METHOD FOR TEMPERATURE-DEPENDENT GAS DETECTION USING A GAS-SELECTIVE MEMBRANE

Description

[0001] The invention relates to a method for gas detection using a gas-selective membrane whose permeability for a specific gas is temperature-dependent.

[0002] For gas detection with such gas-selective membranes, it is known to adjust the membrane temperature to a temperature value at which the membrane's gas permeability for the gas type to be detected is maximum. For this purpose, the membrane is typically heated when the gas permeability is maximum at a temperature greater than room temperature.

[0003] The amount of gas permeating through the membrane depends on the membrane temperature and the partial pressure difference between the gas upstream (i.e., in front of the membrane) and the gas downstream (i.e., behind the membrane). The gas to be detected 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. A total pressure measurement of the gas is advantageous if the membrane is highly selective for a specific gas type.

[0004] In temperature-dependent gas detection using a gas-selective membrane, the gas measurement signal is superimposed by an offset signal that depends, for example, on the temperature or the age of the gas sensor. To determine the offset signal, the gas upstream of the sensor can be removed by evacuation using pumps to measure the offset signal in a vacuum without gas. DE102016200270 discloses the implementation of offset-reduced gas detection using two membranes, one of which is exposed to the measurement gas and the other is not.

[0005] From Akihiko Tanioka: "Temperature and Pressure Dependence of Gas Permeability through Inhomogeneous Polymer Membranes", Bulletin of the Institute for Chemical Research, 1992, pp. 178-187, membranes are known that exhibit different gas permeability for certain gases at different temperatures.

[0006] The invention is based on the object of creating a method for gas detection with a gas-selective membrane in which the influence of the offset signal on the measurement signal is reduced.

[0007] The method according to the invention is defined by the features of patent claim 1.

[0008] The gas detection according to the invention takes place with the aid of a gas-selective membrane with temperature-dependent permeability for the gas to be detected. A temperature device is designed to change the temperature of the membrane. The temperature device is preferably a heater. However, a cooling device is also conceivable if the permeability of the membrane is maximum at a temperature that is lower than the ambient temperature of the membrane. A detector is provided and designed to detect a measurement signal depending 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.

[0009] According to the method according to the invention, the temperature of the membrane is changed using the temperature device in order to set the membrane temperature successively in chronological order to at least two different values ​​that are not equal to zero. At least one first measured value is recorded from the measurement signal of the detector at a first point in time at which the membrane temperature assumes a first temperature value. Subsequently, at a second point in time different from the first, at least one second measured value of the measurement signal is recorded, wherein the membrane temperature at the second point in time assumes a second temperature value different from the first temperature value. The difference is calculated from the two recorded measured values. The assessment of whether a gas to be detected is present and has been measured is carried out on the basis of the difference signal thus formed.

[0010] The permeability of the gas-selective membrane for the gas to be detected should be greater at one of the two membrane temperatures than at the other. For example, the permeability may be greater at the first membrane temperature than at the second. Preferably, the permeability is maximum at the first temperature.

[0011] If the membrane permeability is lower at the second membrane temperature than at the first membrane temperature, the offset component in the second measured value is smaller than in the first measured value. In the first measured value, however, the component of the measurement signal generated by the increased permeability of the gas to be detected is larger. The offset component is reduced by calculating the difference.

[0012] Preferably, the membrane temperature is changed periodically such that the membrane temperature alternates between the two temperature values ​​at periodically recurring intervals, with the measured values ​​for the respective temperatures being recorded at at least two different intervals. The first measured value is thus recorded at subsequent intervals whenever the membrane temperature reaches the first temperature value. Accordingly, the second measured value is recorded at successive intervals whenever the membrane temperature reaches the second temperature value.

[0013] Changing the membrane temperature can be achieved by controlling it between two non-zero temperature values. Temperature control can be achieved by measuring the membrane temperature using a temperature measuring device and heating the membrane with a heating device to adjust the membrane temperature from a lower temperature value to a higher temperature value.

[0014] When calculating the difference between the two measured values ​​of subsequent intervals, averages of the measured values ​​over subsequent intervals can be calculated and used. For example, the difference can be calculated from the average of the first measured values ​​from at least two different intervals and the second measured value lying between these two first measured values. Alternatively, the difference can also be calculated from the average of the second measured values ​​from at least two different intervals and the first measured value lying between the two second measured values.

