HK1037027B - Pressure-measuring cell, pressure-measuring device and method for monitoring pressure within a chamber - Google Patents

Description

Technical field

The invention relates to a pressure measuring cell, a pressure measuring device incorporating such a pressure measuring cell and a method for monitoring the pressure in a chamber by means of such a pressure measuring cell or pressure measuring device.

State of the art

There are known pressure measuring cells of the type (e.g. EP-A-0 351 701) which contain only a capacitive measuring element consisting of a support plate and a membrane which bear electrically conductive layers. Although such measuring elements can measure between about 10-6 mbar and 10 bar of gas with high accuracy, the entire range cannot be measured with a single measuring element.

Pressure measuring cells, such as those designed to measure heat conduction, are also known to be used, e.g. according to Pirani. In such pressure measuring cells, at least one heating element, usually a measuring wire, is heated electrically and the pressure is determined from the heating power via the pressure-dependent thermal conductivity of the gas. In this way, the pressure can be measured in a range from about 10-3 mbar to some 100 mbar. Above some 10 mbar, however, the convective heat transfer predominates, so that the measurement of upstream currents is affected and strongly dependent.

It is also known to use ionization measuring cells for pressures below 10-2 mbar and with further reduced accuracy to 10-1 mbar, whose function is based on the measurement of a particle current density.

For large pressure ranges, for example from about 10-6 mbar to about 100 mbar, it is usual to use at least two different, locally separate pressure measuring cells, each of which is also independently equipped with signal processing equipment. For example, two or more pressure measuring cells may be used, each containing a capacitive measuring element suitable for measuring a sub-area of the described type. These and similar solutions are, however, expensive due to the technical effort involved.

However, it is also known to combine different pressure measuring devices in one device. For example, EP-A-0 658 755 is known to be a pressure measuring device in which a heat conduction measuring device according to Pirani and a 5 cold cathode measuring device are connected to a pressure measuring device, the former covering an upper and the latter a lower pressure range.

It is also known that a mechanical measuring element with a bulky structure is connected to a pyrane-type thermal conduction measuring element (US-A-3 064 478).

Description of the invention

The present invention is based on the task of specifying a pressure measuring cell which has a large measuring range - preferably from about 10 to 6 mbar up to a few bar - and which is at the same time simple in construction, compact and inexpensive and not susceptible to contamination.

The purpose is to specify a pressure measuring device which has the same characteristics as the pressure measuring cell of the invention and which can be easily, in any position and quickly installed, and a pressure measuring device which combines these advantages with a very large measuring range with high accuracy and stability.

Finally, a method for monitoring pressure in a chamber which is sufficiently accurate and stable over a wide measuring range is to be specified.

The pressure measuring cell according to the invention combines in a compact, easy to handle and cost-effective form a capacitive measuring element with a heat conduction measuring element. The upper pressure range above about 0,1 mbar is taken over by the gas-independent capacitive measuring element, the lower pressure range from about 10-6 mbar to about 10 mbar by the heat conduction measuring element, with an average in the cross section, for example. This allows pressures between about 1 mbar and some bar gas-independent and with high accuracy (i.e. about 1%) to be measured, while at the same time the measuring range with sufficient measuring accuracy generally reaches about 10-6 mbar.

The proximity of the two measuring elements ensures that they are always exposed to the same conditions. The pressure measuring cell is versatile and can also be designed to be suitable for relative pressure measurements in the first part of the measuring range. The arrangement is small and compact, e.g. with a diameter of 35 mm or less. The thermal conductivity measurement can be optimized in the specified pressure range with a heating element of small size, e.g. a short heating wire.

The pressure measuring devices of the invention have the advantages of the pressure measuring cell of the invention and are handy and easy to install.

The method of the invention allows the pressure in a chamber to be monitored in a long-term stable manner despite the offset problems of capacitive measuring cells.

Brief description of the drawings

The following figures, which are merely an example, give a detailed account of the invention. Fig. 2a,Fig. 3a,Fig. 3a,Fig. 3a,Fig. 3a,Fig. 3a,Fig. 3b,Fig. 3b,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7b,Fig. 7a,Fig. 7a,Fig. 7b,Fig. 7a,Fig. 7a,Fig. 7b,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,Fig. 7a,F

Ways of carrying out the invention

The pressure measuring cell of the invention is arranged in a vessel or chamber and (Fig. 1) is constructed in such a way that, in principle, a capacitor C of a capacity C, the value of which depends on the gas pressure, is placed between a mass connection A0 and a capacity connection AC. Its value is known to be monitored by means of an appropriate evaluation circuit, which derives a first pressure signal from it. The first pressure signal is derived via a linear linear line from the original value, which reflects the value of the capacity C. The offset of the capacity measuring elements, which usually affects long-term stability, can be compensated by a further method described below.

