A second objective is to optimize the entire measurement system for a maximum measurement signal within each pressure range, particularly when using a MEMS chip as a Pirani pressure sensor. The method described here allows measurements at temperature differences of, for example, up to 250 K (application example for a specific MEMS Pirani sensor) at very low pressures. The temperature value mentioned here is merely an example and depends on the specific MEMS pressure sensor used. This is a feedback system in which the pressure range is approximately determined during startup, enabling measurement at the maximum permissible voltage applied to the Pirani sensor. This increases the signal strength within the respective pressure range compared to conventional measurement methods.

[0038] Fig. Figure 1 shows a schematic diagram of the method according to the invention. The process steps and procedures described below aim, firstly, to extend the measuring range of a Pirani pressure sensor so that, in particular, lower pressures than with the Pirani sensors commonly used in the prior art can be measured. Secondly, the method is intended to ensure that changing environmental conditions, especially changing temperatures in the vacuum of the medium or of a nearby container wall, can be compensated for. The method thus also aims to stabilize the measurement process. This means, in particular, that a switch between the first and second measuring modes can be triggered both depending on the pressure of the surrounding vacuum and independently of the pressure.

[0039] In the presentation in Fig. 1. Initially, a given initial pressure is assumed, at which a given Pirani pressure sensor is designed for pressure measurement. The Pirani pressure sensor can be operated by applying a variable electrical power P. Simultaneously, the actual measurement process takes place using the Pirani sensor. This process consists, for example, of determining the temperature T of the measuring resistor of the Pirani sensor at a specific power input and then calculating the pressure in the surrounding vacuum from this temperature. As previously explained, the temperature is determined by measuring the current electrical resistance R of the measuring resistor and converting this value into the temperature value.

[0040] During the measurement process of the Pirani pressure sensor, two limiting cases can occur in connection with the specific measurement sequence, in which the Pirani pressure sensor is subjected to maximum stress. In the first limiting case, a maximum temperature Tmax of the sensor's measuring resistor is reached. This maximum temperature is predefined and set as a value in a control unit for operating the Pirani sensor. In such a case, the Pirani sensor continues to operate in measurement mode M2. This mode is configured such that the maximum temperature Tmax in the Pirani sensor's measuring resistor is kept constant, and during the further course of the measurement, the power required by the Pirani sensor to maintain this maximum temperature Tmax is determined as a measured quantity for the pressure prevailing in a vacuum and converted into the corresponding pressure value.

[0041] However, a second limiting case is also possible, in which the measuring resistor of the Pirani sensor is subjected to a maximum power Pmax. This maximum power Pmax is determined simply from the voltage drop across the measuring resistor and the current flowing through it. It is also predefined in the control unit for operating the Pirani sensor and results, for example, from the specific Pirani sensor model used. In such a case, the Pirani sensor is operated in measurement mode M1. Here, the applied power is fixed at the maximum value Pmax by the control unit, and the temperature T of the measuring resistor is determined and evaluated as a measurement parameter for calculating the pressure in the surrounding vacuum.

[0042] A key idea is to operate a given Pirani pressure sensor at either a maximum temperature or a maximum power input using the method described here, in order to maximize the measurement signal obtained and to extend the measuring range of the Pirani pressure sensor.

[0043] It should be emphasized that the designations "first measurement mode M1" and "second measurement mode M2" do not indicate any hierarchy between the two modes. The Pirani sensor is used under specific conditions, with the power applied to the sensor's measuring resistor being gradually increased in an initial process. The Pirani sensor is operated in measurement mode M1 or measurement mode M2 ​​depending on whether the maximum power Pmax or the maximum temperature Tmax at the measuring resistor has been reached.

[0044] The switch between the first measurement mode M1 and the second measurement mode M2 ​​can occur especially when the pressure is reduced in a vacuum. Fig. Figure 2 shows a corresponding example. In the representation in Fig. 2. Initially, a volume is assumed in which a sufficiently high pressure prevails, such as that found in the atmospheric pressure range or in a rough vacuum. Within this range, the Pirani sensor is first subjected to a specific electrical power P. The electrical power is fixed at a maximum power Pmax, which is based on the upper load limit of the Pirani sensor.

