METHOD FOR OPERATING A GROUP OF PRESSURE SENSORS

Abstract

Method for operating a group 1 of pressure sensors which are arranged in such a manner that they can measure the pressure in a common measurement volume 2, wherein the group of pressure sensors comprises at least a first pressure sensor 1′ with a first pressure measurement range and a second pressure sensor 1″ with a second pressure measurement range, wherein the first and second pressure measurement ranges overlap in an overlap pressure measurement range, wherein the first and second pressure sensors are each based on an indirect pressure measurement principle and are configured to output a measurement signal calibrated to a reference gas, and wherein the method comprises the steps of: a) providing calibration data specific to the type of gas for the first measurement signal and for the second measurement signal, which calibration data describe a dependence of the first and second measurement signals on the effective pressure and on a list of types of gas, respectively; b) recording a first and a second measured value of the first and second measurement signals, respectively; c) determining a resultant type of gas which best matches the combination of the recorded first measured value and the recorded second measured value, taking into account the first and second calibration data. In one variant, a resultant pressure which is independent of the type of gas is additionally determined. The invention is also directed to an apparatus for earring out the method and to a computer program product.

Claims

1. Method for operating a group (1) of pressure sensors (1′, 1″) which are arranged to measure the pressure in a common measurement volume (2), wherein the group of pressure sensors comprises at least a first pressure sensor (1′) having a first pressure measurement range (4′) and a second pressure sensor (1″) having a second pressure measurement range (4″), wherein the first and second pressure measurement ranges overlap in an overlap pressure measurement range (6), wherein the first pressure sensor (1′) is based on a first indirect pressure measurement principle and is adapted to output a first measurement signal calibrated to a reference gas (G.sub.ref), wherein the second pressure sensor (1″) is based on a second indirect pressure measurement principle and is adapted to output a second measurement signal calibrated to the reference gas, and wherein the method comprises the steps of: a) providing (101) gas-type-specific first calibration data (K.sub.1[G.sub.i]) for the first measurement signal and gas-type-specific second calibration data (K.sub.2[G.sub.i]) for the second measurement signal, wherein the first and second calibration data describe a dependence of the first and second measurement signals, respectively, on the effective pressure (p.sub.eff) and on a gas type in the common measurement volume for a list of gas types comprising at least one first gas type (G.sub.1) which is different from the reference gas; b) substantially simultaneously recording (102) a first measured value (p.sub.1) of the first measurement signal and recording a second measured value (p.sub.2) of the second measurement signal; c) determining (103) a resultant gas type (G*) as that gas type in the list of gas types which best matches the combination of the recorded first measured value (p.sub.1) and the recorded second measured value (p.sub.2), taking into account the first and second calibration data.

2. Method according to claim 1, comprising the additional step of d) determining (104) a resultant pressure (p*) as a function of the recorded first measured value (p.sub.1) and the first calibration data for the resultant gas type and/or as a function of the recorded second measured value (p.sub.2) and the second calibration data for the resultant gas type.

3. Method according to claim 1, wherein the first and second pressure sensors are vacuum pressure sensors.

4. Method according to claim 1, wherein the first pressure sensor (1′) is a Pirani sensor.

5. Method according to claim 1, wherein the second pressure sensor (1″) is a hot-cathode ionization vacuum gauge, especially of the Bayard-Alpert type.

6. Method according to claim 1, wherein the second pressure sensor (1″) is a cold cathode ionization vacuum gauge, in particular an inverted magnetron.

7. Method according to claim 1, wherein the first and second gas-type specific calibration data (K.sub.1[G.sub.i], K.sub.2[G.sub.i]) are each defined by a first and second factor (C.sub.1[G.sub.i], C.sub.2[G.sub.i]), respectively, by which the first measurement signal and the second measurement signal, respectively, must be multiplied to obtain the effective pressure.

8. Method according to claim 7, wherein a list of quotients is formed by forming a quotient (Q[G.sub.i]) for each gas type from the list from the first factor for the respective gas type and the second factor for the respective gas type, wherein a recorded quotient (Q) is formed as a quotient of the recorded first measured value (p.sub.1) and the recorded second measured value (p.sub.2), and wherein in step c) it is determined to which of the quotients (Q*) from the list of quotients the recorded quotient comes closest.

