Method for temperature-dependent gas detection using gas selective membranes

Abstract

A method for detecting a gas using a gas-selective membrane (12), a temperature device (14) and a detector (20), the temperature device (14) being arranged to change the temperature of the membrane (12), the detector (20) being arranged to obtain a measurement signal (S) based on the amount of gas passing through the membrane (12), characterized in that it comprises the following steps: - using the temperature device (14) to change the temperature of the membrane (12), - at the moment (t) when the membrane temperature takes a first temperature value (T1), using the detector (20) to obtain at least one first measurement value (H n , H n+1 , H n+2 ), - at the moment when the membrane temperature takes a second temperature value (T2) different from the first temperature value, using the detector (20) to obtain at least one second measurement value (L n , L n+1 ), - calculating the difference between the two measurement values, and - using this difference to evaluate whether there is a gas to be measured.

Description

Technical Field

[0001] The invention relates to a method for gas detection using a gas selective membrane whose permeability to a specific gas depends on the temperature. Background Art

[0002] For gas detection using such gas selective membranes, it is known to set the membrane temperature to a value at which the gas permeability of the membrane to the gas type to be detected is at a maximum. To this end, the membrane is usually heated when the gas permeability is at a maximum at a temperature above room temperature.

[0003] The amount of gas that permeates through the membrane depends on the temperature of the membrane and the difference in partial pressure of the gas upstream (i.e. in front of the membrane) and downstream (i.e. behind the membrane). The gas to be measured downstream of the membrane is detected using a detector, which can be a gas measuring device, such as a mass spectrometer, or a pressure measuring device that measures the total pressure of the gas. If the membrane is highly selective for a particular type of gas, it is advantageous to measure the total pressure of the gas.

[0004] In temperature-dependent gas detection using gas-selective membranes, the measurement signal of the gas is superimposed by an offset signal which depends, for example, on the temperature or the age of the gas sensor. In order to determine the offset signal, the gas upstream of the sensor can be removed by evacuating it using a pump so that the offset signal is measured in a vacuum environment without gas. Summary of the invention

[0005] One of the objects of the present invention is to provide a method for detecting gas using a gas selective membrane, wherein the influence of an offset signal on a measurement signal is reduced.

[0006] The gas detection according to the invention is carried out using a gas selective membrane which has a temperature-dependent permeability to the gas to be detected. The temperature device is configured to change the temperature of the membrane. Preferably, the temperature device is a heating device. However, if the permeability of the membrane is at a maximum below the ambient temperature of the membrane, a cooling device is also conceivable. The invention also provides a detector which is configured to detect a measurement signal which is dependent on the amount of gas passing through the membrane. The detector can be a gas measuring device, such as a mass spectrometer, or a pressure measuring device, such as a total pressure measuring device.

[0007] According to the method of the invention, the temperature of the membrane is changed using a temperature device so that the temperature of the membrane is adjusted successively to at least two values ​​different from zero in a chronological order. At least one first measurement value is obtained from the measurement signal of the detector at a moment when the membrane temperature adopts a first temperature value. Thereafter, at least a second measurement value of the measurement signal is obtained at a second moment different from the first moment, the membrane temperature at the second moment adopting a second temperature value different from the first temperature value. A difference is calculated from the two obtained measurement values. Based on the calculated difference signal, it is evaluated whether the gas to be measured is present and has been measured.

[0008] The permeability of the gas selective membrane to the gas to be measured at one of the two membrane temperatures should be higher than the permeability at the other membrane temperature. For example, the permeability at a first temperature value of the membrane temperature may be higher than that at a second temperature value. Preferably, the permeability is maximum at the first temperature value.

[0009] If the membrane has a lower permeability at the second membrane temperature than at the first membrane temperature, the offset ratio in the second measured value is smaller than in the first measured value. On the other hand, in the first measured value, the proportion of the measurement signal caused by the measured gas is higher due to the increased permeability. By calculating the difference, the offset ratio is reduced.

[0010] Preferably, the film temperature is changed regularly so that the film temperature alternately adopts two temperature values ​​in a periodically repeated interval, and the measurement value at each temperature is obtained in at least two different intervals. Therefore, when the film temperature adopts a first temperature value, a first measurement value is obtained in each interval of the continuous interval. Correspondingly, when the film temperature adopts a second temperature value, a second measurement value is obtained in each interval of the continuous interval.

[0011] The variation of the membrane temperature can be performed by controlling it between two temperature values ​​different from zero. The temperature control can be performed by measuring the membrane temperature using a temperature measuring device and heating the membrane using a heating device to control the membrane temperature from a lower temperature value to a higher temperature value.