[0045] The measuring resistor of the Pirani sensor assumes an equilibrium temperature, which is established through heat conduction with the surrounding medium. This equilibrium temperature is measured. It is a measure of the pressure of the medium. This is the measuring principle of measurement mode M1.

[0046] As the pressure decreases further, i.e., as the vacuum increases, the thermal conductivity of the surrounding medium also decreases. With a fixed maximum power output (Pmax), the temperature in the measuring resistor rises accordingly as the pressure of the surrounding medium decreases. Therefore, the measuring range of the Pirani sensor is limited by a maximum temperature at low pressures.

[0047] In the method according to the invention, the measuring range is extended to lower pressures. As soon as the control unit registers a maximum temperature Tmax at the measuring resistor, it switches to measuring mode M2. This mode consists of fixing the maximum temperature Tmax achievable by the Pirani sensor. Measuring mode M2 ​​then proceeds such that, as the vacuum pressure continues to decrease, the Pirani sensor is controlled in such a way that the existing maximum temperature Tmax at the measuring resistor is maintained. The power P supplied to the Pirani sensor and required to maintain the maximum temperature now serves as the measured quantity of the pressure. This power is determined and used as the basis for determining the pressure in the vacuum according to an existing calibration.

[0048] Because the thermal conductivity of the medium surrounding the measuring resistor decreases further with decreasing pressure, and thus the measuring resistor is less able to dissipate heat to the surrounding medium, less electrical power is required to maintain a constant temperature Tmax of the measuring resistor. This means that even with a further reduction in pressure in the vicinity of the Pirani sensor, the sensor can be operated at its maximum temperature while simultaneously extending the measuring range of the Pirani sensor to lower pressures. As a result, the available signal strength for evaluation remains sufficiently high across all pressure ranges.

[0049] The Pirani sensor's control unit automatically selects between measurement mode M2 ​​and measurement mode M1. The criterion for choosing between M1 and M2 is which of the two maximum values, Pmax or Tmax, is reached first at a given pressure with increasing power input. If the maximum power Pmax is reached first, the Pirani sensor operates in measurement mode M1; if the temperature at the Pirani sensor reaches Tmax first, measurement mode M2 ​​is activated.

[0050] The control of the Pirani sensor thus aims to operate a given Pirani sensor as close as possible to the limits of the operating parameters Pmax and Tmax that are permissible on the device side.

[0051] This operating mode is particularly useful when the Pirani pressure sensor is operated in a vacuum environment with a variable pressure, where the pressure varies over several orders of magnitude. The method is, for example, applicable in a pressure range of 10 -6 mbar up to 10 3 The method is applicable in mbar, i.e., between the pressure range of high vacuum and normal atmospheric pressure. It is particularly suitable for monitoring the pressure in a volume that is alternately evacuated and repressurized.

[0052] An example of this is in the Fig. 3 and Fig. Figure 4 shows the pressure and temperature profiles in the Fig. 5 and Fig. 6 shown.

[0053] In the example in Fig. 3. Initially, a starting pressure in the atmospheric range is assumed, whereby the pressure continuously decreases. Fig. Figure 4 shows the opposite pressure curve, starting from a low pressure, for example a high vacuum, and increasing the pressure in the measuring volume until atmospheric pressure is reached.

[0054] In Fig. Step 3 begins with a high pressure, for example, atmospheric pressure, which is then reduced. As described, the Pirani pressure sensor initially operates in measurement mode M1. As the pressure continues to decrease, particularly when a high vacuum is reached, the control unit switches to measurement mode M2.

[0055] In the representation in Fig. 3. Between the pressure range in which measurement mode M1 is executed and the pressure range in which measurement mode M2 ​​is operated, there is a transition range U. In this transition range, it is possible to operate the Pirani pressure sensor in both measurement mode M1 and measurement mode M2. Which of the two measurement modes is actually executed in the transition range depends in particular on the direction in which the pressure ranges in the measurement volume are traversed. In the example in Fig. 3. Initially, the measurement operation is based on measurement mode M1, and this measurement mode is maintained throughout the transition range U. Only at the lower end, the lowest pressure of the transition range U, does the control unit switch the operation of the Pirani pressure sensor to measurement mode M2.

