DE9416564U1 - Device for determining the composition of gas mixtures - Google Patents

Description

Device for determining the composition of gas mixtures

The invention relates to a device for determining the composition of a gas mixture according to the preamble of claim 1.

Such a device is used, for example, to determine, in particular to qualitatively determine, the composition of air (humidity measurement), generator gas, mixed gas, water gas, town gas, combustion gases, explosion gases, mine gas, etc. or in physiological examinations (breathing, assimilation).

Likewise, a device for determining the composition of gas mixtures can be used, for example, in a gas filling system, in which, for example, air is displaced in a given volume by the appropriate inflow of a different gas of a known type. To control the filling process with the gas filling system, a sensor is required that can detect the composition of the displaced gas and react to it.

Mechanical resonator sensors are generally suitable for this purpose, especially because they react sufficiently quickly to changes in the composition of the gas mixture.

For example, it is known from GM 91 00 101.3 and the publication at the 7th GMA/ITG conference "Sensor technology and application", Bad Nauheim, 16-18 March 1994 by the authors "A. Bartels, W. Halder, H. Müller and G. Lindner" to use flexural oscillators as resonator sensors. The method described in the publications requires an oscillating system in the form of a mechanical flexural oscillator to measure the composition of a gas mixture.

[File:ANM\PE2903B1 .doc] Description, 26.08.94 Gassensorik Peotor GmbH

kens, in which the bending beam is introduced into the gas mixture to be determined and the qualitative composition can be determined via the damping-dependent resonance frequency of the system. The damping of the resonance frequency is a measure of the pressure, viscosity, flow rate and type of gas surrounding the resonator sensor. The cause of this physical damping effect is the inert mass of the gas molecules accelerated by the moving resonator. A change in the physical gas parameters leads to a shift in the resonance frequency due to the damping, i.e. the resonance frequency drops almost linearly with increasing density. To determine this shift or decrease, the excitation frequency and the oscillation frequency of the system must therefore be measured. The density of the gas to be measured depends on the gas pressure and the mass of the gas atoms or gas molecules at a given temperature.

If the composition of gas mixtures is determined at constant pressure and constant temperature, the mechanical resonator sensors can be used to measure changes in the atomic or molecular mass of the gas components in the gas mixture. If the gas components involved are known and differ in their atomic or molecular masses, the qualitative composition of a gas mixture can be determined in this way.

The resonator sensor described in the above-mentioned publications is designed in the form of a bending beam clamped on one side, which is represented by a thin glass plate. This glass plate is covered with a piezoelectric polyvinylidene fluoride film (PVDF film) that is metallically coated for contact, which acts as an actuator and deflection sensor for the bending vibration. Using a suitable control circuit, the resonator is adjusted to the resonance frequency of a

[File:ANM\PE290381 .doc] Description.j

Gas Sensorik Pector GmbH

selected vibration state. In addition to the physical parameters of the gas, it should be noted that a change in the temperature of the resonator itself also influences the resonance frequency.
5

For a metrological application of this known device, it is therefore necessary to minimize or compensate for the influence of parameters that distort the measurement result with regard to the gas composition, such as simultaneous changes in pressure and temperature, in order to avoid subsequent correction of the measurement results using complex correction or compensation circuits or independent measurements for determining a calibration curve.

It is therefore an object of the invention to provide a device for determining the composition of gas mixtures using resonators of the above type, which enable the composition of gas mixtures to be measured independently of pressure and temperature.

This object is achieved with a device according to claim 1.

According to the invention, the device has a measuring chamber, a reference chamber and a compensation device for setting the same temperature and pressure in the measuring chamber and in the reference chamber. The measuring chamber can be filled with the gas mixture to be determined and the reference chamber with a reference gas. A second, preferably identically constructed resonator sensor is installed in the hermetically sealable reference chamber, which interact with the measuring chamber via the compensation device to compensate for pressure and temperature changes and are thermally coupled.

