Method and measuring apparatus for determining physical properties of gas

Abstract

A method using a gas reservoir and a critical nozzle for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures, the method includes: flowing a gas or gas mixture under pressure from the gas reservoir through the critical nozzle; measuring pressure drop in the gas reservoir as a function of time; determining a gas property factor (*), dependent on physical properties of the gas or gas mixture, based on the measured values of the pressure drop; and determining a desired physical property or quantity relevant to combustion based on the gas property factor (*) through correlation.

Claims

1. A method for determining physical properties and/or quantities relevant to combustion of gas and/or gas mixtures, the method comprising: flowing a gas or gas mixture from a gas reservoir or into a gas reservoir with the gas or gas mixture flowing under pressure through a critical nozzle and past a microthermal sensor, wherein the same mass flow is applied to the critical nozzle and the microthermal sensor; measuring pressure drop or pressure increase respectively in the gas reservoir as a function of time; determining a first gas property factor (*), which is dependent on a first group of physical properties of the gas and/or gas mixture, on the basis of measured values of the pressure drop or pressure increase; determining a second gas property factor (), which is dependent on a second group of physical properties of the gas or gas mixture, from a flow signal generated by the microthermal sensor; determining the thermal conductivity () of the gas and/or gas mixture using the microthermal sensor; and determining a physical property and/or quantity relevant to combustion from the first and second gas property factor (*, ) and the thermal conductivity () through correlation.

2. The method according to claim 1, in which the first gas property factor (*) is derived from the time constant of the pressure drop on the basis of an exponential decline of the measured pressure, or in which the first gas property factor (*) is derived from a proportionality constant of the pressure increase on the basis of a linear increase of the measured pressure.

3. The method according to claim 1, in which the first gas property factor (*) is derived from a time constant of the pressure drop or pressure increase respectively on the basis of an adiabatic decline or increase of the measured pressure.

4. The method according to claim 1, in which the second gas property factor () contains the quotient of heat capacity (c.sub.p) divided by thermal conductivity () of the gas or gas mixture, or is dependent on the same.

5. The method according to claim 1, in which the first and/or the second gas property factor are derived by additionally measuring the nozzle inlet pressure (p.sub.Nozzle) and/or the temperature (T) or initial temperature (T.sub.0) and by omitting all gas-unrelated variables.

6. The method according to claim 1, wherein the gas property factors (*, ) are validated by comparing the values for the total volume of the gas or gas mixture released from or fed into the gas reservoir by: measuring the pressure and temperature in the gas reservoir at the start and end of the pressure drop or pressure increase reading and by determining the standard volume released or fed respectively by reference to the known volume of the gas reservoir; summing up the standard flow measured with the microthermal sensor during the time interval between the start and end of the pressure drop or pressure increase reading; comparing the standard volume released or fed respectively to the summed up standard volume; and in case of a discrepancy, adjusting the first and/or the second gas property factor (*, ) and/or adjusting the pressure signal and/or the standard flow variable of the microthermal sensor.

7. The method according to claim 1, in which the flow signal of the microthermal sensor is calibrated by at least: calibrating the flow signal of the microthermal sensor for a specific calibration gas or gas mixture; determining the ratio (/*) of the second gas property factor, determined on the basis of the flow signal of the microthermal sensor, to the first gas property factor for an unknown gas or gas mixture; and comparing the standard volume values from the reading of the pressure drop or pressure increase and the reading of the summed up standard flow of the microthermal sensor, which are then used to adjust the ratio of the second gas property factor to the first, and to adapt the value for the second gas property factor ().

8. The method according to claim 1 where the desired physical property is the density or the thermal conductivity or the heat capacity or the viscosity of the gas or gas mixture, and/or where the quantity relevant to combustion is the energy content or the calorific value or the Wobbe index or the methane number or the air requirement of the gas or gas mixture.

9. The method according to claim 1, wherein a polytropic index (n) is determined on the basis of measured values of the pressure drop or pressure increase respectively, and the physical property and/or quantity relevant to combustion is further determined from the polytropic index (n).

10. The method according to claim 1, where the desired physical property or the quantity relevant to combustion (Q) is determined by aid of a correlation function
Q=.sub.corr(,*,)=const.Math..sup.r.Math.*.sup.s.Math..sup.t or
Q=.sub.corr(,*,,n)=const.Math..sup.r.Math.*.sup.s.Math..sup.t.Math.n.sup.u, wherein r, s, t and u are exponents, and const is a constant.

11. A measuring apparatus configured to perform the method of claim 1, the measuring apparatus comprising: an analyzer unit configured to determine the first gas property factor, the second gas property factor, the thermal conductivity, and the physical property and/or quantity relevant to combustion, the gas reservoir that is equipped with a pressure sensor, the critical nozzle that is configured to be coupled, via a gas duct, to the gas reservoir allowing the gas or gas mixture to flow under pressure, and the microthermal sensor.

12. The measuring apparatus according to claim 11, comprising a compressor to increase the pressure in the gas reservoir or a vacuum pump to generate low pressure in the gas reservoir.

13. The measuring apparatus according to claim 11, wherein the gas reservoir is equipped with a heat exchanger to approximate isothermal conditions or with a heat insulation to limit heat exchange in the adiabatic or near adiabatic case.