[0056] In the representation in Fig. 4. The pressure range is traversed in reverse direction. That is, it starts at a low pressure, for example, in the high vacuum range, and the pressure is slowly increased until it finally reaches atmospheric pressure. In this case, the Pirani pressure sensor initially operates in measurement mode M2. As the pressure increases, the control unit continues to operate the Pirani pressure sensor in mode M2 ​​and maintains this mode throughout the transition range U. Only at the upper limit, i.e., the highest pressure of the transition range U, does the control unit switch to measurement mode M1.

[0057] The Fig. 5 and Fig. Figure 6 each shows a diagram illustrating the electrical resistance R of the measuring resistor of the Pirani pressure sensor and the power P applied to the measuring resistor. The resistance R is a direct measure of the temperature of the measuring resistor. The aforementioned measurement modes M1 and M2 are shown, as well as the maximum temperature Tmax fixed in measurement mode M2 ​​and the maximum power Pmax fixed in measurement mode M1. The function m1 present in measurement mode M1 represents a first calibration function m1, while the function m2 present in measurement mode M2 ​​represents the calibration function m2 applicable to measurement mode M2. The diagrams in the Fig. 5 and Fig. Figure 6 illustrates that the pressure measurement ranges are determined here via the calibration functions m1 and m2 and the associated resistance and power values ​​R and P.

[0058] The two measuring modes, M1 and M2, depend on the pressure in the measuring volume. At a specific switching pressure pU, the control unit switches from measuring mode M1 to measuring mode M2, or vice versa. The switching pressure pU is shown in the diagram at Fig. 5 out of 10 0 mbar. This is merely an example value, intended only for illustrative purposes.

[0059] Fig. Figure 6 further illustrates the transition range U. One of the two available measurement modes can be executed, and it is also possible, for example, to activate one measurement mode when increasing the pressure and the other when decreasing it. This allows for a comparison of the measured pressure in the transition range between the two measurement modes M1 and M2 under different pressure profiles. In this example, there is no clearly defined transition pressure pU. Within this range, for instance, it is possible that neither the maximum temperature Tmax nor the maximum power Pmax will be clearly recorded, or that they may fluctuate due to changes in ambient conditions. Here, there is no clearly defined switchover pressure pU, but rather the transition range U.

[0060] The transition range, in particular, results in switching hysteresis in the combined resistance and power-pressure diagram when traversing different pressure ranges. For example, the pressure in the measuring volume can initially be increased from a low pressure to a higher pressure. In the low pressure range, the Pirani pressure sensor initially operates in measuring mode M2. At a specific switching pressure p1, the measuring mode changes to measuring mode M1 as described.

[0061] Conversely, it is also possible to lower the pressure from a higher pressure range to a lower pressure range. In this case, the Pirani pressure sensor is initially operated in measuring mode M1, and the measuring mode switches to measuring mode M2 ​​at a switching pressure p2. The switching pressures p1 and p2 typically differ from each other. This constitutes the switching hysteresis of the control method according to the invention, which occurs primarily when the pressure in the measuring volume is traversed.

[0062] The transition range U between the two switching pressures p1 and p2 thus results from a switching hysteresis within the control sequence of the Pirani pressure sensor executed by the control unit. The width of the transition range U depends on a number of conditions, in particular on the thermal conductivity of the specific gas present in the receiver.

[0063] The width of the transition range also depends on the speed at which the pressure within the measuring volume changes, as well as the time required for switching, adjusting, and stabilizing the respective measuring mode. A comparatively rapid pressure change leads to a correspondingly large switching hysteresis and thus a correspondingly wide transition range. However, if the pressure in the measuring volume is changed slowly and cycles through a series of states, reaching a quasi-equilibrium between the inertia of the Pirani pressure sensor's control system and the pressure of the medium, the width of the transition range can be significantly reduced.

[0064] Fig. Figure 7 shows an example block diagram for the connection of a control unit with a Pirani pressure sensor. Fig. Figure 7 shows an exemplary Pirani pressure sensor 1 with a measuring resistor 2 arranged in the area of ​​the pressure sensor. The Pirani pressure sensor 1 is located in a surrounding measuring volume 3.