{File:ANM\PE2903B1.doc] Description, 26.08.94

Gas sensors

Pector GmbH

Because the compensation device ensures that the same pressure always prevails in both chambers and that the temperature of the two resonators corresponds to the gas filling, the difference in the resonance frequencies of the two resonators is then only dependent on the gas composition in the measuring chamber, but not on pressure and temperature fluctuations. By obtaining the measured variable from the difference frequency of the resonators, the costly and time-consuming pressure and temperature compensations that usually had to be used to determine the gas composition can be omitted according to the invention. In addition, the entire device only needs to be calibrated when it is first put into operation.

In addition to the elimination of additional compensation devices, a further advantage of the device according to the invention is that commercially available PVDF resonator sensors, which are easy and inexpensive to manufacture, can be used.

Further embodiments of the invention are the subject of the subclaims.

If the gas mixture is composed of only a first and a second component according to claim 2, whereby in particular the second component is composed of several qualitatively known gases according to claim 3, for example, the proportions of the first component of the gas mixture can be determined. The device according to the invention is thus able to qualitatively determine the proportions of an unknown component in relation to a component which is composed of several gases according to claim 3 from the difference frequency of the two resonators. In particular, if the reference gas filled in the reference chamber is equal to the second component of the gas mixture according to claim 4, then advantageously a

[File:ANM\PE2903B1 .doc] Description, 26.08.94 Gassensorik Pector GmbH

A simple relationship, such as linearity, exists between the difference frequency and the concentration of the gas component to be investigated.

According to claim 5, the device according to the invention can also advantageously determine the composition of a flowing gas mixture without having to change the method for determining the components of the gas mixture. Only the measuring chamber is designed as a flow channel, with the second resonator sensor protruding into the flow channel. In addition to determining the gas composition, the device according to the invention also makes it possible to measure gas flow changes without knowing the individual gas components, particularly when the resonator sensor is flowing from the front onto its large surface. In addition, even if the individual gas components are known, an absolute determination of the gas flow can be carried out after prior calibration.

Preferably, the flow channel of the measuring chamber according to claim 6 is designed so that a laminar flow flows around the second resonator sensor in order to obtain a clear measurement result, which would otherwise lead to a falsified result if there were turbulence in the area of the resonator sensor.

According to claim 7, the first and/or second mechanical resonator sensor advantageously has a base plate, on which a material is applied to at least one surface, which causes the base plate to vibrate. If the resonator sensors are held in the chambers via sensor holders, further damping of the resonator sensors can be reduced due to the specially designed sensor holders.

By manufacturing the base plate from glass according to claim 8, a cost-effective material is used, where

[File:ANM\PE2903B1 .doc] Description, 26.08.94 Gassensorik Pector GmbH

•lul.

with a sufficient quality (Q) for determining the gas composition of the resonator sensor. If the base plate is made of crystalline silicon according to claim 9, the quality is increased three times compared to a glass base plate. In order to increase the breaking strength of the resonator sensor, spring steel, aluminum or copper-beryllium bronze can also be used as a material for the base plate, whereby a quality sufficient for determining the difference frequency of the resonator sensors is still obtained. In order to excite the resonator sensors to oscillate, according to claim 10' the base plate is coated with a piezo material/ which in turn is covered by a contact layer. With this piezo material, an easy-to-use electrical excitation mechanism can be achieved.

In particular, it is advantageous if, according to claim 11, the piezo material is a piezoelectric polyvinylidene fluoride film (PVDF film). This film is commercially available and can be glued to the base plate.

The sensitivity is optimized by selecting suitable vibration modes of the resonator sensors. Preferably, according to claim 12, the mechanical resonator sensors are dimensioned so that a 2/0 mode is preferably excited. When this mode is excited, the highest quality of the resonator was achieved with the base plate made of glass. The bending oscillators described as resonator sensors in GM 91 00 101.3 and in the publication at the 7th GMA/ITG conference "Sensor technology and application", Bad Nauheim, March 16-18, 1994 by the authors "A. Bartels, W. Halder, H. Müller and G. Lindner" are expressly included in the disclosure content of the application.