[0015] To change the membrane temperature, a parameter that influences the membrane temperature can be set and changed on the temperature device. Changing this parameter, resulting in a change in the membrane temperature from one temperature value to the next, should only occur after a period of at least 2 seconds and preferably after a period of approximately 5 to 15 seconds.

[0016] The difference between the various membrane temperature values, which are set and changed using the temperature device, should be between approximately 2 K and 10 K, and preferably between approximately 3 K and 6 K. This applies in particular to the difference between the first temperature value and the second temperature value. The temperature device is preferably designed as a heater, with at least the first membrane temperature value being greater than the temperature of the membrane's surroundings. Before a subsequent, new membrane temperature value is set, the heater can be switched off for a few seconds, with the set first temperature value and the set second membrane temperature value each being above the membrane's ambient temperature.The measured values ​​of the measuring signal are advantageously recorded in each affected interval only after a few seconds, preferably after about 2 to 5 seconds, after a new temperature value has been set on the temperature device or the parameter influencing the membrane temperature has been changed.

[0017] In the method according to the invention, preferably no pump is used to generate a pressure difference between the pressure of the gas upstream of the membrane and the pressure of the gas downstream of the membrane. The method according to the invention can be used in particular for detecting a gas in a room of a building.

[0018] An embodiment of the invention is explained in more detail below with reference to the figures. They show: Figure 1a schematic representation of the gas detector, Figure 2a schematic representation of the measurement signal and Figure 3a schematic representation of the temperature and control devices according to Fig. 1 .

[0019] In Fig. 1 The gas detection device 10 is shown schematically with a gas-selective membrane 12 with temperature-dependent permeability. The membrane 12 is heated by a temperature device 14 in the form of a heater with heating elements arranged on the membrane 12. The heater is controlled via a control device 16. The control device 16 is part of the temperature device 14. Parameters are entered via the control device 16 that influence the power of the heater and thus the heating effect on the membrane 12. As a result, a parameter can be changed via the control device 16 that influences the heater 14 and changes the temperature of the membrane 12. The heater 14 is attached to the membrane 12 in such a way that gas flows from the direction of the arrow 18 in Fig. 1 through the heater 14 and through the membrane 12. With respect to the flow direction according to the arrow 18 in Fig. 1 A detector 20 in the form of a pressure measuring device is arranged downstream, ie, behind the membrane 12. Gas permeating through the membrane 12 enters the detector and increases the measured pressure there. Alternatively, the detector 20 can also be a gas detector, e.g., in the form of a mass spectrometer.

[0020] In Fig. 3 It can be seen that the temperature device 14 has a temperature sensor 14a for measuring the membrane temperature and a membrane heater 14b for heating the membrane 12 and that the control device 16 has a temperature setting device 16a for inputting the membrane temperature to be set via the membrane heater 14b and a control logic 16b which controls the heating power of the membrane heater 14b depending on the membrane temperature measured with the temperature sensor 14a and the membrane temperature specified via the temperature setting device 16a.

[0021] The detector 20 is connected to an evaluation unit 22, which receives and evaluates the measurement signal from the detector 20. With the aid of the evaluation device 22, the measured values ​​of the measurement signal generated by the detector 20 are recorded and the inventive subtraction of the measured values ​​is carried out.

[0022] In Fig. 2 the measurement signal S of the detector 20 is plotted against time t in seconds. At time t = 0, the heater 14 is activated via the control device 16 and a first parameter P 1 is specified, which causes the heater 14 to heat the membrane 12 to a first membrane temperature T 1 that is greater than the room temperature in the environment of the gas detection device 10. For example, the temperature T 1 can be 80 °C. The temperature T 1 is the temperature at which the membrane 12 has a comparatively high permeability for the gas to be detected, in the present embodiment, helium.

