Device for measuring the physical properties of gases

Abstract

Device for measuring the concentration of oxygen in a gas sample, with - at least one modulatable magnetic flux source (4, 5; 4', 5', 4'', 5'') which has an air gap (3; 3', 3'') to which a gas sample can be introduced, - at least one controllable current source (50, 51, 52; 50, 51', 51'', 52', 52'') for generating current or voltage signals, each coupled to the at least one modulatable magnetic flux source to generate a modulatable magnetic flux within the air gap (3) or air gaps (3', 3''), - at least two measuring points (9a, 9b) which are arranged at least partially within the air gap (3) or air gaps (3', 3''), - wherein each measuring point (9a, 9b) has an electrically controllable, temperature-dependent heating structure (6a, 6b, 8a, 8b), - wherein each measuring point (9a, 9b) is coupled to a variable current source to heat the associated heating structure (8a, 8b) to an operating temperature, and - wherein each measuring point (9a, 9b) is coupled to a measuring circuit to measure heat conduction measurement signals generated by the associated heating structure (6a, 6b) under different operating conditions in order to selectively determine the oxygen concentration compared to other admixtures.

Description

[0001] The present invention generally relates to a device for measuring the physical properties of gases. More specifically, the invention relates to a device for measuring the concentrations of paramagnetic gases in a gas sample, such as the concentrations of oxygen and other admixed gases in the breathing gas of a patient being ventilated and / or anesthetized.

[0002] Paramagnetic methods are often used to determine the oxygen concentration in gases. These methods are based on the fact that oxygen molecules are paramagnetic due to their permanent magnetic dipole moment, whereas most other gases are diamagnetic. It is well known that the thermal conductivity of paramagnetic gases (e.g., O2 and NO) changes under the influence of magnetic fields. The reason for this behavior is apparently the fact that paramagnetic gases possess a permanent magnetic moment, which, however, is normally not apparent externally due to the thermal molecular motion of the gas molecules. However, a sufficiently strong external magnetic field ensures that the magnetic dipole moments of the individual molecules are aligned.On the one hand, this causes a change in susceptibility, resulting in an increase in magnetic flux. On the other hand, a certain molecular arrangement develops in the gas, which limits the ability to transfer thermal energy to neighboring molecules through collisions. This slightly changes the thermal conductivity of the gas.

[0003] In a known measuring device based on this phenomenon, the gas sample to be examined is placed in a cylindrical vessel, along the longitudinal axis of which is a thin measuring wire heated to a working temperature. If the thermal conductivity of the gas changes due to an external magnetic field, this causes a change in the resistance of the measuring wire, which can be determined using a measuring bridge.

[0004] In medical technology, complex fresh gas mixtures are sometimes used to ventilate patients under anesthesia. In most cases, these mixtures contain a binary basic mixture of oxygen and nitrogen, nitrous oxide or xenon, and one of the common inhalation anesthetics (e.g., desflurane, sevoflurane, isoflurane, enflurane, halothane). For patient monitoring, it is often necessary to determine gas concentrations during the patient's expiration phase. During the expiration phase, the gas mixture contains, in addition to the aforementioned gases, carbon dioxide, water vapor, and possibly other metabolic products such as ethanol, methane, and acetone. With regard to the relevant gas concentrations, the focus here is primarily on oxygen, carbon dioxide, and the anesthetic, as well as their temporal dynamics.There is therefore a need for cost-effective measuring devices that detect these gases with the required resolution and with as little cross-sensitivity as possible. Typically, several independent sensors optimized for the respective target gas are used for this purpose.

[0005] For example, the use of multi-channel infrared-optical analyzers with thermal radiation sources is known for measuring infrared-active gases. These analyzers are capable of analyzing a gas mixture for its absorption properties at multiple wavelengths. Based on the at least partially recorded spectra, the individual gaseous components can then be determined with regard to their concentration, provided they have sufficient and specific IR absorption. However, gases such as oxygen, nitrogen, helium, and xenon cannot be detected using this method.

[0006] Sensors based on infrared laser diodes are also known. Due to their narrow-band emission characteristics, they are capable of resolving the equally narrow absorption lines of oxygen. However, in order to perform a concentration measurement with sufficient accuracy using this method, a minimum absorption length is required due to the small absorption cross sections, which results in an unfavorable sensor size. In addition, interactions occur between the gases involved, which may require a correction of the measured O2 concentration values. Furthermore, this method is not suitable for the direct determination of the other gas components. Finally, this method, like the previously mentioned methods, is relatively expensive due to the high-quality optical components, especially since the laser diodes used are subject to aging effects that limit their service life.