9. Method according to claim 1, wherein in step c) for each gas from the list of gases, starting from the recorded first measured value (p.sub.1), based on the first and second gas-type-specific calibration data, it is determined what value is expected for the second measurement signal if this gas were present in the common measurement volume, and wherein the smallest deviation of this value from the recorded second measured value (p.sub.2) is used as a criterion for determining the resultant gas type.

10. Method according to claim 1, wherein a plurality of pairs of a first measured value of the first pressure sensor and a second measured value of the second pressure sensor are each recorded as the pressure in the common measurement volume changes, wherein the gas type that best matches the combination of the recorded plurality of pairs is selected when determining the resultant gas type (G*).

11. Method according to claim 1, wherein it is checked on the basis of the recorded first measured value (p.sub.1), on the basis of the recorded second measured value (p.sub.2) or on the basis of the resultant pressure (p*) whether the pressure present in the common measurement volume lies in the overlap pressure measurement range and wherein the resultant pressure and/or the resultant gas type are rejected as invalid if this is not the case.

12. Apparatus (10) for carrying out the method according to claim 1, wherein the apparatus comprises a group (1) of pressure sensors arranged such that they are capable of measuring the pressure in a common measurement volume (2), wherein the group of pressure sensors comprises at least a first pressure sensor (1′) having a first pressure measurement range (4′) and a second pressure sensor (1″) having a second pressure measurement range (4″), wherein the first and second pressure measurement ranges overlap in an overlap pressure measurement range (6), wherein the first pressure sensor (1′) is based on a first indirect pressure measurement principle and the second pressure sensor (1″) is based on a second indirect pressure measurement principle, and wherein the apparatus comprises means (5) for storing first calibration data and second calibration data.

13. Apparatus (10) according to claim 12, comprising a control unit (12) which is operatively connected to a first measurement signal output (3′) of the first pressure sensor, to a second measurement signal output (3″) of the second pressure sensor and to means (5) for storing first calibration data and second calibration data for processing the measurement signals of the pressure sensors and which is arranged for outputting the resultant gas type and/or the resultant pressure.

14. Computer program product comprising instructions which, when the instructions are executed by a control unit (12) of an apparatus (10) according to claim 13, cause the control unit to perform the steps of a method (100) for operating a group (1) of pressure sensors (1′, 1″) which are arranged to measure the pressure in a common measurement volume (2), wherein the group of pressure sensors comprises at least a first pressure sensor (1′) having a first pressure measurement range (4′) and a second pressure sensor (1″) having a second pressure measurement range (4″), wherein the first and second pressure measurement ranges overlap in an overlap pressure measurement range (6), wherein the first pressure sensor (1) is based on a first indirect pressure measurement principle and is adapted to output a first measurement signal calibrated to a reference gas (G.sub.ref), wherein the second pressure sensor (1″) is based on a second indirect pressure measurement principle and is adapted to output a second measurement signal calibrated to the reference gas, and wherein the method comprises the steps of: a) providing (101) gas-type-specific first calibration data (K.sub.1[G.sub.i]) for the first measurement signal and gas-type-specific second calibration data (K.sub.2[G.sub.i]) for the second measurement signal, wherein the first and second calibration data describe a dependence of the first and second measurement signals, respectively, on the effective pressure (p.sub.eff) and on a gas type in the common measurement volume for a list of gas types comprising at least one first gas type (G.sub.1) which is different from the reference gas; b) substantially simultaneously recording (102) a first measured value (p.sub.1) of the first measurement signal and recording a second measured value (p.sub.2) of the second measurement signal; c) determining (103) a resultant gas type (G*) as that gas type in the list of gas types which best matches the combination of the recorded first measured value (p.sub.1) and the recorded second measured value (p.sub.2), taking into account the first and second calibration data.