14. A method to use a gas reservoir and a critical nozzle for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures, the method comprises: flowing a gas or gas mixture from the gas reservoir or into the gas reservoir with the gas or gas mixture flowing under pressure through the critical nozzle, and, in the case the gas or gas mixture is flowing from the gas reservoir, with a decreasing mass flow of the gas or gas mixture when pressure is decreasing in the gas reservoir; measuring the pressure drop or pressure increase respectively in the gas reservoir as a function of time, wherein a nozzle inlet pressure of the critical nozzle is higher than a critical pressure of the critical nozzle during the measuring of the pressure drop or pressure increase respectively in the gas reservoir; determining a gas property factor (*), which is dependent on the physical properties of the gas or gas mixture, on the basis of measured values of the pressure drop or pressure increase; and determining a desired physical property or quantity relevant to combustion on the basis of the gas property factor (*) through correlation.

15. The method according to claim 14, in which the gas property factor (*) is derived from the time constant of the pressure drop on the basis of an exponential decline of the measured pressure, or in which the gas property factor (*) is derived from a proportionality constant of the pressure increase on the basis of a linear increase of the measured pressure.

16. The method according to claim 14, in which the first gas property factor (*) is derived from a time constant of the pressure drop or pressure increase respectively on the basis of an adiabatic decline or increase of the measured pressure.

17. The method according to claim 14, in which the gas property factor is determined by additionally measuring the nozzle inlet pressure (p.sub.Nozzle) and/or the temperature (T) or initial temperature (T.sub.0) and by omitting all gas-unrelated variables.

18. The method according to claim 14 wherein a polytropic index (n) is determined on the basis of measured values of the pressure drop or pressure increase respectively, and/or wherein the physical property and/or quantity relevant to combustion through correlation is further determined on the basis of the polytropic index (n).

19. The method according to claim 14, wherein the desired physical property or quantity relevant to combustion is determined only on the basis of the gas property factor (*).

20. A measuring apparatus configured to perform the method of claim 14, the measuring apparatus comprising: an analyzer unit configured to determine the gas property factor and to determine the desired physical property or quantity relevant to combustion, the gas reservoir that is equipped with a pressure sensor, and the critical nozzle that is configured to be coupled, via a gas duct, to the gas reservoir allowing the gas or gas mixture to flow under pressure.

21. The measuring apparatus according to claim 20, comprising a compressor to increase the pressure in the gas reservoir or a vacuum pump to generate low pressure in the gas reservoir.

22. The measuring apparatus according to claim 20, wherein the gas reservoir is equipped with a heat exchanger to approximate isothermal conditions or with a heat insulation to limit heat exchange in the adiabatic or near adiabatic case.

23. A method to use a gas reservoir and a critical nozzle for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures, the method comprises: flowing a gas or gas mixture from the gas reservoir or into the gas reservoir with the gas or gas mixture flowing under pressure through the critical nozzle, and, in the case the gas or gas mixture is flowing from the gas reservoir, with a decreasing mass flow of the gas or gas mixture when pressure is decreasing in the gas reservoir; measuring the pressure drop or pressure increase respectively in the gas reservoir as a function of time; determining a gas property factor (*), which is dependent on the physical properties of the gas or gas mixture, on the basis of measured values of the pressure drop or pressure increase; and determining a desired physical property or quantity relevant to combustion on the basis of the gas property factor (*) through correlation, wherein a polytropic index (n) is determined on the basis of measured values of the pressure drop or pressure increase respectively, and/or wherein the physical property and/or quantity relevant to combustion through correlation is further determined on the basis of the polytropic index (n).

24. A measuring apparatus configured to perform the method of claim 23, the measuring apparatus comprising: an analyzer unit configured to determine the gas property factor and the polytropic index and to determine the desired physical property or quantity relevant to combustion, the gas reservoir that is equipped with a pressure sensor, and the critical nozzle that is configured to be coupled, via a gas duct, to the gas reservoir allowing the gas or gas mixture to flow under pressure.

Description

SUMMARY OF DRAWINGS

(1) The invention is explained in more detail below with reference to the drawings. In the drawings:

(2) FIG. 1a shows an exemplary embodiment of a schematic configuration of a measuring apparatus according to the present invention (high-pressure variant),

(3) FIG. 1b shows a variant of the exemplary embodiment shown in FIG. 1a,

(4) FIG. 2 shows a second exemplary embodiment of the schematic configuration of a measuring apparatus according to the present invention (low pressure variant),

(5) FIG. 3 shows an exemplary embodiment of a microthermal sensor for use in a measuring apparatus according to the present invention, and

(6) FIG. 4 shows a graphical illustration of the directly measured density ratio (ordinate) as a function of the correlated density ratio (abscissa) for various gas groups at standard conditions (0 C., 1013.25 mbar).

(7) FIG. 5a shows an exemplary embodiment of a schematic configuration of a measuring apparatus according to a second embodiment of the invention (high-pressure variant),

(8) FIG. 5b shows a variant of the exemplary embodiment shown in FIG. 5a,

(9) FIG. 6 shows a second exemplary embodiment of a schematic configuration of a measuring apparatus according to a second embodiment of the invention (low pressure variant),

(10) FIG. 7 shows a graphical illustration of the directly measured methane content (ordinate) as a function of the correlated methane content (abscissa) for a binary raw biogas (methane and carbon dioxide).