[0007] Although electrochemical sensors represent a cost-effective alternative to IR-optical methods, they only allow for the measurement of oxygen concentration and—with severe limitations—carbon dioxide concentration. Anesthetic gases and noble gases cannot be measured this way.

[0008] In solid electrolyte sensors, such as those known from DE 20 2004 015 400 U1, a solid such as zirconium oxide acts as an ion conductor. While such sensors primarily exhibit good selectivity for oxygen, the high operating temperature required to enable ion conduction can, under unfavorable conditions, trigger decomposition processes in medical gas mixtures. In particular, the halogenated hydrocarbons commonly used in anesthesia are no longer stable at operating temperatures of around 600°C and can produce highly toxic reaction products. Furthermore, nitrous oxide, also used in anesthesia, tends to decompose into nitrogen and oxygen at temperatures above 400°C, although toxic nitrogen oxides can also be produced. The oxygen released during this process then leads to a falsely elevated concentration reading.Using this measuring principle, oxygen concentrations can only be measured effectively in nitrogen / oxygen mixtures. Other gases are not suitable for analysis. However, this method is capable of detecting the equally important parameter of flow velocity if the sensor is used in the main stream.

[0009] Gas sensors based on thermal conductivity are known from the literature. They operate either with heated metal wires or with resistive heating structures applied to the membranes of microstructured silicon elements. These sensors exploit the fact that, for a given electrical heating energy, the excess temperature of the wire or the microstructured heating structure depends on the thermal conductivity properties of the support structure and the gases surrounding the heating device. Such setups allow the unambiguous determination of the concentration ratios of binary gas mixtures if their components exhibit sufficiently different thermal conductivities. Gas mixtures with more than two components cannot be measured using this method.In particular, nitrogen / oxygen mixtures with admixtures of, for example, water vapor or CO2 cannot be meaningfully analyzed due to the similar thermal conductivity values ​​of O2 and N2.

[0010] EP 0 285 833 A2 discloses a thermal conductivity-based gas sensor that utilizes the fact that the thermal conductivity values ​​of gases exhibit certain temperature dependencies, the extent of which depends on the molecular structure of the gas in question. This document proposes successively analyzing the gas sample under investigation at different measurement temperatures and using the thermal conductivity values ​​measured at different temperatures to determine the concentration proportions of the various gases. In principle, this allows for the analysis of mixtures with three or more components. However, this requires linear independence of the measurement data sets, which is normally only achieved to a limited extent. Furthermore, sequential measurement requires a stable composition of the gas mixture, at least for the duration of the analysis.The additional pneumatic devices required for this increase the cost of such a sensor and negatively impact its size. Selective measurement of oxygen concentration is therefore not possible.

[0011] The publications DE 100 37 380 A1, DE 102 51 130 A1, and DE 102 41 244 C1 describe devices that utilize the magnetic field-dependent thermal conductivity of the oxygen content in gas mixtures to determine their concentration. In these devices, the magnetic flux density in the measuring gap of an electromagnet is cyclically varied, and the resulting changing thermal conductivity of the gas mixture is recorded using a thermal conductivity measuring chip also located in the measuring gap. For this purpose, the measuring chip has a heating device on a microstructured membrane, which is used to heat a portion of the membrane to a specific excess temperature, and a temperature measuring unit, e.g., a thermocouple (thermopile), with which this temperature can be determined.Cyclic modulation of the magnetic field in the presence of a paramagnetic gas, such as oxygen, changes the thermal conductivity of the oxygen component in the gas mixture, which in turn leads to a variation in the measured temperature value, which can be determined using a lock-in method, among other methods. Since the magnitude of the temperature fluctuations is also influenced by the thermal conductivity properties of the other gases in the mixture, certain nonlinearities arise in the sensor characteristic curve, which depend on the nature of the gas components involved.

[0012] Furthermore, US Pat. No. 2,944,418 A is known, which describes a device for analyzing gas mixtures that absorb or release oxygen through chemical reactions. The oxygen content of the gas mixture is determined in a measuring chamber using a heating wire surrounded by a magnetic field.

[0013] The present invention is therefore based on the object of providing a device to overcome the above-mentioned disadvantages.