[0065] The measuring resistor 2 is subjected to an electrical power P. As is known, the electrical power P is the product of the voltage drop across the measuring resistor and the current flowing through it.

[0066] A control unit 4 is provided for connecting the Pirani pressure sensor 1. The components contained in the control unit 4, which are described below as examples, can be implemented either as hardware with corresponding circuits or as software.

[0067] The control unit 4 contains a temperature calibration memory 5 with a temperature calibration function m1 and a power calibration memory 6 with a power calibration function m2. The temperature calibration function m1 and the power calibration function m2 essentially correspond to the curves m1 and m2 in the Fig. 5 and Fig. 6. Furthermore, a switching device 7 is provided, in which the control unit 4 can be operated in either measurement mode M1 or measurement mode M2 ​​and has an optimal range for electronic control and evaluation.

[0068] The control unit 4 also contains a measuring unit 8 for the temperature of the measuring resistor 2. This is usually designed as a resistance measuring unit, whereby the value of the electrical resistance R of the measuring resistor 2 determined with this resistance measuring unit is converted into a temperature.

[0069] The control unit 4 also includes a power control unit 9, which regulates the power supply to the measuring resistor 2. Furthermore, a measuring unit 10 is provided, in which the measurement sequence of measurement mode M1 is stored and which controls measurement mode M1, as well as a measuring unit 11, which contains the sequence of measurement mode M2 ​​and which controls the sequence of measurement mode M2. The measuring units 10 and 11, the switching unit 7, and the calibration memories 5 and 6 can be implemented either individually or all together as software, i.e., as a single program code.

[0070] Fig.Figure 8 shows an exemplary MEMS chip 12 as an embodiment for a Pirani pressure sensor. The MEMS chip contains a first conductor track 13 for the measuring resistor and a cavity 14 surrounding the conductor track, which has a direct connection to the surrounding vacuum. The cavity 14 forms the surrounding measuring volume for the conductor track 13. The conductor track 13 is typically applied to a membrane and suspended "freely" within the cavity 14. A thin wall 15 of the membrane separates the conductor track 13 of the measuring resistor from the cavity 14.

[0071] The MEMS chip also features a reference resistor 16, which allows the chip's temperature to be determined independently and on-chip. This resistor also serves to determine the ambient temperature of the medium surrounding the measuring resistor. If necessary, the ambient temperature measured by the reference resistor is used as the basis for the control sequences mentioned above, based on the fixed maximum temperature Tmax. This allows the control system of the Pirani pressure sensor to react immediately to changes in temperature within the vacuum medium. The MEMS chip is located, for example, on a silicon substrate and housed in a TO package.

[0072] The method and the arrangement for carrying out the method have been explained using exemplary embodiments. Further embodiments are possible within the scope of expert knowledge. Additional embodiments are also described in the dependent claims. Reference symbol list M1 first measurement mode m1 Temperature calibration function M2 second measurement mode m2 Power calibration function P electrical power Pmax Maximum power R electrical resistance Temperature Tmax Maximum temperature U transition area 1 Pirani pressure sensor 2 measuring resistors 3 measuring volumes 4 Control unit 5 temperature calibration memories 6 performance calibration memories 7 switching devices 8 Unit of measurement 9 Power control unit 10 measuring units for measuring mode M1 11 Measuring unit for measuring mode M2 12 MEMS chips 13 conductor track for measuring resistor 14 cavities 15 wall 16 Reference resistor