(11) FIG. 8a shows an exemplary embodiment of a schematic configuration of a measuring apparatus according to a third embodiment of the invention with a gas reservoir and a microthermal sensor (high-pressure variant),

(12) FIG. 8b shows a variant of the exemplary embodiment shown in FIG. 8a,

(13) FIG. 9 shows a second exemplary embodiment of a schematic configuration of a measuring apparatus according to a third embodiment of the invention with a gas reservoir and a microthermal sensor (low pressure variant),

(14) FIG. 10 shows a graphical illustration of the classification of natural gas mixtures by reference to thermal diffusivity (ordinate) with simultaneous knowledge of the thermal conductivity A (abscissa).

(15) FIG. 11 shows pressure decay curves calculated for different polytropic indices with the pressure being displayed on a logarithmic ordinate.

DETAILED DESCRIPTION

(16) FIG. 1a shows an exemplary embodiment of a schematic configuration of a measuring apparatus according to the present invention in which the pressure in the main gas duct 1 is higher than the critical pressure for the critical nozzle 6 of the measuring apparatus (high-pressure variation). In the exemplary embodiment, the measuring apparatus consists, in addition to the critical nozzle 6, of an analyzer unit 11, which is configured for performing the method according to the present invention, a gas reservoir 4, which is equipped with a pressure sensor 8 and a microthermal sensor 7 to measure the flow and thermal conductivity, in which case the gas reservoir 4 is connected with the critical nozzle 6 and the microthermal sensor 7 for the measurements.

(17) If required, the measuring apparatus may comprise one or more of the following additional components: a test line 2, which leads to the gas reservoir 4, and which may be connected to a main gas duct 1 during operation, an inlet valve 3, which may be arranged in the test line 2 to control the gas supply to the gas reservoir, an outlet valve 5, installed on the outlet side of the gas reservoir to control the flow of gas from the gas reservoir, an outlet 10 for discharging the gas released from the measuring apparatus, an additional pressure sensor 8, which may be installed on the outlet 10, a temperature sensor 9, which is installed in the gas reservoir, and a compressor 12, which may be installed on the inlet side of the gas reservoir 4 to increase the pressure in the gas reservoir.

(18) An exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures according to the present invention is described below with reference to FIG. 1a. In this method, the gas or gas mixture flows from a gas reservoir 4 through a critical nozzle 6 and past a microthermal sensor 7, with the same mass flow being applied to the critical nozzle and the microthermal sensor. The pressure drop in gas reservoir 4 is measured as a function of time and a first gas property factor *, dependent on a first group of physical properties of the gas or gas mixture, is determined on the basis of the measured values of the pressure drop, with the first gas property factor being derived, for example, from a time constant of the pressure drop. A second gas property factor , dependent on a second group of physical properties of the gas or gas mixture, is calculated from the flow signal of the microthermal sensor 7, with the second gas property factor including, for example, the heat capacity c.sub.p of the gas or gas mixture, or being dependent on the same. Next, the thermal conductivity of the gas or gas mixture is determined with the aid of the microthermal sensor 7, and the desired physical property or quantity relevant to combustion is determined by aid of correlation on the basis of the first and/or second gas property factor *, and the thermal conductivity.

(19) Other advantageous embodiments and variants of the method are described in the preceding sections of the specification. The following description provides additional details on the method that may be used if desired.

(20) Advantageously, the inlet valve 3 and the outlet valve 5 are opened first to allow the gas or gas mixture that is to be measured to flow from the main gas duct 1 through the test line 2 and through the measuring apparatus to ensure that no extraneous gas from a previous measurement remains in the measuring apparatus. The inlet valve and outlet valve can be opened via a control unit. In individual cases, the analyzer unit 11, too, can control the inlet valve and the outlet valve, as shown in FIG. 1a. In this case, the outlet valve 5 closes and the gas reservoir 4, the volume content V of which is known, fills up until the inlet valve 3 is closed. Pressure p and temperature Tin the gas reservoir can be measured with the pressure sensor 8 or the temperature sensor 9, to ensure that the standard volume V.sub.norm of the gas or gas mixture contained in the gas reservoir can be deduced at any time.

(21) $\begin{array}{cc}{V}_{\mathrm{norm}}=\frac{p}{1013.25\mathrm{mbar}}.Math.\frac{273.15K}{T}.Math.V.& \left(17\right)\end{array}$

(22) If the pressure p in the gas reservoir 4 is higher than the pressure p.sub.crit, which is required to critically operate nozzle 6, the outlet valve 5 can be opened again. By preference, the pressure p in the gas reservoir exceeds p.sub.crit by several bars, so that the pressure drop reading can be performed during this phase of overpressure, while nozzle 6 is always operated critically. Outlet valve 5 now closes again, which concludes the pressure drop measurement. By preference, pressure sensor 8 is installed as a differential pressure sensor relative to the outlet 10 of the measuring apparatus. However, it is also possible to provide an additional pressure sensor 8 at the outlet.

(23) During the pressure drop reading, the time-dependent pressure p(t) and the time-dependent temperature T(t) in the pressure reservoir 4 has been measured and recorded by the analyzer unit 11. With these data, the time constant in equation (6) or the gas property factor * in equation (7) is determined in the analyzer unit. At the same time, flow data have been measured with the microthermal sensor 7, which were recorded in turn by the analyzer unit to determine the factor S in equation (9) or the gas property factor in equation (11). Since the inlet valve and the outlet valve close after the pressure drop reading, no gas flows past the microthermal sensor 7 anymore. Now the measurement of the thermal conductivity reading can take place. The thermal conductivity , recorded in turn by the analyzer unit, is determined with the aid of equation (12).