[0014] It is a particular object of the present invention to provide a measuring device to (a) to be able to measure the oxygen concentration in a mixed gas selectively compared to other admixtures, (b) to be able to carry out a continuous determination of the oxygen concentration, (c) to provide a sensor characteristic curve that is linear and independent of gas admixtures, and (d) to be able to determine the concentration of the added gases.

[0015] These and other objects are achieved by a measuring device having the features of claim 1. Advantageous and preferred developments of the measuring device according to the invention are specified in the respective dependent claims.

[0016] Due to the type of measurement described above, the various gases exhibit different dependencies on the operating points (operating temperature of the measuring chip, magnetic flux density at which the O2 measurement is performed, ambient pressure) of the measuring device. The solution to the problem lies in operating the measuring device known from DE 100 37 380 A1 either sequentially with different operating parameters, or preferably in equipping it with at least one additional heat conduction measuring point, wherein this at least one additional heat conduction measuring point is each operated with different operating parameters. In other words, the at least two measuring points of the measuring device according to the invention are operated in parallel with different operating parameters or with different operating points. Both the thermal and the magnetic operating points can be varied.It should be noted that a change in the pressure operating point is also possible.

[0017] Each measuring point has at least one heat conduction measuring unit and a heating device, whereby any electrical heating means can be used as the heating device. Alternatively, the heat conduction measuring unit and heating device can be configured as a combined heating and measuring element, whereby this combined element is preferably a heating wire or a similar heating means that can be heated to a desired temperature by supplying electrical power and whose temperature values ​​can be read out. In the examples mentioned, the heat conduction measuring unit(s) and the heating device(s) or the combined heating and measuring element(s) are arranged in the air gap of a magnetic circuit. These components are preferably integrated in one or more measuring chips located in the air gap of the magnetic circuit. Preferably, a measuring chip with multiple measuring points is used.Such a measuring chip preferably has, for each measuring point, a microstructured membrane with a heating device formed thereon, with which part of the membrane can be brought to a certain temperature by supplying electrical power, and a heat conduction measuring unit, designed, for example, as a thermocouple (thermopile), with which this temperature can be determined.

[0018] As previously explained, the measuring device according to the invention has at least one measuring point, each with a heating device and at least one measuring unit, wherein the measuring points are preferably operated at different operating points. To determine the gas concentration, the respective operating temperatures of the at least one heat measuring unit are determined simultaneously. Alternatively or additionally, the required heating power (or heating current or heating voltage) of the heating device(s), the operating parameters characterizing the respective operating point, and, if applicable, their temporal profiles are recorded.

[0019] Possible operating states or operating modes for operating the measuring device according to the invention are, for example, conceivable: 1. Operation of the measuring point(s) at different heating outputs or operating temperatures, static and dynamic, 2. Operation of the measuring point(s) at different magnetic flux densities, whereby both static and dynamically changing magnetizations are conceivable, and 3. Operating pressure within the gas measuring cell (also static and dynamic).

[0020] These operating conditions can be used individually or in combination, and are understood to mean that the primary static and dynamic temperature measurement signals of the measuring device can be used either directly as a measured value or as a controlled variable. In the latter case, the control signals required for control (e.g., heating power, heating current, heating voltage, or coil current for generating the magnetic field strength) are then evaluated metrologically.

[0021] The present invention will now be described using some embodiments with reference to the figures, which show various embodiments of the sensor according to the invention or the measuring device according to the invention and explain the associated measuring methods. Fig. 1 shows an embodiment of the measuring device according to the invention in the form of an electromagnet in whose air gap a measuring chip is provided; Fig. 2 shows a detailed representation of a measuring chip from Fig. 1 with one measuring point; Fig. 2a shows a detailed representation of a measuring chip from Fig. 1 with two measuring points; Fig. 3 shows the measuring chip with two measuring points from Fig. 2, which is provided in the air gap of a magnet; Fig. 4 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a with constant voltage; Fig. 5 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a with constant current; Fig. 6 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a with constant power; Fig. 7 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, where the heating power is used as a measurement signal; Fig. 8 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, where the heating current is used as a measuring signal; Fig. 9 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, where the heating voltage is used as a measuring signal; Fig. 10 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, where the heating power is used as the heat conduction signal and the thermoelectric voltage as the O2 signal; Fig. 11 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, where the heating current is used as the heat conduction signal and the thermoelectric voltage as the O2 signal; Fig. 12 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, where the heating voltage is used as the heat conduction signal and the thermoelectric voltage is used as the O2 signal; Fig. 13 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, in which a sinusoidal signal is superimposed on the heating voltage; Fig. 14 shows a circuit for controlling one of the measuring points from Fig. 2 and Fig. 2a, in which a pulse-shaped signal is superimposed on the heating voltage; Fig. 15 shows the output signal of the circuit from Fig. 13; Fig. 16 shows the output signal of the circuit from Fig. 14; Fig. 17 shows a circuit for controlling a measuring point in which a heating wire with temperature evaluation is used as a combined heating and measuring element; Fig. 18 shows a further embodiment of the measuring device according to the invention in the form of a magnet, in the air gap of which the measuring chip made of Fig. 2a and which has a stepped pole piece; Fig. 19 shows another embodiment of the measuring device according to the invention in the form of two separate magnetic circuits, in whose air gaps the measuring chip from Fig. 2a is provided for; Fig. 20 shows a modification of the embodiment of Fig. 18, wherein one pole piece is provided with a non-magnetic wedge; Fig. 21 a circuit for controlling a magnetizing coil for the measuring device from Fig. 1 and Fig. 3 shows; and Fig. 22 a circuit for controlling two magnetizing coils for the measuring device from Fig. 19 shows.