[0012] When calculating the difference between two measured values ​​of consecutive intervals, the average value of the measured values ​​in each interval of the consecutive intervals can be calculated and used. For example, the average value of the first measured values ​​of at least two different intervals and the difference between the second measured values ​​of the two first measured values ​​can be calculated. As an alternative, the average value of the second measured values ​​of at least two different intervals and the difference between the first measured values ​​of the two second measured values ​​can also be calculated.

[0013] In order to change the temperature of the membrane, a parameter influencing the membrane temperature can be adjusted and changed on the temperature device. The change of this parameter and the resulting change of the membrane temperature from one temperature value to the next temperature value should occur after a period of at least 2 seconds, preferably within a period in the range of about 5 to 15 seconds.

[0014] The difference between the different temperature values ​​of the film temperature which are set and changed by the temperature device should be between about 2 K and 10 K, preferably between about 3 K and 6 K, which applies in particular to the difference between the first temperature value and the second temperature value. Preferably, the temperature device is designed as a heating device, at least in which the first temperature value of the film temperature is higher than the temperature of the environment in which the film is located. Before the subsequent new temperature value of the film temperature is set, the heating device can be switched off for a few seconds, the first temperature setting value and the second temperature setting value of the film temperature each being higher than the ambient temperature. Advantageously, the measured value of the measurement signal in each relevant interval is acquired only a few seconds later, preferably approximately 2 to 5 seconds later, after the temperature device has set a new temperature value or has changed a parameter which influences the film temperature.

[0015] In the method of the invention, preferably, the pressure difference between the gas pressure upstream of the membrane and the gas pressure downstream of the membrane is generated without using a pump.The method of the invention is particularly useful for detecting gas in a building room. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] An embodiment of the present invention will be described in detail below with reference to the drawings.

[0017] Figure 1 is a schematic diagram of a gas detector;

[0018] Figure 2 is a schematic diagram of the measured signal; and

[0019] Figure 3 yes Figure 1 Schematic diagram of the medium temperature device and control device. DETAILED DESCRIPTION

[0020] Figure 1 A gas detection device 10 is schematically shown, which comprises a gas selective membrane 12 having a permeability that varies with temperature. The membrane 12 is heated by a temperature device 14 in the form of a heating device having a heating element arranged on the membrane 12. The heating device is controlled by a control device 16. The control device 16 is part of the temperature device 14. Parameters that influence the output of the heating device and thus the heating effect of the membrane 12 are input via the control device 16. Thus, the parameters that influence the heating device 14 and change the temperature of the membrane 12 can be changed by the control device 16. The heating device 14 is mounted on the membrane 12, and the gas can be caused to flow along the membrane 12 by means of the heating device 14. Figure 1The direction of the arrow 18 passes through the membrane 12. The detector 20, in the form of a pressure measuring device, is arranged relative to the Figure 1 Downstream of the flow direction of the arrow 18 in the figure, i.e. behind the membrane 12. The gas that permeates the membrane 12 reaches the detector and increases the pressure measured there. As an option, the detector 20 can also be a gas detector, for example, in the form of a mass spectrometer.

[0021] from Figure 3 As can be seen in FIG. 1 , the temperature device 14 includes a temperature sensor 14a for measuring the film temperature and a film heating device 14b for heating the film 12. The control device 16 includes a temperature preset device 16a for inputting the film temperature adjusted by the film heating device 14b and a control logic 16b for controlling the heating output of the film heating device 14b according to the film temperature measured by the temperature sensor 14a and the film temperature preset by the temperature preset device 16a.

[0022] The detector 20 is connected to an evaluation device 22 which receives and evaluates the measurement signal of the detector 20. With the evaluation device 22, measured values ​​of the measurement signal generated by the detector 20 are acquired and differences between the measured values ​​are calculated.

[0023] exist Figure 2 In FIG. 1 , the measurement signal S of the detector 20 is plotted as a function of time t (in seconds). At time t=0, the heating device 14 is activated by the control device 16, and a first parameter P1 is preset, which causes the heating device 14 to heat the membrane 12 to a first membrane temperature T1 that is higher than the room temperature in the environment of the gas detection device 10. For example, the temperature T1 may be 80° C. The temperature T1 is the temperature at which the membrane 12 has a relatively high permeability to the gas to be detected (helium in this embodiment).