(24) Now the (optional) validation of the gas property factor or * respectively takes place in the analyzer unit 11. Thereafter, depending on the desired quantity relevant to combustion Q, the calculation of this value by aid of equation (15) with the previously determined correlation function Q.sub.corr=.sub.corr(, *, ) is made.

(25) If required, it is possible to provide additionally, as shown in FIG. 1b, a compressor 12, installed, for example, on the inlet side of the gas reservoir 4 to increase the pressure in the gas reservoir.

(26) FIG. 2 shows a second exemplary embodiment of the schematic configuration of a measuring apparatus according to the present invention, which is based on low pressure in the gas reservoir. This so-called low pressure variant is advantageous, for example, for the gas supply to end customers. In the second exemplary embodiment, the measuring apparatus comprises, in addition to the gas reservoir 4, a pressure sensor 8 on the gas reservoir, an analyzer unit 11, which is configured to perform a method according to the present invention, a critical nozzle 6 and a microthermal sensor 7 to measure the flow and the thermal conductivity, in which case the gas reservoir 4 is connected with the critical nozzle 6 and the microthermal sensor 7 for the measurement.

(27) If required, the measuring apparatus may comprise one or more of the following additional components: a vacuum pump 12 connected to the gas reservoir 4 to generate low pressure in the gas reservoir, a test line 2 leading to the gas reservoir 4 and which may be connected with a main gas duct 1 during operation, an inlet valve 3, which may be located in the test line 2 to control the gas supply to the gas reservoir, an outlet valve 5, installed on the outlet side of the gas reservoir to control the flow of gas from the gas reservoir, an outlet 10 for discharging the effluent gas from the measuring apparatus, an additional pressure sensor 8, which may be located in the test line 2 or main gas duct, and a temperature sensor 9, which is installed in the gas reservoir 4.

(28) An exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures according to the present invention is described below with reference to FIG. 2. In this method, the gas or gas mixture flows under pressure through the critical nozzle 6 and past the microthermal sensor 7 into the gas reservoir 4, with the same mass flow being applied to the critical nozzle and the microthermal sensor. The pressure increase in the gas reservoir 4 is measured as a function of time, and a first gas property factor *, dependent on a first group of physical properties of the gas or gas mixture, is determined by reference to the measured values of the pressure increase, with the first gas property factor being derived, for example, from a proportionality constant of the pressure increase. A second gas property factor , dependent on a second group of physical properties of the gas or gas mixture, is calculated from the flow signal of the microthermal sensor 7, with the second gas property factor including, for example, the heat capacity c.sub.p of the gas or gas mixture, or being dependent on the same; Next, the thermal conductivity of the gas or gas mixture is determined with the aid of the microthermal sensor 7, and the desired physical property or quantity relevant to combustion is determined by aid of correlation on the basis of the first and/or second gas property factor *, and the thermal conductivity.

(29) Other advantageous embodiments and variants of the method are described in the preceding sections of the specification. The following description provides additional details on the method that may be used if desired.

(30) In a first step, the pressure in gas reservoir 4 is advantageously decreased to such an extent, for example with a vacuum pump 12, that the critical nozzle 6 can be critically operated; in other words, until the pressure in the gas reservoir is less than half the pressure upstream of the critical nozzle. No high vacuum is required. As long as the pressure p and the temperature T can be measured in the gas reservoir 4, it is possible to calculate the gas standard volume that has flown into the gas reservoir. However, it is an advantage if the pressure is by some factor less than required for critical conditions, because this means that the measurement can consume more time accordingly, which makes it possible to determine the proportionality constant more accurately.

(31) For further details on the methods, which may be used if necessary, reference is made to the specification of the first exemplary embodiment, subject to replacement of the term pressure drop by the term pressure increase, where appropriate.

(32) FIG. 3 shows an exemplary embodiment of a microthermal sensor for use in a measuring apparatus according to the present invention. For example, the microthermal sensor 7 may beas shown in FIG. 3an integrated microthermal CMOS hot-wire anemometer that is installed in a section 2 of the test line during normal operation and that can be supplied with a gas or gas mix flow 2a. The microthermal CMOS hot-wire anemometer comprises a substrate 13, which typically contains a membrane 14, which measures only a few micrometers in thickness. Furthermore, the CMOS hot wire anemometer consists of two thermal elements 15.1 and 15.2 and a heating element 16, which can be placed between the two thermo-elements in the direction of the flow. The two thermo-elements 15.1., 15.2 serve to record the resulting temperature generated due to the heat exchange 15.1a, 15.2a in combination with the gas or gas mixture flow 2a.

(33) For further details on the functioning of the CMOS hot wire anemometer, reference is made to D. Matter, B. Kramer, T. Kleiner, T. Suter, Mikroelektronischer Haushaltsgaszhler mit neuer Technologie (Micro-electronic domestic gas meters using new technologies), in Technisches Messen 71, 3 (2004), pp. 137-146.

(34) FIG. 4 illustrates the directly measured density ratio /.sub.ref (ordinate) as a function of the correlated density ratio .sub.corr/.sub.ref (abscissa) for various gas groups at standard conditions (0 C., 1013.25 mbar), in which case the correlated density ratio was determined with a method or a measuring apparatus in accordance with the present invention. A typical H-gas was used as a reference gas.