The first resistor R1 and the second resistor R2 are placed between the mains connection A0 and a first resistor connection AR1 and a second resistor connection AR2 respectively, representing the heating elements of a heat conduction measuring element. The resistors R1, R2 are stabilized and their performance measured by adjusting or monitoring the voltage and current at the resistor connections AR1 and AR2 at different temperatures T1 and T2 respectively, corresponding to certain resistance values.

For the power given by heat transfer from a resistor to the wall of a chamber as a function of the gas pressure p in the chamber, the temperature T of the resistor and the wall temperature TW is given by the formula known from, e.g. H. R. Hidber, G. Süss: 'Pirani manometer with linearized response', Rev. Sci. Instrum. 47/8 (1976), 912-914 ${\text{(1)\hspace{1em}\hspace{1em}\hspace{1em}N(p) = α(εT}}^{\text{4}}{\text{-ε}}_{\text{W}}{\text{T}}_{\text{W}}{\text{}}^{\text{4}}{\text{) + (β(T-T}}_{\text{W}}\text{)/}\sqrt{{\text{T}}_{\text{W}}}{\text{) × p + γ(T-T}}_{\text{W}}\text{)}$

The first term covers the heat transfer by radiation and the last term by thermal conduction in the area of the resistance connections, while the middle term describes the heat transfer by the gas caused by the pressure-dependent thermal conduction for a pressure range of less than 10 mbar, which is of particular interest here.

If N ((p) is measured at the two different temperatures T1 and T2 at which the resistors R1, R2 have been stabilized and the difference is obtained, ${\text{(2)\hspace{1em}\hspace{1em}\hspace{1em}N}}_{\text{1}}{\text{(p)-N}}_{\text{2}}{\text{(p) = αε(T}}_{\text{1}}{\text{}}^{\text{4}}{\text{-T}}_{\text{2}}{\text{}}^{\text{4}}{\text{) + (β(T}}_{\text{1}}{\text{-T}}_{\text{2}}\text{)/}\sqrt{{\text{T}}_{\text{W}}}{\text{) × p + γ(T}}_{\text{1}}{\text{-T}}_{\text{2}}\text{)}$ α, ε, β and γ are constants. ${\text{(3)\hspace{1em}\hspace{1em}\hspace{1em}N}}_{\text{1}}{\text{-N}}_{\text{2}}\text{= A + B × p/}\sqrt{{\text{T}}_{\text{W}}}$ The wall temperature is just at the root. $\sqrt{{\text{T}}_{\text{W}}}$ The first is that the thermal energy of the thermal energy is not the same as the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the thermal energy of the ther

If a standard pressure P already containing the so-called thermal transpiration (see K. F. Poulter et al., Vacuum 33 (1983), 311 and W. Jitschin et al., J. Vac. Sci. Technol. A5 (1987), 372) is defined as the pressure P at which the temperature of the atmosphere is at its lowest, then the pressure P is the pressure at which the temperature of the atmosphere is at its lowest. $\text{(4)\hspace{1em}\hspace{1em}\hspace{1em}P = p ×}\sqrt{{\text{(T}}_{\text{WO}}{\text{/T}}_{\text{W}}\text{),}}$ which corresponds to the pressure p, for example at TW0 = 300°K, gives the same value at ${\text{(5) P = (N}}_{\text{1}}{\text{-N}}_{\text{2}}\text{-A)/K}$ The second pressure signal is derived from the evaluation unit from the voltages and currents at the resistance connections AR1, AR2 as shown in figure 5.

The unit of analysis generates an output signal from the first pressure signal from the capacitive measuring element and the second pressure signal from the thermal conduction measuring element, each of which clearly corresponds to a specific measurement result of the pressure measuring cell. In the upper part of the measuring range, the output signal is determined by the first pressure signal from the capacitive measuring element, in the lower part of the same from the second pressure signal from the thermal conduction measuring element. The transition may be bouncing at a transition value, but this may lead to a slight jump value of the measurement and hysteric effects. Preferably, the transition is carried out in a transition value, e.g. between 0,1 and 10 mbar, respectively, particularly in the EP-558 method 75A-655.