If the device according to the invention is used to determine flowing gas mixtures, it is advantageous that the resonator located in the flow

[Fi!e:ANM\PE29O3B1.docj Description, 26.08.94 Gassensorik Pector GmbH

The sensor has a large surface area that is aligned parallel to the flow direction. This ensures that the influence of different flow velocities on the measurement signal during the measurement process is negligible.

If the measuring chambers and the reference chamber are separated from the equalization device and the equalization device is designed as a flexible, gas-impermeable membrane according to claim 14, pressure equalization can be achieved in a simple manner. If the membrane according to claim 15 has a high heat transfer value, an effective heat exchange takes place between the reference and measuring chambers, whereby the heat equalization between the two gases in the measuring and reference chambers takes place in such a short time that the measurement signal is not significantly influenced.

If the material of the membrane according to claim 16 is inelastic, no further force has to be applied to stretch the membrane during pressure equalization, whereby pressure equalization is guaranteed unadulterated between the reference and measuring chambers and the relationship between the concentration dependence and the shift in the resonance frequency is not unfavorable.

Advantageously, the membrane according to claim 17 is formed from a polyethylene terephthalate film (PETP film), which is available under the trade name "Mylar" from DuPont.

The measuring chamber and the reference chamber can have a common housing, for example in the form of a double chamber system, according to claim 19, which enables a compact design. The measuring chamber and the reference chamber can be arranged close to one another in the housing to save space. These reference and measuring chambers can then

[File:ANM\PE2903B1.doo] Description, 26.08.94 Gassensorik Pector GmbH

♦• ·

I. * I

be separated from each other by the compensation device. If care is taken to ensure that the housing is heat-conductive according to claim 20, the heat exchange between the two resonator sensors can take place. A temperature change of one of the two resonator sensors is transferred accordingly to the other resonator sensor, whereby the measurement signal, i.e. the difference frequency between the two resonator sensors, is independent of the temperature fluctuations of the resonator sensors.

The method used in the device to determine the composition of the gas mixture has the advantage that the individual components of gas mixtures can be determined independently of the absolute values of the natural frequencies or the shifted natural frequencies of the resonator sensor. With appropriate calibration, the composition of the gas mixture can be determined from the difference frequency between the first and second resonator frequencies of the two resonator sensors. Calibration advantageously only needs to be carried out at the beginning of commissioning after installation or conversion. This method therefore makes it possible for the natural frequencies of the resonator sensors to be tuned to the expected difference frequency and thus to be set to natural frequencies of the resonator sensors at which the resonator sensors have sufficient quality or sufficient quality to record the measurement signals.

Further advantageous embodiments are the subject of the remaining subclaims.

An embodiment of the invention is explained in more detail below using schematic drawings. They show

Fig. 1 is a sectional drawing of an embodiment of the device according to the invention;

[File:ANM\PE2903B1.doc] Description, 26.08.94 Gassensorik Pector GmbH

Fig. 2 is a block diagram of the resonator sensor control;

Fig. 3 Diagram of the temperature behavior of both resonator sensors;

Fig. 4 Diagram of the pressure behavior of both resonator sensors; and

Fig. 5 Diagram of the dependence of the difference frequency on a proportion of the gas mixture.

In a sectional view in Figure 1, the device according to the invention is shown in a preferred embodiment, which essentially has a two-part housing 1, with the measuring chamber 5 being contained in one part 3 and the reference chamber 9 in the other part 7. Sensor holders 11 are installed in the measuring chamber 5 and the reference chamber 9. A first resonator sensor 13 is mounted on the sensor holder 11 in the measuring chamber 5 and a second resonator sensor 14 is mounted on the sensor holder 11 in the reference chamber 9. The two resonator sensors 13 and 14 are connected via electrical leads to corresponding controls outside the device according to the invention. Seals 17 on the sensor holders 11 seal the measuring chamber 5 and the reference chamber 9 in a gas-tight manner. The measuring chamber 5 is connected via a passage 19 to an extended measuring chamber 21, the design of which is coordinated with the reference chamber 9. Between the extended measuring chamber 21 and the reference chamber 9, a compensation device in the form of a compensation membrane 2 3 is arranged, which is fixed by a clamping ring 25. The inner diameter of the clamping ring 25 is matched to the design of the extended measuring chamber 21 and the reference chamber 9. The reference chambers 9, the clamping ring 2 5 and the extended measuring chamber 21 form a cylindrical