[0023] As a result, the measuring signal S increases, for example in the form of an electrical measuring current, as in Fig. 2 shown, to a maximum value which is measured at a time t of approximately 10 seconds as the first measured value H n. The parameter of the heating device 14 is then changed from P 1 to P 2 via the control device 16, which results in the membrane temperature falling from the value T 1 to the value T 2. The second membrane temperature T 2 is lower than the first membrane temperature T 1 and is nevertheless above the room temperature in the environment of the gas detection device 10. In particular, the second membrane temperature T 2 is a temperature at which the permeability of the membrane 12 for the gas to be detected is lower than at T 1 .

[0024] Therefore, the measurement signal S drops from its local maximum due to the temperature T 1 to a local minimum due to the temperature T 2 and is measured at a time t of approximately 20 seconds as the second measured value L n. Subsequently, the parameter P 2 is changed back to P 1 via the control device 16, thus starting another cycle of periodic repetition of the temperature change. The resulting first interval is Fig. 2 referred to as n and extends from 0 to a time t of 20 seconds. At time t of 20 seconds, the subsequent interval n+1 of the periodic repetition of the temperature switching begins. Changing the parameter of the control device 16 from P 2 to P 1 results in an increase in the membrane temperature towards the first membrane temperature T 1 . At a time of approximately 30 seconds, the first measured value of the second interval n+1 is then measured as H n+1, before the parameter is changed back to P 2 and, as a result, the membrane temperature is reduced to T 2 . The resulting measurement signal is measured as the second measured value L n+1 of the second interval n+1. The second interval then ends at 40 seconds and the third interval n+2 begins.

[0025] A linear drift of the offset of the measurement signal can be eliminated as follows: The first measured values ​​H n , H n+1 , H n+2 etc. and the second measured values ​​L n , L n+1 etc. of the respective intervals n, n+1, n+2 etc. are recorded by the evaluation device 22. The evaluation device 22 forms a difference between the first measured value and the second measured value, wherein in the present embodiment the mean value is formed from the first measured values ​​H n , H n+1 of subsequent intervals n, n+1 as (H n + H n+1 ) / 2 and the second measured value L n recorded between the two first measured values ​​H n , H n+1 is subtracted from this mean value. This results in the difference signal Δ S n of an interval n, where n is a natural number greater than 0, as: Δ S n = (H n + H n+1 ) / 2 - L n .

[0026] Alternatively, it is also conceivable that the averaging of the second measured values ​​L n , L n+1 of two consecutive intervals n, n+1 can be carried out as (L n + L n+1 ) / 2, whereby the first measured value for the difference calculation is taken as the first measured value H n+1 that was recorded between the two second measured values ​​L n , L n+1. The resulting signal difference Δ S n = H n + 1 − L n + L n + 1 / 2 .

[0027] In principle, it is also conceivable that the mean values ​​of both the first and the second measured value can be calculated, whereby the mean values ​​can also be calculated over two or more intervals.

[0028] The second measured value L n , L n+1 is recorded in each interval at a time when the membrane temperature has been set to the second temperature T 2 , thus minimizing the permeability of the membrane 12. Thus, the second measured value L n , L n+1 approximately corresponds to the offset signal that does not result from gas passing through the membrane 12.

[0029] The evaluation unit 22 makes an assessment as to whether the measurement signal S results from a gas to be detected and in what quantity this gas is present, based on the difference signal Δ S n, because the influence of the offset signal on the difference signal is reduced.

[0030] The membrane temperature is controlled via the control device 16 and the heater 14 by first measuring the membrane temperature using a measuring device not shown in the figures. Depending on the temperature measurement result, the control device 16 initiates heating of the membrane 12 using the heater 14 until the higher, desired temperature value is reached.

[0031] The invention is thus based on the principle of periodically changing the membrane temperature in order to distinguish the offset signal from a useful signal resulting from a detected gas. This periodically changes the amount of gas that passes through the membrane at a given partial pressure difference between the gas pressures in front of and behind the membrane. The signal difference in the measurement signal between two temperature levels T 1 , T 2 is proportional to the partial pressure difference and independent of the sensor offset. If the signal difference is measured and the system calibrated accordingly, the desired useful signal is obtained independently of the interfering sensor offset signal.