[0022] Fig. Figure 1 shows a preferred embodiment of the measuring device (or sensor) 1 according to the invention for measuring the oxygen concentration or the concentration of another paramagnetic gas in a gas sample. The measuring device 1 has a measuring chip 2, which in the case shown is arranged in the air gap 3 of an electromagnet 4 provided with a coil 5, so that the measuring point of the measuring chip 2 can be subjected to an electrically controllable magnetic field. Instead of the coil 5, however, a permanent magnet (not shown) can also be provided, with the aid of which a constant magnetic field is generated. The measuring device 1 is further configured so that the gas to be analyzed (gas sample) can flow through the air gap 3 and past the measuring chip 2.

[0023] As detailed in Fig. As shown in Figure 2, the measuring chip 2 according to a first embodiment has a measuring point with at least one electrically controllable heat conduction measuring unit 6, which is preferably designed as a thermocouple (thermopile). The measuring chip 2 can have several heat conduction measuring units 6, which can be arranged at different locations. Furthermore, the measuring point can also be realized by two or more separate individual chips or as a measuring chip with several measuring points, as described in detail with reference to Fig. 2a. The measuring chip 2 can have one or more perforated membrane(s) 7 for the gas sample to enter from the top. Preferably, however, a closed membrane is used, the support frame of which is removed, e.g., by etching, far enough away that the measurement gas can pass through the resulting gap beneath the membrane. Alternatively, the volume below the membrane can also be permanently filled with a poorly heat-conducting gas (e.g., xenon).

[0024] The magnetic field generated by coil 5 is preferably designed as a pure alternating field with a temporal characteristic symmetrical to the zero point. The temporal characteristic is preferably sinusoidal, but can also have other shapes (triangular or rectangular). The magnetic field can alternatively or additionally be amplitude-controlled. In addition to the signal processing options, the amplitude control of the alternating magnetic field also offers the advantage of being able to reduce the electrical power for the measuring points at higher oxygen signal levels along with the magnetization. However, a magnetization with a DC component underlay is also conceivable, in which the modulated field component can be shifted along the magnetization characteristic curve. In order to keep the energy expenditure for magnetization low, it is also conceivable to generate the magnetic field at least partially with a permanent magnet.

[0025] As further stated in Fig. As can be seen in Figure 2, the measuring chip 2 has an electrically controllable heating device 8, which can be configured, for example, as an electrically conductive resistance structure deposited on the membrane or as a heating wire. The heating device is preferably arranged or configured to heat the membrane 7 of the measuring chip 2 to a desired temperature.

[0026] It should be noted that the heat conduction measuring unit 6 and the heating device 8 can be integrated as a temperature-dependent heating structure, i.e., a resistive heating / measuring element in which the temperature measurement is performed using the temperature coefficient. Examples of such heating structures are heating wires or similar heating means with a temperature-dependent resistivity. For reasons of clarity, these temperature-dependent heating structures are illustrated and described in the figures as separate heat conduction measuring units 6 and heating devices 8, respectively. Consequently, where technically feasible, the measuring units 6 and the associated heating devices 8 in the described embodiments can be replaced by integrated temperature-dependent heating structures.

[0027] Unless otherwise stated, the exemplary embodiments described below predominantly refer to arrangements with two measuring points provided on a measuring chip 2. Alternatively, two measuring chips 2 operating in parallel, each with one measuring point, can be used. In both cases, each of the measuring points can be provided with one or more heat conduction measuring units 6.