[File:ANM\PE29O3B1.doc] Description, 26.08.94 Gassensorik Pector GmbH

Space that is cut through by the compensating membrane 23. The compensating membrane 23 can penetrate into the expanded measuring chamber 21 to equalize the pressure. If the two-part housing 1 is joined together, a sealing ring 27 is provided that enables sealing to the outside. Both parts 3 and 7 of the housing 1 are preferably joined together detachably using a screw connection. In the initial position, the compensating membrane 23 protrudes into the reference chamber 9 in a hemispherical shape. If, for example, pressure fluctuations occur that result in a low pressure level in the measuring chamber 5 compared to the reference chamber 9, the compensating membrane 23 moves more or less into the expanded measuring chamber 21 depending on the pressure difference. The size of the compensable pressure difference can be set using the compensating membrane diameter within the framework of the design conditions of the device. The pressure range can be adjusted both via the geometry of the reference or measuring chamber and via the material properties of the compensating membrane, preferably in the range of 300 mbar. As shown in Figure 1, the measuring chamber 5 is shown with a circular cross-section, which was only made with a view to a simple manufacturing method.

In order to reduce the flow resistance of the resonator sensor installed in the measuring chamber to a minimum value compared to a flowing gas mixture, the large surface of the resonator sensor is aligned parallel to the flow direction.

Figure 2 shows a block diagram of the components required for the measurement. Each resonator sensor is controlled separately by preferably identically constructed control circuits or regulators 31, 41. The respective regulator 31, 41 initially excites the actuator side of the resonator sensors via line 15a, whereby a signal from the sensor side of the resonator sensor of the respective

[Fi1e:ANM\PE2903B1.doc] Description, 26.08.94

Gas Sensorik Pector GmbH

• * *·&igr;?

Control is supplied via line 15b. The task of the controls 31, 41 is to keep the resonator sensor in resonance and to track it. The resonance frequencies of the resonator sensor in the reference and measuring chambers, which are tapped from the controls 31, 41 at points Al and A2 respectively, are subtracted in a computer 51 and the measured value is obtained from this, which is displayed via an output device 53. The control of the resonator sensors to oscillate at the resonance frequency is made possible by the fact that a phase-locked control loop (PLL module) 33, 43 has a voltage-controlled oscillator 35, 45, the output signal of which excites the resonator sensor to oscillate via an amplifier 37, 47 using line 15a. The oscillation frequency of the resonator sensor is detected by the sensor side of the resonator sensor 13, whereby the oscillation frequency is passed on in the form of an electrical signal to a comparator 39, 49, which forwards the signal via a phase shifter 34, 44 to a phase detector 36, 46 integrated in the phase-locked control loop 33, 43. The signal is then fed back from the phase detector 36, 46 via a low-pass filter to the voltage-controlled oscillator 35, 45. The entire control is therefore based on the fact that the phase shift between the forced oscillation of the resonator sensor compared to the exciting force of the oscillator 35, 45 is determined as a function of their frequency, whereby the phase is regulated back to a 90° phase shift.

Figure 3 shows the temperature behavior of the resonator sensors of the device according to the invention. It can be seen that the resonance frequencies of the resonator sensors show the same behavior as the temperature increases. The difference between the two frequencies remains constant as the temperature changes and therefore has no influence on the measured value.