(35) The measuring apparatus described above for determining physical properties and/or quantities relevant to combustion of a gas or gas mixture belongs to a new category, namely Measurement of the pressure drop or pressure increase in a gas reservoir, wherein the gas flows through a critical nozzle, as well as measurement of thermal conductivity and of flow with the aid of a microthermal sensor, and data validation by summation of the flow values. The components used are inexpensive, which makes it possible to develop new markets, where currently no gas quality sensors are being used for cost reasons. From an accuracy perspective, only a few limitations compared to more expensive, commercially available devices are to be expected, since in this case, too, at least three independent measured variables are being used for the correlation.

(36) Furthermore, the invention comprises in a second embodiment the use of a gas reservoir and a critical nozzle for determining physical properties and/or quantities relevant to combustion of a gas or gas mixture, or a method in which a gas reservoir and a critical nozzle for determining physical properties and/or quantities relevant to combustion of a gas or gas mixture are used, wherein the gas or gas mixture flows under pressure from the gas reservoir through the critical nozzle; in this case, the pressure drop in the reservoir is measured as a function of time, a gas property factor *, dependent on the physical properties of the gas or gas mixture, which is derived, for example, from a time constant of the pressure drop, is determined on the basis of the measured variables of the pressure drop, and a desired physical property or quantity relevant to combustion is determined from the gas property factor * through correlation.

(37) The second embodiment of the invention described above can also be seen as a distinct, independent invention.

(38) FIG. 5a shows an exemplary embodiment of a schematic configuration of a measuring apparatus according to the second embodiment of the present invention in which the pressure in the main gas duct 1 is higher than the critical pressure for the critical nozzle 6 of the measuring apparatus (high-pressure variation). In the exemplary embodiment the measuring apparatus, in addition to the critical nozzle 6, consists of an analyzer unit 11, which is configured for carrying out a method according to the second embodiment of the invention, and a gas reservoir 4, which is equipped with a pressure sensor 8, in which case the gas reservoir 4 is connected to the critical nozzle 6 for measurement purposes.

(39) If required, the measuring apparatus may comprise one or more of the following additional components: a test line 2, which leads to the gas reservoir 4, and which may be connected to a main gas duct 1 during operation, an inlet valve 3, which may be located in the test line 2 to control the gas supply to the gas reservoir, an outlet valve 5, installed on the outlet side of the gas reservoir to control the flow of gas from the gas reservoir, an outlet 10 for discharging the effluent gas from the measuring apparatus, an additional pressure sensor 8, which may be installed on the outlet 10, a temperature sensor 9, which is installed in the gas reservoir, and a compressor 12, which may be located on the inlet side of the gas reservoir 4 to increase the pressure in the gas reservoir.

(40) An exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures according to the second embodiment of the invention is described below with reference to FIG. 5a. In this exemplary embodiment, the gas or gas mixture flows from the gas reservoir 4 through the critical nozzle 6. The pressure drop in gas reservoir 4 is measured as a function of time and a first gas property factor *, dependent on a first group of physical properties of the gas or gas mixture, is determined on the basis of the measured values of the pressure drop, with the gas property factor being derived, for example, from a time constant of the pressure drop. Furthermore, a desired physical property or quantity relevant to combustion is determined on the basis of the gas property factor * by aid of correlation.

(41) Advantageously, in the second embodiment of the invention, binary gas mixtures are analysed in regard to their content of the two components forming the gas mixture, since the gas property factor * is intrinsically a continuous function of the gas content x % or (1x %). With the knowledge of content x % or (1x %), it is then possible to determine physical properties and/or quantities relevant to combustion of the binary gas mixture from sets of tables or by aid of corresponding calculation programs. Of course, it is also possible to directly correlate these physical properties and/or quantities relevant to combustion of the binary gas mixture with the gas property factor *.

(42) In an embodiment of the method, it is thus possible to determine the percentage of a component contained in a binary gas mixture, in which case the variable to be correlated corresponds either to the percentage of the component in the composition (x %) and/or any other physical property of the binary gas mixture.

(43) Other advantageous embodiments and variants of the method are described in the preceding sections of the specification. The following description provides additional details on the method that may be used if desired.

(44) Advantageously, the inlet valve 3 and the outlet valve 5 are opened first to allow the gas or gas mixture that is to be measured to flow from the main gas duct 1 through the test line 2 and through the measuring apparatus to ensure that no extraneous gas from a previous measurement remains in the measuring apparatus. The inlet valve and outlet valve can be opened via a control unit. In individual cases, the analyzer unit 11, too, can control the inlet valve and the outlet valve, as shown in FIG. 5a. In this case, the outlet valve 5 is closed and the gas reservoir 4, the volume content V of which is known, fills up until the inlet valve 3 is covered. Pressure p and temperature Tin the gas reservoir can be measured with the pressure sensor 8 or the temperature sensor 9, to ensure that the standard volume V.sub.norm of the gas or gas mixture contained in the gas reservoir can be deduced at any time.

(45) $\begin{array}{cc}{V}_{\mathrm{norm}}=\frac{p}{1013.25\mathrm{mbar}}.Math.\frac{273.15K}{T}.Math.V.& \left(17\right)\end{array}$

(46) If the pressure p in the gas reservoir 4 is higher than the pressure p.sub.crit, which is required to critically operate nozzle 6, the outlet valve 5 can be opened again. By preference, the pressure p in the gas reservoir exceeds p.sub.crit by several bars, so that the pressure drop reading can be performed during this phase of overpressure, while nozzle 6 is always operated critically. Outlet valve 5 now closes again, which concludes the pressure drop measurement. By preference, pressure sensor 8 is installed as a differential pressure sensor relative to outlet 10 of the measuring apparatus. However, it is also possible to provide an additional pressure sensor 8 at the outlet.