A pressure measuring device of the invention in a first embodiment is shown in Fig. 2a.b. At the end of a protective tube 1 with a flange 2 through which the protective tube 1 can be connected to a corresponding chamber connection, a pressure measuring cell 3 in a first embodiment is suspended elastically and gastightly by means of an elastic spring 4 in the form of a ring. The spring 4 is directly welded with a support plate 5 of the pressure measuring cell 3 which also performs the function of a base body and is transferred with the protective tube 1. It equates the different thermal distances of the steel-made protective plate 1 and the support plate 5, which preferably consists of at least 95% AlO3 ceramic.

The inner surface of the support plate 5 facing the inside of the protective tube 1 is fitted with a membrane 6 which is gas-tightly connected and separated from the support plate 5 by a circular glass seal 7 and which contains a reference vacuum 8 which may be welded. The membrane 6 is preferably made of a ceramic containing preferably Al2O3 mm or more. Its thickness is preferably between 10 μm and 1000 mm, in particular between 10 μm and 250 μm. Its diameter is preferably between 2 mm and 15 mm, in particular between 4 mm and 9 mm.

The two opposite surfaces of the support plate 5 and the membrane 6 each carry an electrically conductive layer which passes through the support plate 5 with a connector 9 corresponding to the A0 mass connection or a connector 10 corresponding to the AC capacity connection to which the outer face is connected. The support plate 5 and the membrane 6 connected to it thus form the capacitive measuring element, with the said electrically conductive layers as capacity C. In the preferred ceramic form of the support plate 5 and the membrane 6, the capacitive measuring element formed by the same is also corrosive in atmospheric conditions.

In addition, two heating elements are provided, arranged next to membrane 6, which represent the electrical resistors R1, R2 and thus form the thermal conduction measuring element. They are formed as measuring wires 11, 12 which, at a slightly greater distance than membrane 6, run parallel to the inner surface of the support plate 5 by a central support 13, which is electrically connected to the connector 9, starting at a right angle to the support plates 14; 15 and are preferably made of nickel, platinum or tungsten or an alloy containing at least one of these metals, so that they are resistant to corrosion by aggressive gases. The support plates are connected to the conductors 5 by means of a further layer, which is connected to the corresponding resistors of the measuring plate. AR1 and AR2 (the value of the resistor) are placed between them, so that they are not exposed to heat and corrosion by means of a layer of heat and heat.

In the upper part of the measuring range, from about 0,1 mbar to about 10 bar, the measuring result is derived, as already explained, wholly or partially from the value of the capacity C, which is formed by the electrically conductive layers on the carrier plate 5 and membrane 6 separated by the reference vacuum 8. This value depends on the bending of the membrane 6, which is in turn a function of pressure. Below the lower limit of the range, the influence of the pressure on the bending of the membrane 6 is so small that the A values of the same cannot be detected with sufficient precision. Therefore, in the lower part of the measuring range, from about 10-6 mbar to about 10 mbar, the M signal or tephra printer is derived from the measuring power of 12 mbar, in particular from the cavity of 50 mbar.

The pressure measurement can be performed with only one heating wire, which is alternately brought to the different temperatures T1, T2 or only works at one temperature. In the latter case, however, stronger influences of the wall temperature must be taken into account. In some cases, depending on the application area, other materials can also be used than the specified materials or another resistance element, e.g. a strip, a thin film film, a microchip or a microchip-based Pirani element on a wafer.

The pressure measuring device's protective tube is then closed by the base plate. Finally, the pressure measuring cell may also be designed to be suitable for relative measurements in the upper part of the measuring range by drilling through the base plate into the reference volume between it and the membrane, which in this case is not a vacuum.

In the second embodiment, the pressure measuring cell, which is essentially the same as in the first embodiment and corresponds to the diagram shown in Figure 1, has a separate base, namely a base plate 17, preferably made of ceramic or another electrically insulating material.

The connection between base plate 17 and support plate 5 is made by a pin 18 carried and anchored in the centre by base plate 17 and the back of the support plate 5 is attached to it in such a way that it is attached to it along a diameter. At the same time it is connected by a sleeve 19 to the electrically conductive layer on the same. Its protruding section through base plate 17 forms the connection 9 corresponding to the mass connection A0. The electrically conductive layer on membrane 6 is connected to the connection 10 corresponding to the capacity connection AC by means of a connection pin carried through base plate 17 corresponding to the capacity connection.

The measuring wires 11, 12 are arranged on the side of the support plate 5 facing the membrane 6 so that it protects the membrane 6 from thermal effects of the membrane 5. They extend from one end of the tube 19 through which they are electrically connected to the pin 18, running horizontally against the base plate 17, where they are connected to the supports 14, 15.