[FNe:ANM\PE2903B1 .doc] Description, 26.08.94 Gassensorik Pector GmbH

Figure 4 shows the pressure equalization effect of the membrane. It can be seen that when the pressure changes, an immediate pressure equalization takes place in both chambers. The resonance frequencies of the resonator sensors change to the same extent and in the same way when the pressure increases and when the pressure drops. This ensures that the pressure fluctuations do not influence the measured variables, i.e. the difference frequency of the resonance frequencies.

Figure 5 shows the difference frequency of the resonator sensors as a function of a proportion, in this case SFg, which is dissolved in air. Since air has also been filled into the reference chamber, the linear dependence of the measured variable on the composition of the glass mixture is documented.

The device shown in Figure 1 is designed in a compact manner, whereby it may also be advantageous to separate the measuring chamber 5 and the reference chamber 9 from one another. In this case, it is only necessary to take into account that an additional compensation device may have to be created which can achieve pressure or temperature compensation.

Claims (1)

IFile:ANM\PE2903A1 .doc] Claims· · 8§.9ftt994 Gas sensors Pector GmbH Protection claims 1. Device for determining the composition of a gas mixture with: a measuring chamber which can be filled with the gas mixture and which has a first mechanical resonator sensor, a reference chamber that can be filled with a reference gas, which has a second mechanical resonator sensor, and a compensation device for setting the same temperature and pressure in the measuring chamber and in the reference chamber. 2. Device according to claim 1, characterized in that the gas mixture is composed of a first and a second component. Device according to claim 2, characterized in that the second component is composed of several gases which are qualitatively known. Device according to claim 2 or 3, characterized in that the reference gas is equal to the second component of the gas mixture. 5. Device according to one of claims 1 to 4, characterized in that the measuring chamber is designed as a flow channel and the second resonator sensor projects into the flow channel. Device according to claim 5, characterized in that the flow channel is designed to maintain a laminar flow. . '..Drink",.T- : [File:ANM\PE2903A1,doc] Claim^ Gas sensors Peotor GmbH 7. Device according to one of claims 1 to 6, characterized in that the first and/or the second mechanical resonator sensor has a base plate to which at least one surface has been applied a material by which the base plate is excited to oscillate, the resonator sensors being held in the chambers via sensor holders. 8. Device according to claim 7, characterized in that the base plate is made of glass. 9. Device according to claim 7, characterized in that the base plate consists of crystalline silicon, spring steel, aluminum or copper-beryllium bronze. 10. Device according to one of claims 7 to 9, characterized in that the material applied to the base plate is a piezoelectric material which is coated with an electrically conductive contact layer. 11. Device according to claim 10, characterized in that the piezoelectric material is a polyvinylidene fluoride film (PVDF film) which is coated with a metallic contact layer. 12. Device according to one of claims 7 to 11, characterized in that the mechanical resonator sensors are dimensioned such that preferably a 2/0 mode can be excited. 13. Device according to one of claims 1 to 12, characterized in that the large surface of the resonator sensor is aligned parallel to the flow direction. [File:ANM\PE2903A2 ] Claims, gas sensors PeotorGmbH 14. Device according to one of claims 1 to 13, characterized in that the compensating device comprises a movable gas-impermeable membrane which separates the measuring chamber from the reference chamber. 15. Device according to claim 14, characterized in that the membrane has a high heat transfer value (k). 16. Device according to one of claims 14 or 15, characterized in that the membrane is dimensioned so that its deformation as a result of a pressure difference between the measuring and reference chambers does not result in elastic restoring forces in the membrane. 17. Device according to claim 15, characterized in that the membrane is a polyethylene terephthalate film (PETP film). Device according to one of claims 1 to 17 , characterized by a device for forming the difference frequency between a first resonance frequency of the first mechanical resonator sensor and a second resonance frequency of the second mechanical resonator sensor. 19. Device according to one of claims 1 to 18, characterized in that the measuring chamber and the reference chamber have a common housing. 20. Device according to claim 19, characterized in that the housing is heat-conducting.