[0028] Fig. Figure 2a shows an embodiment with a measuring chip 2 having two adjacent measuring points 9a, 9b, each provided with a heat conduction measuring unit 6a, 6b and a heating device 8a, 8b. As shown in Fig. As further shown in Figure 2a, the measuring units 6a, 6b and the heating devices 8a, 8b are provided on a cover layer 7 made, for example, of Si3N4. This cover layer 7 can be exposed by etching from the underside at the locations where the measuring units 6a, 6b and the heating devices 8a, 8b are provided. To allow access of the measuring gas to be analyzed to both sides of the measuring points 9a, 9b, either the membrane 7a, 7b can be partially removed by etching, or the measuring chip 2 is partially etched thin, allowing gas access from the front. As explained above, the measuring units and the associated heating devices can each be replaced by integrated temperature-dependent heating structures.

[0029] In Fig. 3 shows a preferred embodiment in which a chip 2 is used, which has two measuring points 9a, 9b, each with a heat conduction measuring unit 6a, 6b and an associated heating device 8a, 8b. Fig. 3 the upper pole piece 4a and the lower pole piece 4b of the electromagnet 4 Fig. 1. In the air gap 3 between the two pole pieces, the measuring chip 2 is mounted on the surface of the lower pole piece, and the measuring chip 2 has two adjacent measuring points 9a, 9b, each of which contains a heat conduction measuring unit 6a, 6b and an associated heating device 8a, 8b (not shown), as in Fig. 2a. As explained above, the two measuring points 9a, 9b can be integrated in one chip 2 ( Fig. 2a). Alternatively, two or more chips, each with a measuring point, can be provided. It is of course also possible that the Fig. Chip 2 shown in Figure 2a has more than two measuring points.

[0030] Preferably, the two measuring points 9a, 9b are approached in parallel (i.e., simultaneously) with two different operating points. In principle, however, the different operating points can also be approached sequentially, in which case, as already mentioned, the gas mixture must be kept constant for the duration of the measurement.

[0031] If the two measuring points 9a, 9b are operated simultaneously at different operating points (i.e. different heating outputs, different magnetic flux densities or different operating pressures), then different voltages (O2 signals) are received from the two measuring units 6a, 6b, which correspond to the respective measured O2 values ​​at the two measuring points. For example, periodic O2 fluctuations can be separated from the respective base signals using simple filter devices. The two resulting periodic O2 signals and the non-periodic base signals are then related to one another and evaluated in order to determine the current oxygen concentration with high accuracy. By correlating the two periodic O2 signals, freed of the base signals, with the non-periodic base signals, non-linearities can also be reduced.The concentrations of admixed gases can also be determined in the same way.

[0032] In a first operating mode, the heating devices 8a, 8b of the two measuring points 9a, 9b are operated at different temperature operating points, whereby there are a total of three basic types of control (in the circuit arrangements, the magnetization device is partially omitted to simplify the illustration and only the control for one of the two measuring points is shown).

[0033] The following describes the operation at different heating outputs or operating temperatures (static and dynamic).

[0034] In the first type of control, as in Fig. 4, Fig. 5 and Fig. 6, the two measuring points 9a, 9b, of which only one is shown, are supplied with a constant heating voltage ( Fig. 4), with a constant heating current ( Fig. 5) or with a constant heating output ( Fig. 6), each of which corresponds to the respective thermal operating point of the heating devices 8a, 8b. The required values ​​are determined once in air and then kept constant (calibration). The measurement signals for the heat conduction 20 of the gas flowing past the measuring points 9a, 9b and the resulting periodic oxygen signals 21 are found here in the voltages of the heat conduction measuring units 6a, 6b.

[0035] The following describes the circuit structure of Fig. 4, constant voltage control, is described in more detail. The heating device 8a, 8b is connected to a DC voltage source 12 via an amplifier 10 (connected as an impedance converter) and a voltage divider 11 with a variable tap. The output signal of the heat conduction measuring unit 6a, 6b is passed through a low-pass filter 13 or a high-pass filter 14 to output the heat conduction signal 20 or the O2 signal 21, respectively.

[0036] The circuit structure of Fig. 5, constant current control, differs from the structure of Fig. 4 merely in that the amplifier 10 is connected as a non-inverting amplifier, wherein a part of the output voltage of the amplifier is fed back to the inverting input of the amplifier via a voltage divider consisting of heating device 8a, 8b and shunt 15.