Description

[0051] Exemplary embodiments of the present invention are explained in further detail below with reference to figures, wherein:

[0052] FIG. 1 schematically shows an apparatus for carrying out the method;

[0053] FIG. 2 shows a flowchart of the method according to the invention;

[0054] FIG. 3 schematically shows possible relative positions of first and second pressure measurement ranges and resulting overlap pressure measurement range;

[0055] FIG. 4 shows the dependence of a pressure determined by a Pirani sensor on the type of gas in a double-logarithmic diagram;

[0056] FIG. 5 schematically shows first and second gas-dependent calibration curves.

[0057] FIG. 1 schematically shows an exemplary apparatus 10 for carrying out the method. The apparatus comprises a group 1 of pressure sensors, having at least one first pressure sensor 1′ and one second pressure sensor 1″, which can measure pressures in a common measurement volume 2. The measurement volume 2 can in particular be a partial volume of a vacuum chamber, as indicated schematically by the dash-dotted outlined area. The first pressure sensor 1′ is set up to forward a first measurement signal p.sub.1 from a first measurement signal output 3′ to a control unit 12. The second pressure sensor 1″ is set up to forward a second measurement signal p.sub.2 from a second measurement signal output 3″ to the control unit 12. The active connections drawn with dashed lines can be implemented, for example, by wire, and they can also be implemented, for example, by radio signals (Bluetooth, etc.) or optical signal transmission. Dashed arrows show the flow of information between the elements of the apparatus. The apparatus comprises means 5 for storing gas-dependent calibration data, which can be transmitted to the control unit. A resultant gas type G* and a resultant pressure p* can be output by the control unit.

[0058] The parts of the shown apparatus or the complete apparatus can be installed in a common housing. In particular, the group of pressure sensors and the control unit may be combined in a common housing to form a pressure sensor unit. Additionally, a means for storing calibration data may optionally be housed in the common housing.

[0059] FIG. 2 shows a flowchart of the method 100 according to the invention. The method comprises the steps of [0060] a) providing 101 gas-type-specific first calibration data K.sub.1[G.sub.i] for the first measurement signal and gas-type-specific second calibration data K.sub.2[G.sub.i] for the second measurement signal, wherein the first and second calibration data describe a dependence of the first and second measurement signals, respectively, on the effective pressure p.sub.eff and on a gas type in the common measurement volume for a list of gas types comprising at least one first gas type G.sub.i which is different from the reference gas; [0061] b) substantially simultaneously detecting 102 a first measured value p.sub.1 of the first measurement signal and detecting a second measured value p.sub.2 of the second measurement signal in the overlap pressure measurement range; [0062] c) determining 103 a resultant gas type G* as the gas type in the list of gas types that best matches the combination of the recorded first measured value p.sub.1 and the recorded second measured value p.sub.2, by taking into account the first and second calibration data.

[0063] Steps 101, 102 and 103 are carried out sequentially, with the necessary calibration data already being made available before the start (START) of the procedure. At the end (END) of the method, the resultant gas G* is known.

[0064] The optional step d) is shown by a dashed rectangle, which, if additionally executed, leads to a variant of the method, which also provides a resultant pressure as output. With this additional step, the resultant pressure p* is also known at the end of the method.

[0065] The additional step d) involves determining 104 a resultant pressure p* as a function of the recorded first measured value p.sub.1 and the first calibration data for the resultant gas type and/or as a function of the recorded second measured value p.sub.2 and the second calibration data for the resultant gas type. Based on the resultant gas type known from step c), the corresponding set of calibration data is thus used to translate the measured values of the pressure sensors into the gas-type-independent effective pressure.

[0066] FIG. 3 shows in FIG. 3.a) and FIG. 3.b) schematically two possibilities of the relative position of the first 4′ and second 4″ pressure measurement range of the first 1′ and second 1″ pressure sensor of the group of pressure sensors on a pressure axis p. The pressure axis p is to be understood schematically here, it could be for example a linear axis or also a logarithmic axis. High pressures are drawn further up on the axis than lower pressures. An overlap pressure measurement range 6 exists, in which the first 4′ and the second 4″ pressure measurement ranges overlap. The reading of the first and second measurement signals in step a) of the method takes place while the pressure in the common measurement volume is in this overlap pressure measurement range 6. In FIG. 3.b) the case is shown in which the second pressure measurement range 4″ lies completely within the first pressure measurement range 4′, so that the overlap pressure measurement range is identical to the second pressure measurement range 4′.