The basic principle of all versions is that the measuring wires are mounted close to the membrane so that they are not subjected to different pressures even when pressure gradients occur, but on the other hand do not have a thermal effect on the membrane in a disturbing way. For this reason, the distances between the membrane and the measuring wires should not be greater than 5 mm, but also not less than 2 μm. A shielding, which is provided by the first version by means of the 16 protection and the second version by means of the 5 supporting platform, is also an advantage.

The pressure measuring cells of the invention are also very suitable, owing to their compactness, as part of special purpose pressure measuring devices which, for example, contain an additional measuring element to extend the measuring range downwards or to improve accuracy or stability at low pressures.

In the first embodiment of a pressure measuring device (Fig. 4), a magnetron 23, i.e. a cold cathode measuring element of a known structure, is placed in tube 1 with a pin 24 and a permanent magnet coaxially surrounding the pressure measuring tube 1 25. At the open end of the pressure measuring device, an opening is opened for the pin 24 and closed by the base plate 17 of the pressure measuring cell 3, which corresponds to the second embodiment described above.

Another pressure measuring device (Fig. 5) has a triode 26, i.e. a cathode-ray measuring element, in addition to the pressure measuring cell 3, which again corresponds to the second embodiment described. The pressure measuring cell 3 and the triode 26 are mounted on a common support plate 27 made of ceramic or another electrically insulating material, which closes the protective tube 1 at one end and through which the connections of both are made. The triode 26 is the open end of the pressure measuring tube 1 surrounded by flange 2, i.e. the pressure measuring cell 3 is pre-set. The triode 26 makes the pressure measuring device accurate and very stable in the range from 10-1 mbar to 10-5 mbar or 10-6 mbar. The triode 26 is also a robust device.

Another pressure measuring device (Fig. 6) has a Bayard-Alpert element 28 as its cathode-ray measuring element, which is characterized by a relatively high resistance to pollution.

In any case, the additional measuring element provides an additional pressure signal which is also sent to the evaluation unit and can be used to determine the output signal for a specific part of the measuring range.

The advantageous properties of pressure measuring cells according to the invention or pressure measuring devices incorporating them can be further improved by introducing, for example, 30o pressurized pressors to be processed from outside, i.e. at atmospheric pressure p0 (Fig. 8a), whereby the pressure of the capacitor element, which affects the long-term stability, is often compensated for. When monitoring the pressure in chambers where the same is cyclically changed, this can be done regularly, e.g. in a nozzle 29 of a connected system (Fig. 7), into which objects, e.g. 34 pressurized pressors to be processed at atmospheric pressure p0 (Fig. 8a), are introduced further, after which the pressure of the nozzle 29 p is reduced to 30 pts on a transport pressure vessel, a 30 pts or more of the pressure is transferred from 30 pts to 30 pts.

It is very important that the lock 29 is not opened until the pressure in the lock has approached atmospheric pressure p0 to a large extent - usually between 2% and 5% depending on the sensitivity of the objects to be worked - otherwise disruptive currents will be generated and the valves will be heavily loaded, which can lead to abrasion and the corresponding contamination.

This problem is solved by compensating for the offset as soon as the measured value falls below a threshold value ps, which is usually not higher than 100 mbar, preferably not higher than 50 mbar or even 30 mbar. In this case, the output signal determined by the evaluation circuit from the first pressure signal is compared with one determined by the same from the second pressure signal. If an offset deviation occurs, the line used to transmit the first pressure signal is shifted so that the output signal transmitted by it matches the output signal transmitted by the second pressure signal. This is repeated in a continuous or continuous manner as long as the pressure p increases.

The offset of the capacitive measuring element can therefore be determined to approximately 1 mbar. Due to the otherwise very high accuracy of the capacitive measuring element, its measuring accuracy in its measuring range is also approximately 1 mbar, which results in a 0.1% deviation at atmospheric pressure. This accuracy is usually far enough. If the actual pressure is reached in the heat conduction measurement, the result is even better.