[0037] When designing the circuit of Fig. 6, constant power control, the output (voltage) of amplifier 10 connected to the first contact of heating device 8a, 8b is coupled to the first input of a multiplier 16, whose second input (current) is connected to the second contact of the heating device via an inverting amplifier 17 and two series resistors. The output of multiplier 16 is coupled to the inverting input of amplifier 10.

[0038] As in Fig. 7, Fig. 8 and Fig. As shown in Figure 9, in a second type of control, the respective thermal operating points are regulated to constant values ​​independent of the gas composition. The output voltages of the heat conduction measuring units 6a, 6b are used as control variables, and the heating voltages, see Fig. 4, the heating currents, see Fig. 5, or the heating output, see Fig. 6, tracked. In this case, the measurement signals are transmitted by the required heating voltages 24, heating currents 23, and heating powers 22.

[0039] Fig. 10, Fig. 11, and Fig. 12 shows a third preferred type of control, which is a mixture of the two aforementioned methods and combines the advantages of operation at constant temperature levels with comparatively simple (because slow) temperature control. Here, the output voltages of the heat conduction measuring units 6a, 6b are used as control variables, and the heating voltages, heating currents, or heating powers are adjusted so that the operating temperatures remain constant on average over time. The constant temperature averages ensure stable measurement conditions, regardless of the type of gas mixture, while the rapidly changing, modulation-induced oxygen signals 25 remain directly measurable as temperature fluctuations without causing significant operating point shifts due to their lower amplitude.The control signals are conditioned using electronic low-pass filters in such a way that the gas mixture-related (and slower) temperature changes are regulated without disturbing the faster periodic, magnetic field-related thermal conductivity changes (oxygen measurement signal).

[0040] In another type of control, the heating devices 8a, 8b are at least partially additionally controlled with a time-variable heating power component. This component can, for example, be a sinusoidal heating power component 26 ( Fig. 13) or a pulse-shaped heating power component 27 ( Fig. 14). In the case of an arrangement in which the heating and temperature measuring elements are identical (e.g., hot wire), by evaluating the temporal relationship between the introduction of the heating power and the change in temperature at the heating element, a statement can be made about the heat dissipation dynamics into the gas, which is essentially determined by the heat conduction and heat capacity values ​​of the gas components. When using spatially separated temperature sensors, as is appropriately used for microstructured heat conduction measuring units 6a, 6b, this effect is more pronounced. When applying a sinusoidal temperature modulation to the heating element ( Fig. 15), this can be measured as a phase shift and amplitude ratio between heating power 30 and temperature measurement signal 31. If a pulsed change in the heater temperature is applied, curve 32, Fig. 6, then the properties of the gas can also be deduced from the decay behavior of the temperature sensor, curve 33. The attenuation of the temperature signals can also be used for evaluation.

[0041] In Fig. Figure 13 shows an example of such an arrangement for the operation of a microstructured heating device with sinusoidal superposition and control with a constant heating voltage. The circuit also has a lock-in amplifier 28 and a phase detector 29. The sinusoidal additional signal is additively superimposed on the constant base voltage and, depending on the type and concentration of the mixed gases, leads to a time-delayed reaction at the thermocouple. This operating mode can be applied to all Fig. 4 to 12. For the operation of a measuring unit with a heating wire, the change in resistance of the heating device must be evaluated to obtain a suitable temperature signal, whereby the temperature coefficient is used to adjust the wire temperature R D = f(ϑ). This is shown schematically in Fig. 17 shown.

[0042] In Fig. Figure 14 shows a circuit with pulse-shaped superposition. Since the spectral frequency components contained in the impulse response of the temperature signal cover a certain bandwidth and overlap with the signal components of the conventional heat conduction measurement and those of the O2 measurement, complex filtering of the raw signals is necessary, which is conveniently performed in a computer. Temporal synchronization of the magnetization signal with the pulse signal simplifies signal processing, just as it does with the Fig. 13 shown variant with superimposed sine signal 26.

[0043] The following explains the operating mode at different magnetic flux densities, whereby both static and dynamically changing magnetizations are conceivable in this operating mode.