(47) During the pressure drop reading, the time-dependent pressure p(t) and the time-dependent temperature T(t) in the pressure reservoir 4 has been measured and recorded by the analyzer unit 11. With these data, the time constant in equation (6) or the gas property factor * in equation (6) and the gas property factor * in equation (7) or equation (7) is determined in the analyzer unit.

(48) Depending on the desired quantity relevant to combustion Q, this value is now calculated on the basis of equation (15) with the previously determined correlation function Q.sub.corr=.sub.corr(* ) in analyzer unit 11.

(49) If required, it is possible to provide additionally, as shown in FIG. 5b, a compressor 12, installed, for example, on the inlet side of the gas reservoir 4 to increase the pressure in the gas reservoir.

(50) FIG. 6 shows a second exemplary embodiment of the schematic configuration of a measuring apparatus according to the second embodiment of the invention, which is based on low pressure in the gas reservoir. This so-called low pressure variant is advantageous, for example, for the gas supply to end customers. In the second exemplary embodiment, the measuring apparatus, in addition to the gas reservoir 4, comprises a pressure sensor 8, installed on the gas reservoir, an analyzer unit 11, which is configured for carrying out a method according to the second embodiment of the invention, and a critical nozzle 6, in which case the gas reservoir 4 is connected to the critical nozzle 6 for measurement purposes.

(51) If required, the measuring apparatus may comprise one or more of the following additional components: a vacuum pump 12 connected to the gas reservoir 4 to generate low pressure in the gas reservoir, a test line 2 leading to the gas reservoir 4 and which may be connected with a main gas duct 1 during operation, an inlet valve 3, which may be located in the test line 2 to control the gas supply to the gas reservoir, an outlet valve 5, installed on the outlet side of the gas reservoir to control the flow of gas from the gas reservoir, an outlet 10 for discharging the effluent gas from the measuring apparatus, an additional pressure sensor 8, which may be located in the test line 2 or main gas duct, and a temperature sensor 9, which is installed in the gas reservoir 4.

(52) Another exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas and mixtures according to the second embodiment of the invention is described below with reference to FIG. 6. In this exemplary embodiment, the gas or gas mixture flows under pressure through the critical nozzle 6 into the gas reservoir 4. The pressure increase in the gas reservoir 4 is measured as a function of time, and a gas property factor *, dependent on a first group of physical properties of the gas or gas mixture, is determined by reference to the measured values of the pressure increase, with the gas property factor being derived, for example, from a proportionality constant of the pressure increase. A desired physical property or quantity relevant to combustion is determined on the basis of the gas property factor * by aid of correlation.

(53) Other advantageous embodiments and variants of the method are described in the preceding sections of the specification. The following description provides additional details on the method that may be used if desired.

(54) In a preceding step, the pressure in gas reservoir 4 is advantageously decreased to such an extent, for example with a vacuum pump 12, that the critical nozzle 6 can be critically operated; in other words, until the pressure in the gas reservoir is less than half the pressure upstream of the critical nozzle. No high vacuum is required. As long as the pressure p and the temperature T can be measured in the gas reservoir 4, it is possible to calculate the gas standard volume that has flown into the gas reservoir. However, it is an advantage, if the pressure is by some factor less than strictly required for critical conditions, because this means that the measurement proceeds during more time accordingly, which makes it possible to determine the proportionality constant more accurately.

(55) For further details on the methods, which may be used if necessary, reference is made to the specification of the first exemplary embodiment, subject to replacement of the term pressure drop by the term pressure increase, where appropriate.

(56) FIG. 7 illustrates the directly measured methane content n.sub.CH4 (ordinate) as a function of the correlated methane content n.sub.CH4 corr (abscissa) for a binary raw biogas, composed of methane and carbon dioxide, at standard conditions (0 C., 1013.25 mbar), in which case the correlated methane content was calculated with a method or a measuring apparatus in accordance with the second embodiment of the invention. A typical H-gas was used as a reference gas. The desired variable Q (in this case, the methane content n.sub.CH4 corr in x %) is advantageously determined with the aid of the correlation function Q.sub.corr=a+b.Math.*+c.Math.*.sup.2+d.Math.*.sup.3, in the illustrated example, numerically as a=7.82, b=22.7, c=20.4 and d=6.45.

(57) The measuring apparatus described above for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures belongs to a new category, namely Measurement of the pressure drop or pressure increase in a gas reservoir, wherein the gas flows through a critical nozzle. The components used are inexpensive, which makes it possible to develop new markets, where currently no gas quality sensors are being used for cost reasons. From an accuracy perspective, only a few limitations compared to more expensive, commercially available devices are to be expected, since in this case only one independent measured value, instead of three, is used for the correlation.

(58) In addition, the invention encompasses in a third embodiment the use of a gas reservoir and of a microthermal sensor calibrated for a specific calibration gas or gas mixtures to determine physical properties and/or quantities relevant to combustion of gas or gas mixtures; in this set-up a gas reservoir and a microthermal sensor calibrated for a specific calibration gas or gas mixture for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures are used, with the gas or gas mixture flowing under pressure from the gas reservoir past the microthermal sensor, in which case the volume flow v.sub.x.Math.A, determined by the microthermal sensor calibrated for a specific calibration gas or gas mixture, is summed up and compared to the gas volume released from the gas reservoir; from the comparison of the two volumes, a gas property factor S/v.sub.x, dependent on the physical properties of the gas or gas mixture, is determined, in which v.sub.x represents the flow rate of the released gas volume and in which a desired physical property or quantity relevant to combustion is determined from the gas property factor, which may consist, for example, of S/v.sub.x=c.sub.p.Math./ (see equation (9)), through correlation.