[0067] FIG. 4 shows the dependence of a pressure determined by means of a Pirani sensor on a certain type of gas in a double-logarithmic representation. In the horizontal direction, the “effective” pressure p.sub.eff is plotted. In the vertical direction, the pressure p (mbar) read at a Pirani sensor is plotted as a function of the effective pressure p.sub.eff (mbar) for different types of gas, each with a separate curve; see the label for each curve in the area at the top right of the diagram. The illustrated pressure range extends on both axes from 10-mbar to 10.sup.2 mbar, i.e. over 5 orders of magnitude. In this case, the Pirani sensor is calibrated to show the pressure p.sub.eff for the gas type air, i.e. the pressure curve to air (Air) is a straight line on the diagonal in the double-logarithmic plot. Each of the curves shown is thus a gas-type-dependent calibration curve. The first pressure sensor can be a Pirani sensor, for example, so the set of curves shown in FIG. 4 can represent the gas-type-specific first calibration data.

[0068] In a pressure range below about 1 mbar, the effect of the gas type can be described by a factor between p.sub.eff and the pressure p measured with the Pirani sensor, which shows up in the double-logarithmic diagram as an offset of the curves. As the inventors have recognized, the essential information of these calibration curves in over about two decades can already be described sufficiently accurately by the aforementioned factor, so that a table with the corresponding factors is a very memory-saving form of gas-type-specific initial calibration data.

[0069] Similarly, second calibration data for the second pressure sensor can be provided as a set of curves or as a table of factors.

[0070] FIG. 5 shows exemplary and schematic calibration curves for three gases, for gas G.sub.1 (solid line), for gas G.sub.2 (dashed with short lines) and gas G.sub.3 (dashed with long lines). In the left half, the calibration curves for a first pressure sensor are shown as a function of the first measurement signal p.sub.1. In the right half the calibration curves for a second pressure sensor are shown as a function of the second measurement signal p.sub.2. The effective pressure p.sub.eff corresponding to the respective measurement signal is plotted on the vertical axis, wherein a certain vertical position in the left diagram corresponds to the same effective pressure as in the right diagram. The calibration curves are to be understood as illustrative examples which clarify the principle underlying the invention. For example, the diagrams may be double-logarithmic representations. The gas-type dependence of the first sensor is different from the gas-type dependence of the second sensor. In the second pressure sensor, variations in the slope and curvature of the curves are apparent, which are not pronounced in the calibration curves of the first sensor in this example.

[0071] A white triangle shows the determined first measurement signal of the first pressure sensor on the p.sub.1 axis. At the same time, the second pressure sensor determined the second measurement signal displayed as a black triangle on the p.sub.2 axis. With auxiliary lines starting from the white triangle it is indicated which effective pressure one would expect in the common measurement volume depending on the gas type and which second measurement signal would be expected at this effective pressure. Gas G.sub.3 fits the actually measured values in the best manner, therefore gas G.sub.3 is defined as resultant gas G*. The criterion for this can be e.g. distance on the—optionally logarithmic—p.sub.2 axis. On the calibration curve for G*=G.sub.3, the resultant pressure p* can now be read on the p.sub.eff axis.

[0072] In the event that several such pairs of first and second measurement signals are to be compared, a sum of squared distances, for example, is a suitable criterion to determine the best fitting gas. The role of first and second measurement signal can be reversed in that also starting from the actually measured second measurement signal expected measurement signals on the p.sub.1 axis are determined and there—alternatively or additionally—the distance from the measured first measurement signal is determined as a criterion for the best fitting gas.