Claims

[1] Method for operating a Pirani pressure sensor (1) with a measuring resistor (2) and a measuring volume (3) surrounding the measuring resistor, wherein the measuring resistor is supplied with an electrical power (P) and an equilibrium temperature of the measuring resistor (2) is established in the medium of the measuring volume, wherein a control unit (4) performs a switching between a first measuring mode (M1) and a second measuring mode (M2) during the pressure measurement, wherein in the first measuring mode (M1) the measuring resistor is supplied with a defined maximum power (Pmax), wherein the temperature (T) of the measuring resistor is determined as a measured quantity of the pressure to be determined, and in the second measuring mode (M2) the temperature (T) of the measuring resistor is fixed at a maximum temperature (Tmax), wherein the power (P) required to maintain the maximum temperature (Tmax) is determined as a measured quantity of the pressure to be determined. [2] Method according to claim 1 characterized by , that during the operation of the Pirani pressure sensor, the pressure-dependent measurement signal is maximized by operating the Pirani pressure sensor in the first measurement mode (M1) and / or in the second measurement mode (M2) at a destruction limit defined by the device, wherein the destruction limit in the first measurement mode (M1) corresponds to the fixed maximum power (Pmax) at the measuring resistor (2) and the destruction limit in the second measurement mode (M2) corresponds to the fixed maximum temperature (Tmax) at the measuring resistor (2). [3] Method according to claim 1 or 2, characterized by the following adjustment procedure steps - Changing the pressure within the measuring volume (3) while simultaneously increasing the electrical power supply to the measuring resistor (2) and measuring the temperature (T) in the measuring resistor, - Operational control of the measuring resistor by means of a control unit (4) until a threshold state is reached with a defined maximum temperature (Tmax) and / or a defined maximum supplied power (Pmax) using the first measuring mode (M1) or the second measuring mode (M2), wherein - in the first measurement mode (M1) in a first pressure range, when the maximum power (Pmax) is reached, the power input is fixed at the maximum value (Pmax) and the pressure in the measurement volume is determined via a measurement of the temperature (T) in the measuring resistor as a pressure-dependent measured quantity (G) and - A switch to the second measurement mode (M2) occurs when the temperature (T) in the measuring resistor reaches the maximum temperature (Tmax), whereby in the second measurement mode the temperature (T) is fixed at the maximum temperature (Tmax) and the power (P) supplied to the measuring resistor at this maximum temperature (Tmax) is determined as a pressure-dependent measured quantity. [4] Method according to any one of claims 1 to 3, characterized by , that the first measurement mode (M1) is performed at a higher first pressure range and the second measurement mode (M2) at a lower second pressure range, wherein the control unit automatically switches between the first measurement mode (M1) and the second measurement mode (M2), the pressure ranges and switching being possible in both directions. [5] Method according to any one of the preceding claims, characterized by , that the control unit (4) executes the first measurement mode (M1) or the second measurement mode (M2) when passing through the pressure ranges in a transition range U, whereby a switching hysteresis is realized depending on the direction traversed within the transition range U. [6] Method according to any one of the preceding claims, characterized by, that as a Pirani pressure sensor (1) an arrangement consisting of a MEMS chip (12) with a cavity (14) arranged on the MEMS chip (12) as a measuring volume and a first conductor structure (13) as a measuring resistor is used. [7] Arrangement for operating a Pirani pressure sensor (1), comprising a measuring resistor (2), a measuring volume at least partially surrounding the area of ​​the measuring resistor (2), a control unit (4) for controlling the operation of the measuring resistor, comprising a control for supplying electrical power to the measuring resistor, a measuring unit for recording the resistance value of the measuring resistor (2) with a first temperature calibration memory (5) for a stored temperature calibration function (m1) for converting the resistance value into a temperature value and the temperature value into a value for a measuring pressure at a predetermined power supply, where the control unit (4) includes a power measuring unit for electrical power applied to the measuring resistor (2), a power calibration memory (6) with a stored power calibration function (m2) for converting electrical power supplied at a fixed temperature into a measuring pressure, wherein the control unit has a switching means (7) between a pressure measurement via the temperature calibration function (m1) and a pressure measurement via the power calibration function (m2). [8] Arrangement according to claim 7, characterized by , that the control unit (4) has a program code for implementing the temperature calibration function (m1), the power calibration function (m2), pressure calculation and / or the switching device (7). [9] Arrangement according to one of claims 7 to 8, characterized by, that the Pirani pressure sensor (1) is designed as a MEMS chip (12) in which the measuring resistor is provided as a first conductor track (13), a cavity (14) adjacent to the first conductor track as a measuring volume and a second conductor track as a reference thermometer (16).

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