[0044] A first variant of this operating mode works with a modified magnet system (compare Fig. 3), in which the measuring points 9a, 9b located in the air gap 3 can be supplied with different magnetic flux densities. This can be achieved, for example, by means of stepped pole pieces in the air gap ( Fig. 18), or by a second, independently controllable magnetic system ( Fig. 19). In the first case from Fig. 18, a section 4c made of a non-magnetic material is inserted into the upper pole piece 4a, resulting in a fixed flux density ratio. As can be seen from Fig. As can be seen in Figure 18, the left part of the pole shoe 4a is located above the measuring point 9a, while the right section 4c of the pole shoe, made of non-magnetic material, is located above the measuring point 9b. As a result, the measuring points 9a and 9b are subjected to different flux densities. It is obvious that the magnetic flux is conducted through a coil (as in Fig. 1) or can be generated by a permanent magnet.

[0045] In the second case ( Fig. 19), a variable flux density is achieved over the two measuring points by providing two electromagnets 4' and 4", which can be variably magnetized via two separately controllable coils 5' and 5". The measuring points 9a, 9b are provided in the respective air gaps of the electromagnets 4' and 4", respectively. However, this variant requires increased technical effort. Instead of the two coils 5', 5", two permanent magnets of different strengths can also be used, whereby in this way a similar effect as in Fig. 18 is reached.

[0046] The Fig. The amplitude-controlled magnetization operation shown in Figure 19 can also be realized with a single, non-stepped magnet system (i.e., only one electromagnet instead of the two magnets 4' and 4") shown and a single measuring point. The amplitude control of the magnetization can be used to control the magnetic field-modulated oxygen signal to a constant predetermined value independent of the oxygen concentration. The concentration signal is then represented by the amplitude of the magnetization current.

[0047] As mentioned above, with a fixed stepped pole piece (see Fig. 18), a stable magnetization gradation can also be realized with a permanent magnet. Furthermore, a wedge-shaped air gap can be used in conjunction with a linear array of measuring points 9a-n or heat conduction measuring devices, whereby a multitude of measured values ​​can be obtained at different flux densities. The wedge-shaped air gap can be easily realized by beveling the lower surface of the pole piece 4a facing the measuring points 9a-n. Alternatively, instead of the cuboid element 4c made of non-magnetic material (see Fig. 18) a wedge-shaped element 4c' made of non-magnetic material may be used, as shown in Fig. 20. It is obvious that the magnetic flux can be generated with a permanent magnet (constant magnetic field) or with a coil for generating an amplitude-modulatable magnetic field.

[0048] An electronic control system is shown schematically Fig. 21 and Fig. 22. The evaluation circuits shown in Fig. 4 to 14 shown arrangements can be used. In Fig. 21, the magnetizing coil 4 is supplied with an amplitude-modulated alternating voltage by means of an alternating voltage source 50 via a voltage regulator 51 and an amplifier 52. In Fig. 22 two magnetizing coils 4' and 4" (see Fig. 19) is supplied with an amplitude-modulatable alternating voltage by means of an alternating voltage source 50 via two voltage regulators 51', 51" and associated amplifiers 52', 52", each of which is supplied with an amplitude-modulatable alternating voltage.

[0049] Overall, a multitude of measured variables for the gas can be obtained in this way, which exhibit varying linear independence and are difficult to calculate analytically. Therefore, it is suggested to use multivariate regression methods for this purpose. All of the described procedures can also be performed with individual elements if the different operating points are controlled sequentially rather than in parallel. In this case, however, it must be ensured that the gas composition remains unchanged during the analysis. LIST OF REFERENCE SYMBOLS 1 measuring device 2 measuring chips 3 Air gap 4 Electromagnet 5 coil 6 Heat conduction measuring unit 7 membrane(s) 8 Heating device 9 measuring points 10 amplifiers 11 voltage divider 12 DC voltage source 13 Low pass 14 high pass 15 Shunt 16 multipliers 17 inverting amplifier 20 Heat conduction signal 21 Oxygen signal 22 heating output 23 Heating current 24 heating voltage 25 oxygen signal 26 sinusoidal heating power component 27 pulse-shaped heating power component 28 lock-in amplifiers 29 Phase detector 30 heating output 31 Temperature measurement signal 32 Heater temperature 33 Decay behavior of the temperature sensor 50 AC voltage source 51 voltage regulators 52 amplifiers