[0024] Therefore, the measurement signal S increases, for example, by Figure 2 The measured current increases to a maximum value at a time t of about 10 seconds, which is the first measured value H n Afterwards, the parameter of the heating device 14 is changed from P1 to P2 by the control device 16, which causes the membrane temperature to drop from the value T1 to the value T2. The second membrane temperature T2 is lower than the first membrane temperature T1, but still higher than the room temperature in the environment of the gas detection device 10. In particular, the permeability of the membrane 12 to the gas to be measured at the second membrane temperature T2 is lower than its permeability at T1.

[0025] Therefore, the measurement signal S drops from the local maximum value at temperature T1 to the local minimum value at temperature T2 and is measured as the second measurement value L at a time t of about 20 seconds. nSubsequently, the control device 16 changes the parameter from P2 back to P1, thereby starting another period of regular repetition of temperature changes. The first interval thus generated is Figure 2 The first measurement value of the second interval n+1 is determined as H at about 30 seconds, which extends from 0 to t at 20 seconds. At t = 20 seconds, the next periodic temperature switching interval n+1 begins. The parameter of the control device 16 is changed from P2 to P1, resulting in the film temperature rising to the first film temperature T1. At about 30 seconds, the first measurement value of the second interval n+1 is measured as H n+1 , and then the parameter is changed to P2 again, so that the film temperature is reduced to T2. The resulting measurement signal is measured as the second measurement value L of the second interval n+1 n+1 Then, the second interval ends at 40 seconds and the third interval n+2 begins.

[0026] The linear drift of the offset of the measured signal can be eliminated by:

[0027] The first measured value H n , H n+1 , H n+2 ... and the second measured value L of each interval n, n+1, n+2, ... n , L n+1 ...is acquired by the evaluation device 22. The evaluation device 22 calculates the difference between the first measurement value and the second measurement value, wherein, in the present embodiment, the first measurement value H of the subsequent interval n, n+1 is calculated. n , H n+1 Calculate the average value (H n +H n+1 ) / 2 and subtract from it the two first measured values ​​H n , H n+1 The second measurement value L obtained between n Therefore, the differential signal ΔS in interval n is n , n is a natural number greater than 0, the result is:

[0028] ΔS n =(H n +H n+1 ) / 2-L n .

[0029] As an alternative, it is also conceivable to use the second measured value L of two consecutive intervals n, n+1 n , L n+1 Calculate the mean, that is, (L n +L n+1 ) / 2, where the first measured value is used to calculate the two second measured values ​​L n , L n+1 The first measurement value H obtained between n+1The signal difference obtained here is: ΔS n =H n+1 –(L n +L n+1 ) / 2.

[0030] In general, it is also conceivable that an average value of the first measured value and the second measured value can be calculated, wherein it is also possible that the calculation of the average value takes place in two or more intervals respectively.

[0031] The second measured value L n , L n+1 is obtained in each interval, during which the temperature of the membrane has been set to the second temperature T2, so that the permeability of the membrane 12 is minimum. Therefore, the second measurement value L n , L n+1 respectively roughly correspond to a deviation signal which is not caused by gas passing through the membrane 12 .

[0032] Because the influence of the offset signal on the differential signal has been reduced, the evaluation unit 22 determines whether the measurement signal S comes from the gas to be measured and the amount of the gas based on the differential signal ΔSn.

[0033] The control of the film temperature by the control device 16 and the heating device 14 is achieved by first measuring the film temperature using a measuring device not shown in the figure. According to the temperature measurement result, the control device 16 uses the heating device 14 to heat the film 12 until a higher, desired temperature value is reached.

[0034] The invention is therefore based on the principle of periodic variation of the membrane temperature in order to distinguish the offset signal from the useful signal generated by the detection gas. Thus, the amount of gas passing through the membrane is periodically varied, with a pre-set partial pressure difference of the gas pressure before and after the membrane. The difference in the measured signal between the two temperature levels T1, T2 is proportional to the partial pressure difference and has nothing to do with the offset of the sensor. When the signal difference is measured and the system is calibrated accordingly, the desired useful signal is obtained without being influenced by the interfering offset signal of the sensor.

[0035] In this embodiment, the difference between the two temperature levels T1 and T2 is only 5K. n +H n+1 The signal deviation ΔS between two temperature levels can be obtained by taking the average value of the two low temperature levels and the intermediate signal Ln at the lower temperature level. In principle, the signal can also be obtained by taking the average value of the two low temperature levels and the high temperature level in between. In this way, the linear drift of the background is also compensated. This drift is Figure 2 It can be seen that the signal amplitude has a negative gradient decreasing from left to right.

[0036] The method of the invention is preferably used for ambient air monitoring. Here, the sensor does not necessarily have to react quickly. Likewise, active air delivery is ultimately not required.