(59) The third embodiment of the invention described above can also be seen as a distinct, independent invention.

(60) FIG. 8a shows an exemplary embodiment of the schematic configuration of a measuring apparatus in accordance with the third embodiment of the invention in case the main gas duct 1 is under pressure (high-pressure variant). In the exemplary embodiment, the measuring apparatus consists of an analyzer unit 11, which is configured for carrying out the method in accordance with the third embodiment of the invention, a gas reservoir 4, which is equipped with a pressure sensor 8 and a microthermal sensor 7 to measure the flow and thermal conductivity, in which case the gas reservoir 4 is connected to the microthermal sensor 7 for measurement purposes.

(61) If required, the measuring apparatus may comprise one or more of the following additional components: a test line 2, which leads to the gas reservoir 4, and which may be connected to a main gas duct 1 during operation, an inlet valve 3, which may be located in the test line 2 to control the gas supply to the gas reservoir, an outlet valve 5, installed on the outlet side of the gas reservoir to control the flow of gas from the gas reservoir, an outlet 10 for discharging the effluent gas from the measuring apparatus, an additional pressure sensor 8, which may be installed on the outlet 10, a temperature sensor 9, which is installed in the gas reservoir, and a compressor 12, which may be located on the inlet side of the gas reservoir 4 to increase the pressure in the gas reservoir.

(62) An exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures in accordance with the third embodiment of the invention is described below with reference to FIG. 8a. In the method, the gas or gas mixture flows under pressure from the gas reservoir 4 past the microthermal sensor 7, calibrated for a specific calibration gas or gas mixture, in which case the volume flow v.sub.x.Math.A is summed up and compared to the gas volume released from the gas reservoir; from the comparison of the two volumes a gas property factor S/v.sub.x, dependent on the physical properties of the gas or gas mixture, is determined, in which v.sub.x represents the flow rate of the released gas volume, and in which the desired physical property or quantity relevant to combustion is determined from the gas property factor, which may consist, for example, of S/v.sub.x=c.sub.p.Math./ (see equation (9)), through correlation.

(63) In an advantageous embodiment of the method, the thermal conductivity of the gas or gas mixture is determined additionally with the aid of the microthermal sensor 7.

(64) Advantageously, with the third embodiment of the invention, natural gas mixtures are examined as to their classification as H-gases or L-gases (gases with a high (H) or low (L) calorific value), since the gas property factor, which may consist, for example, of S/v.sub.x=c.sub.p.Math./ (see equation (9)), corresponds to the reciprocal value of the thermal diffusivity of the gas mixture, with the aid of whichtogether with the thermal conductivity , which can be measured separately with the microthermal sensora distinction between H-gas group and L-gas group can be made.

(65) The classification of a natural gas mixture as belonging to the H-gas or L-gas group can be determined, for example, by identifying the gas property factor (S/v.sub.x) with the reciprocal value of the thermal diffusivity c.sub.p.Math./, and wherein the classification is made, subject to thermal conductivity, on the basis of a limit value for the thermal diffusivity; above the limit value, a gas mixture is classified as L-gas, and below the limit value, as H-gas.

(66) Thus, in an embodiment variant of the method, the thermal conductivity of the gas or gas mixture is determined additionally with the aid of the microthermal sensor 7, and a classification of the measured gas as H-gas or L-gas is made in conjunction with the gas property factor S/v.sub.x=c.sub.p.Math./.

(67) Other advantageous embodiments and variants of the method are described in the preceding sections of the specification. The following description provides additional details on the method that may be used if desired.

(68) Advantageously, the inlet valve 3 and the outlet valve 5 are opened first to allow the gas or gas mixture that is to be measured flow from the main gas duct 1 through the test line 2 and through the measuring apparatus to ensure that no extraneous gas from a previous measurement remains in the measuring apparatus. The inlet valve and outlet valve can be opened via a control unit. In individual cases, the analyzer unit 11, too, can control the inlet valve and the outlet valve, as shown in FIG. 8a. In this case, the outlet valve 5 is closed and the gas reservoir 4, the volume content V of which is known, fills up until the inlet valve 3 is closed. Pressure p and temperature Tin the gas reservoir can be measured with the pressure sensor 8 or the temperature sensor 9, to ensure that the standard volume V.sub.norm of the gas or gas mixture contained in the gas reservoir can be deduced at any time.

(69) $\begin{array}{cc}{V}_{\mathrm{norm}}=\frac{p}{1013.25\mathrm{mbar}}.Math.\frac{273.15K}{T}.Math.V.& \left(17\right)\end{array}$

(70) The outlet valve 5 can now be opened again. By preference, the pressure p in the gas reservoir 4 is higher than the downstream pressure after the gas reservoir by such a rate that the timespan in which the gas from the gas reservoir 4 flows past the microthermal sensor 7 is long enough to ensure that the volume flow v.sub.x.Math.A can be summed up with sufficient accuracy. Outlet valve 5 now closes again, which concludes the flow measurement. By preference, pressure sensor 8 is installed as a differential pressure sensor opposite outlet of the measuring apparatus. However, it is also possible to provide an additional pressure sensor 8 at the outlet.