List of reference marks

1Protective tube2flanged3pressure measuring cell4spring5supporting plate6membrane7glass sealing8reference vacuum9, 10connectors11, 12measuring wires13 - 15protective ring17base plate18pin19lashe20, 21connectors23magnetron24pin25permanent magnet26triode27supporting plate28bayard-alpert element29slot30wafer31valve32intermediate chamber33valve34processing chamber

Claims (25)

1. Pressure sensor having a capacitive measuring element which comprises a support plate (5) and a ceramic membrane (6) preceding the same at a distance and connected gas-tight to it, both being provided with electrically conducting layers, characterized in that said pressure sensor has a thermal conductivity measuring element with at least one heating element which is connected to the support plate (5) directly or via a base member supporting both measuring elements, where the membrane (6) is shielded from radiation emitted by the at least one heating element.
2. Pressure sensor according to Claim 1, characterized in that the distance between the membrane (6) and the at least one heating element is not more than 5 mm.
3. Pressure sensor according to Claim 1 or 2, characterized in that the membrane (6) consists of a ceramic which contains in particular Al₂O₃, and its thickness is between 10 µm and 1000 µm, in particular between 10 µm and 250 µm.
4. Pressure sensor according to any of Claims 1 to 3, characterized in that the diameter of the membrane (6) is between 2 mm and 15 mm, preferably between 4 mm and 9 mm.
5. Pressure sensor according to any of Claims 1 to 4, characterized in that a gettered reference vacuum (8) is enclosed between the support plate (5) and the membrane (6).
6. Pressure sensor according to any of Claims 1 to 5, characterized in that each heating element is in the form of a measuring wire (11, 12), a tape, a thin-film arrangement or a microchip.
7. Pressure sensor according to any of Claims 1 to 6, characterized in that it has at least two heating elements heatable independently of one another.
8. Pressure sensor according to any of Claims 1 to 7, characterized in that the at least one heating element contains at least 1% of nickel, platinum or tungsten.
9. Pressure sensor according to any of Claims 1 to 8, characterized in that the at least one heating element is a distance away from and parallel to the support plate (5).
10. Pressure sensor according to any of Claims 1 to 9, characterized in that the at least one heating element is arranged at a larger distance from the support plate (5) than the latter beside the membrane (6).
11. Pressure sensor according to any of Claims 1 to 10, characterized in that the shielding of the membrane (6) is ensured by a protective ring (16) which surrounds said membrane and is arranged on the support plate (5) and outside which the at least one heating element is arranged.
12. Pressure sensor according to any of Claims 1 to 10, characterized in that the at least one heating element is arranged on that side of the support plate (5) which faces away from the membrane (6).
13. Pressure sensor according to any of Claims 1 to 12, characterized in that the support plate (5) is fastened on a pin (18) which is passed through a base member preferably in the form of a baseplate (17), approximately perpendicularly to said base member.
14. Pressure measuring apparatus having a pressure sensor (3) according to any of Claims 1 to 13, characterized in that it comprises a protective tube (1) which surrounds the pressure sensor (3).
15. Pressure measuring apparatus according to Claim 14, characterized in that the protective tube (1) is closed at one end by the support plate (5).
16. Pressure measuring apparatus according to Claim 15, characterized in that the support plate (5) is suspended in an elastic and sealing manner in the protective tube (1).
17. Pressure measuring apparatus according to any of Claims 14 to 16, characterized in that the pressure sensor (3) is arranged together with at least one further measuring element for pressure measurement in the protective tube (1).
18. Pressure measuring apparatus having a pressure sensor (3) according to any of Claims 1 to 13, characterized in that it is arranged together with at least one further pressure measuring element on a common holder.
19. Pressure measuring apparatus according to Claim 17 or 18, characterized in that the at least one further measuring element is a cold-cathode element.
20. Pressure measuring apparatus according to Claim 17 or 18, characterized in that the at least one further pressure measuring element is a hot-cathode element, in particular a triode (26) or a Bayard-Alpert element (28).
21. Method for monitoring the pressure in a chamber by means of a pressure sensor according to any of Claims 1 to 13 or a pressure measuring apparatus according to any of Claims 14 to 20, characterized in that in each case an output signal representing the measured result is generated, which is determined, at least when the measured result is above a transition value or transition range, on the basis of a first pressure signal originating from the capacitive measuring element and, when the pressure falls below a threshold value (p_s), in each case any offset of the first pressure signal is compensated in such a way that the determination of the output signal on the basis of the first pressure signal leads to the same result as its determination on the basis of a second pressure signal which originates from the thermal conductivity measuring element.
22. Method according to Claim 21, characterized in that the compensation of the offset is repeated as long as the pressure is below the threshold value (p_s) and is decreasing, but not in the case of increasing pressure.
23. Method according to Claim 21 or 22, characterized in that the threshold value (p_s) is not greater than 50 mbar, preferably not greater than 30 mbar.
24. Method according to any of Claims 21 to 23, characterized in that the output signal is determined, at least when the measured result is below the transition value or transition range, on the basis of the second pressure signal originating from the thermal conductivity measuring element.
25. Method according to any of Claims 21 to 24, characterized in that the chamber is a lock (29).