[0073] The method according to the invention and all its embodiments may be combined with an additional step of zeroing at least one of the pressure sensors. In various types of pressure sensors, the measurement signal generated for a given effective pressure is subject to drift over time. This effect can be eliminated by zeroing, which further increases the accuracy of the method according to the invention. The zero-point measurement signal is preferably determined at an effective pressure which is at least one to two decades below the measurement range of the pressure sensor to be zeroed. Checking whether a sufficiently low pressure is present can be carried out in various ways. For example, to zero a Pirani sensor, the achievement of a sufficiently low pressure can be checked with an ionization vacuum gauge whose pressure measurement range extends to at least two decades below the measuring range of the Pirani sensor. As another example, when zeroing a Bayard-Alpert type ionization vacuum gauge, reaching a sufficiently low pressure can be checked by an extractor-type ionization vacuum gauge. Reaching a sufficiently low pressure for zeroing a pressure sensor can also be achieved, for example, by suitable method steps, such as by pumping down the common measurement volume for a long time. The achievement of a sufficiently low pressure can alternatively also be derived from operating parameters of a vacuum pump operatively connected to the common measurement volume of the pressure sensors.

[0074] It should be noted that the drift of the pressure sensor and the gas-type dependence are two separate phenomena. For example, even if the pressure sensor is always reset to zero after each change of gas type, there is still a gas-type dependency.

[0075] Returning to the way in which a list of gas types, which may include a list of gas mixtures, can be handled, the following illustrative examples are given. For example, in the case of a Pirani sensor, the idea is to summarize the contributions of the thermal conductivity of each component of the gas mixture to the total thermal conductivity. For example, with formula 11 from the publication

[0076] K. Jousten, On the gas species dependence of Pirani vacuum gauges, Vac. Sei. Technol. A 26, 3, May/June 2008,

[0077] Jousten gives a formula which takes into account effective accommodation coefficients and heat capacities of each gas species involved in a mixture. Alternatively, also suitable for Pirani sensors, with formulas 17 and 18 from the publication

[0078] Ikhsan Setiawan et al, Critical Temperature Differences of a Standing Wave Thermoacoustic Prime Mover with Various Helium-Based Binary Mixture Working Gases, 2015 J. Phys.: Conf. Ser. 622 012010,

[0079] Setiawan gives a formula, which based on the gas fraction, thermal conductivities and molar masses of each gas species involved, indicates the thermal conductivity of the gas mixture. Both of the above formulas are suitable for generating a table of gas types in the form of gas mixtures, or also for using a fraction of one or more gas types as a continuous parameter. The latter is advantageous for the “best fit” procedures as described above.

[0080] A similar, but somewhat more complex procedure is possible with ionization vacuum gauges. Here, the energy distribution of the electrons, the ionization potential of the gases, the fragmentation of the gases and any recombination are important as possible influencing variables for the prediction of an ion current. If the sensitivity S, which is defined as the ratio of ion current at the ion collector on the one hand and electron emission current and pressure on the other, is known from experimental data or simulations, the combined ion current can be determined as the sum weighted with partial pressures of the gas types involved. Pressures and partial pressures are considered as the difference to the residual pressure, collector currents as the difference to the collector current at the residual pressure.

LIST OF REFERENCE SIGNS

[0081] 1 Group of pressure sensors [0082] 1′ First pressure sensor [0083] 1″ Second pressure sensor [0084] 2 Common measurement volume [0085] 3′ First measurement signal output [0086] 3″ Second measurement signal output [0087] 4′ First pressure measurement range [0088] 4″ Second pressure measurement range [0089] 5 Means for storing calibration data [0090] 6 Overlap pressure measurement range [0091] 12 Control unit [0092] G* Resultant gas type [0093] p.sub.1 First measurement signal [0094] p.sub.2 Second measurement signal [0095] p.sub.eff Effective pressure [0096] p* Resultant pressure [0097] 100 Method [0098] 101 Step a) Providing calibration data [0099] 102 Step b) Recording of the first and second measurement [0100] signals [0101] 103 Step c) Determining the resultant gas type [0102] 104 Step d) Determining the resultant pressure [0103] START Start of method [0104] END End of method