Claims

[1] Device for measuring the concentration of oxygen in a gas sample, with - at least one modulatable magnetic flux source (4, 5; 4', 5', 4'', 5'') having an air gap (3; 3', 3'') to which a gas sample can be supplied, - at least one controllable current source (50, 51, 52; 50, 51', 51'', 52', 52'') for generating current or voltage signals, each coupled to the at least one modulatable magnetic flux source in order to generate a modulatable magnetic flux within the air gap (3) or air gaps (3', 3''), - at least two measuring points (9a, 9b) which are arranged at least partially within the air gap (3) or air gaps (3', 3''), - wherein each measuring point (9a, 9b) has an electrically controllable, temperature-dependent heating structure (6a, 6b, 8a, 8b), - wherein each measuring point (9a, 9b) is coupled to a variable current source to heat the associated heating structure (8a, 8b) to a working temperature, and - wherein each measuring point (9a, 9b) is coupled to a measuring circuit for measuring heat conduction measuring signals generated by the associated heating structure (6a, 6b) under different operating conditions in order to determine the oxygen concentration selectively with respect to other admixtures. [2] Device according to claim 1, wherein the temperature-dependent heating structures each comprise an electrically controllable heating device (8a, 8b) and a heat conduction measuring unit (6a, 6b). [3] Device according to claim 1, wherein the at least one modulatable magnetic flux source comprises an electromagnet (4; 4', 4'') coupled to a coil (5; 5', 5''). [4] Device according to one of the preceding claims, in which the modulatable magnetic flux has a temporal course symmetrical to the zero point with, for example, a sinusoidal, triangular or rectangular shape, and in which the magnetic flux can additionally be controlled in amplitude. [5] Device according to one of the preceding claims, in which the at least one modulatable magnetic flux source (4, 5; 4', 5', 4'', 5'') has two pole shoes (4a, 4b) between which the air gap (3; 3', 3'') is formed, wherein the at least two measuring points (9a, 9b) are attached to one of the pole shoes (4a, 4b). [6] Device according to one of the preceding claims, in which the at least one modulatable magnetic flux source (4, 5; 4', 5', 4'', 5'') has two pole shoes (4a, 4b) between which the air gap (3; 3', 3'') is formed, wherein at least one of the pole shoes (4a) has a section made of non-magnetic material (4c), whereby a reduced magnetic flux is generated in sections in the air gap, and wherein at least one of the measuring points (9b) is arranged in the region of the reduced magnetic flux. [7] Device according to one of the preceding claims, in which the at least one modulatable magnetic flux source (4, 5; 4', 5', 4'', 5'') has two pole shoes (4a, 4b) between which the air gap (3; 3', 3'') is formed, wherein at least one of the pole shoes (4a) has a substantially wedge-shaped section (4c') made of non-magnetic material, whereby regions with different magnetic fluxes are generated in the air gap, and wherein the at least two measuring points (9a, 9b) are arranged in regions of different magnetic fluxes. [8] Device according to one of the preceding claims, in which the electrically controllable heating device (8) is designed as a resistance wire or heating wire. [9] Device according to one of the preceding claims, wherein the measuring chip (2) has a membrane (7) and the heating device (8) is arranged or designed to heat the membrane (7) to a desired temperature. [10] Device according to one of the preceding claims, in which the at least two measuring points (9a, 9b) are arranged on a measuring chip (2). [11] Device according to one of the preceding claims, in which the at least two measuring points (9a, 9b) are operated simultaneously at different temperature operating points. [12] Device according to one of claims 1 to 10, wherein the at least two measuring points (9a, 9b) are operated sequentially at different temperature operating points. [13] Device according to claim 11 or claim 12, wherein the at least two measuring points (9a, 9b) are operated during a measuring period with a constant heating voltage, with a constant heating current or with a constant heating power, each of which matches the respective thermal operating point of the heating devices (8a, 8b). [14] Device according to claim 11 or claim 12, wherein the thermal operating points of the at least two measuring points (9a, 9b) are controlled to constant values ​​during a measuring period, the output voltages of the heat conduction measuring units (6a, 6b) being used as control variables and the heating voltages, the heating currents or the heating powers being tracked. [15] Device according to claim 14, in which the required heating voltages, heating currents or heating powers are the carriers of the measuring signals. [16] Device according to one of claims 11 to 15, in which during a measuring period the heating devices (8a, 8b) are additionally controlled with a time-variable heating power component, which component can be sinusoidal or pulse-shaped. [17] Device according to one of claims 11 to 16, wherein the magnetization signal is adjusted so that at least one measurement signal is controlled to a constant preset value. [18] Device according to claim 17, wherein the required magnetization amplitude is the carrier of the measurement signal. [19] Device according to one of claims 11 to 16, wherein the amplitude of the magnetic field generated by the at least one modulatable magnetic flux source is controlled to a constant predetermined value independently of the concentration of the gas to be measured.

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