[0032] The difference between the two temperature levels T 1 , T 2 is only 5 K in the present embodiment. The signal swing Δ S between the two temperature levels can be obtained from the averaging of two adjacent high temperature signals, H n + H n+1 , and the intermediate signal of the lower temperature level L n . In principle, it is also possible to obtain the signal from the averaging of two low levels and the intermediate high level. In this way, a linear drift of the background is also compensated. The drift is in Fig. 2 recognizable as a negative slope of the dashed lines of the signal amplitudes decreasing from left to right.

[0033] The method according to the invention is preferably used for indoor air monitoring. The sensor does not necessarily have to react quickly. Active air supply is also not absolutely necessary.

[0034] The membrane temperature T 1 , T 2 is varied in order to change the permeability of the membrane 12 and thus the sensor sensitivity of the gas detection device 10. This change in sensitivity is accompanied by a change in the measurement signal S, in this case a current change, which is proportional to the partial pressure of the gas to be detected (helium). The variation in the membrane temperature is achieved by changing the parameter P 1 , P 2 . The parameters are set to the lowest possible values ​​in order to generate the smallest possible continuous sensor current (measurement signal S) at atmospheric pressure in order not to unnecessarily reduce the service life of the sensor. The change in the parameter P 1 , P 2 takes place in the present embodiment according to Fig. 2every 10 seconds, switching back and forth between parameters P 1 and P 2. After changing the parameter, a delay of approximately 8 seconds is observed before the respective measured value H n , L n is measured, in order to allow the time required for heating control to elapse.

Claims

1. A method for the detection of gas using a gas-selective membrane (12) having a temperature-dependent permeability, a temperature device (14) that is designed to change the temperature of the membrane (12), and a detector (20) that is designed to detect a measurement signal on the basis of the amount of gas passing through the membrane (12), the measurement signal being proportional to the partial pressure of a gas to be detected characterized by the following steps: - changing the temperature of the membrane (12) using the temperature device (14), - acquiring at least one first measuring value (Hn, Hn+1, Hn+2) using the detector (20) at a time (t) at which the membrane temperature adopts a first temperature value (T1), - acquiring at least one second measuring value (Ln, Ln+1) using the detector (20) at a time (t) at which the membrane temperature adopts a second temperature value (T2) different from the first temperature value, wherein the permeability of one of the two temperature values (T1, T2) is greater than of the other temperature value, - calculating the difference between the two measuring values, and - using the difference to assess whether a gas to be detected is present.

2. The method according to claim 1, characterized in that the change of the membrane temperature is performed periodically such that the membrane temperature alternately adopts the two temperature values (T1, T2) at periodically recurring intervals (n, n+1), the measuring values (Hn, Ln, Hn+1, Hn+1, Ln+1) for the respective temperatures being acquired during at least two different intervals (n, n+1).

3. The method according to claim 2, characterized in that the difference is calculated between the means value of the first measuring values (Hn, Hn+1) of at least two different intervals (n, n+1) and the second measuring value (Ln) between the two first measuring values (Hn, Hn+1).

4. The method according to claim 2, characterized in that the difference is calculated between the means value of the second measuring values (Ln, Ln+1) of at least two different intervals (n, n+1), and the first measuring value (Hn+1) between the two second measuring values (Ln, Ln+1).

5. The method according to any one of the preceding claims, characterized in that changing the temperature is affected by measuring and controlling the temperature, the temperature control being performed between two temperature values (T1, T2) greater than zero.

6. The method according to any one of the preceding claims, characterized in that changing the temperature from one temperature value (T1) to the next temperature value (T2) is performed after at least 2 seconds and preferably after approx. at least 5-15 seconds.

7. The method according to any one of the preceding claims, characterized in that the difference between the set temperature values (T1, T2) of the membrane temperature is between approx. 2-10 K and preferably between about 3-6 K.

8. The method according to any one of the preceding claims, characterized in that the measuring values of the measuring signal (S) are acquired in each interval only after at least approx. 2-5 seconds, after a new temperature value has been set for the temperature device (14).

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

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

11. The method according to any one of the preceding claims, characterized in that no pump is used to create a differential pressure between the pressures of the gas upstream of the membrane (12) and the gas downstream of the membrane (12).

12. The method according to any one of the preceding claims, characterized in that the method is used to detect a gas in a room of a building.

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