[0037] The purpose of the variation of the membrane temperatures T1, T2 is to change the permeability of the membrane 12 and thus the sensor sensitivity of the gas detection device 10. The change in sensitivity is accompanied by a change in the measurement signal S, in this case a change in the current, which is proportional to the partial pressure of the gas to be measured (helium). The variation of the membrane temperature is achieved by varying the parameters P1, P2. Here, the parameters are set to values ​​as low as possible in order to produce a continuous sensor current (measurement signal S) that is as small as possible at atmospheric pressure, so as not to unnecessarily shorten the service life of the sensor. Figure 2 In this embodiment, the change of parameters P1 and P2 occurs every 10 seconds, switching back and forth between parameters P1 and P2. After changing the parameters, it is necessary to wait for about 8 seconds before measuring the respective measured values ​​Hn and Ln next time, so as to wait for the time required for controlling heating.

Claims

1. A method for detecting gas using a gas selective membrane (12), a temperature device (14) and a detector (20), wherein the temperature device (14) is configured to change the membrane temperature of the gas selective membrane (12), and the detector (20) is configured to detect a measurement signal based on the amount of gas passing through the gas selective membrane (12). It is characterized in that The following steps are involved: - using the temperature device (14) to change the membrane temperature of the gas selective membrane (12); - at a moment (t) when the film temperature adopts a first temperature value (T1), obtaining at least one first measurement value (H) using the detector (20) n , H n+1 , H n+2 ); - at a moment (t) when the film temperature adopts a second temperature value (T2) different from the first temperature value (T1), acquiring at least one second measurement value (L) using the detector (20) n , L n+1 ); - calculating the at least one first measurement value (H n , H n+1 , H n+2 ) and the at least one second measurement value (L n , L n+1 ) and - using the difference to assess the presence of a gas to be measured, wherein the gas selective membrane has a temperature-dependent permeability to the gas to be measured, and wherein the permeability of the gas selective membrane to the gas to be measured at the first temperature value is different from the permeability at the second temperature value.

2. The method according to claim 1, characterized in that The film temperature is periodically changed so that the film temperature alternately adopts the first temperature value (T1) and the second temperature value (T2) in a periodically repeated interval (n, n+1), wherein the measured values ​​(H) at each temperature are obtained in at least two different intervals (n, n+1). n , L n , H n+1 , L n+1 ).

3. The method according to claim 2, characterized in that Calculate the first measurement value (H) of at least two different intervals (n, n+1) n , H n+1 ) and the average of the two first measurements (H n , H n+1 ) between the second measurement value (L n ) 4. The method according to claim 2, characterized in that: Calculate the second measurement value (L) of at least two different intervals (n, n+1) n , L n+1 ) and the average of the two second measurements (L n , L n+1 ) between the first measurement value (H n+1 ) 5. The method according to claim 1, characterized in that: Changing the temperature is achieved by measuring and controlling the temperature, wherein the temperature is controlled between the first temperature value (T1) greater than zero and the second temperature value (T2).

6. The method according to claim 2, characterized in that The film temperature alternately adopts the first temperature value (T1) and the second temperature value (T2) for a period of at least 2 seconds.

7. The method according to claim 1, characterized in that The difference between the first temperature value (T1) and the second temperature value (T2) of the film temperature is between 2-10K.

8. The method according to claim 2, characterized in that: After setting a new temperature value for the temperature device (14), the first measurement value (T1) and the second measurement value (T2) of the measurement signal (S) in each interval are acquired only 2-5 seconds later.

9. The method according to claim 8, characterized in that The temperature device (14) is a heating device, wherein the first temperature value (T1) of the membrane temperature is higher than the ambient temperature of the gas selective membrane (12).

10. The method according to claim 9, characterized in that The heating device (14) is deactivated before setting a new temperature value for the membrane temperature, wherein both the first temperature value (T1) and the second temperature value (T2) are higher than the ambient temperature.

11. The method according to claim 1, characterized in that: A pressure difference is generated between the gas pressure upstream of the gas selective membrane (12) and the gas pressure downstream of the gas selective membrane (12) without using a pump.

12. The method according to any one of claims 1 to 11, characterized in that: The method is used for detecting gases indoors in a building.

13. The method according to any one of claims 1 to 11, characterized in that: The detector (20) is a gas measuring device or a pressure measuring device.

14. The method according to claim 2, characterized in that The film temperature alternately adopts the first temperature value (T1) and the second temperature value (T2) in a period of 5-15 seconds.

15. The method according to claim 1, characterized in that The difference between the first temperature value (T1) and the second temperature value (T2) of the film temperature is between 3-6K.

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