(71) Flow data have been measured with the microthermal sensor 7 during the flow measurement and recorded by the analyzer unit 11 to determine factor S in equation (9). Since the inlet valve and the outlet valve close after the flow reading, no gas flows past the microthermal sensor 7 anymore. Now the measurement of the thermal conductivity reading can take place. The thermal conductivity , recorded in turn by the analyzer unit, is determined with the aid of equation (12).

(72) With these data, the volume flow is summed up in the analyzer unit 11 to form volume V.sub.sum and to compare it to the gas volume V.sub.diff released from the gas reservoir. Based on the comparison of these two volumes, it is now possible to determine a gas property factor S/v.sub.x, dependent on the physical properties of the gas or gas mixture, in which v.sub.x represents the flow rate derived from the released gas volume. For practical reasons, the volumes for the comparison are converted to standard conditions for the purposes of the comparison by aid of equation (17), with the result that v.sub.x consists of
v.sub.x=v.sub.x.Math.V.sub.diff.sup.norm/V.sub.sum.sup.norm(18)
with the released gas volume V.sub.diff.sup.norm converted to standard conditions and the accumulated volume converted to standard conditions V.sub.sum.sup.norm. Thereafter, depending on the desired quantity Q relevant to combustion, this value is now calculated in the analyzer unit 11 with the aid of equation (15) with the previously determined correlation function Q.sub.corr=.sub.corr(S/v.sub.x), or the value of S/v.sub.x is being used to classify, in conjunction with the thermal conductivity , a natural gas mixture in the category H-gas or L-gas.

(73) If required, it is possible to provide additionally, as shown in FIG. 8b, a compressor 12, installed, for example, on the inlet side of the gas reservoir 4 to increase the pressure in the gas reservoir.

(74) FIG. 9 shows a second exemplary embodiment of the schematic configuration of a measuring apparatus according to the third embodiment of the invention, which is based on low pressure in the gas reservoir. This so-called low pressure variant is advantageous, for example, for the gas supply to end customers. In the second exemplary embodiment, the measuring apparatus comprises, in addition to the gas reservoir 4, a pressure sensor 8 on the gas reservoir, an analyzer unit 11, which is configured to carry out a method according to the third embodiment of the invention and a microthermal sensor 7 to measure the flow and the thermal conductivity, in which case the gas reservoir 4 is connected to the microthermal sensor 7 for the purposes of the measurement.

(75) If required, the measuring apparatus may comprise one or more of the following additional components: a vacuum pump 12 connected to the gas reservoir 4 to generate low pressure in the gas reservoir, a test line 2 leading to the gas reservoir 4 and which may be connected with a main gas duct 1 during operation, an inlet valve 3, which may be located in the test line 2 to control the gas supply to the gas reservoir, an outlet valve 5, installed on the outlet side of the gas reservoir to control the flow of gas from the gas reservoir, an outlet 10 for discharging the effluent gas from the measuring apparatus, an additional pressure sensor 8, which may be located in the test line 2 or main gas duct, and a temperature sensor 9, which is installed in the gas reservoir 4.

(76) Another exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas and mixtures in accordance with the third embodiment of the invention is described below with reference to FIG. 9. In this exemplary embodiment, the gas or gas mixtures flows at a pressure that is typically higher than the downstream pressure after the gas reservoir by such a rate that the timespan in which the gas from the gas reservoir 4 flows past the microthermal sensor 7 is long enough to ensure that the volume flow v.sub.x.Math.A can be summed up with sufficient accuracy. The summed-up volume flow V.sub.sum, is compared to the gas volume V.sub.diff released from the gas reservoir, and from the comparison of the two volumes, a gas property factor S/v.sub.x, dependent on the physical properties of the gas or gas mixture, is determined, in which v.sub.x represents the flow rate of the released gas volume, and in which the desired physical property or quantity relevant to combustion is determined from the gas property factor, which may consist, for example, of S/v=c.sub.p.Math./ (see equation (9)), through correlation.

(77) A further exemplary embodiment of the method for determining physical properties and/or quantities relevant to combustion of gas and mixtures in accordance with the third embodiment of the invention is described below with reference to FIG. 9. In this exemplary embodiment, the gas or gas mixtures flows under pressure past the microthermal sensor 7 into the gas reservoir 4. The volume flow v.sub.x.Math.A of the gas or gas mixture, determined on the basis of the flow rate (v.sub.x) measured with the microthermal sensor, is summed up and the summed up volume flow compared to the gas volume V.sub.diff fed into the gas reservoir. From the comparison of the two volumes, a gas property factor S/v.sub.x, dependent on the physical properties of the gas or gas mixture, is derived, in which v.sub.x represents the flow rate determined from the fed gas volume, and a desired physical property or quantity relevant to combustion is determined from the gas property factor through correlation.

(78) Thus, in an advantageous embodiment of the method, the thermal conductivity of the gas or gas mixture is determined with the aid the microthermal sensor 7, and a classification of the measured gas as H-gas or L-gas is made, for example, in conjunction with the gas property factor S/v.sub.x=c.sub.p.Math./.

(79) For other advantageous embodiments and variants of the method, and for further details on the methods, which may be used if required, reference is made to the preceding sections of the specification, subject to replacement of the term pressure drop by the term pressure increase, where appropriate.

(80) FIG. 10 illustrates how a classification as H-gas or L-gas can be made by means of known thermal conductivities (abscissa) and thermal diffusivities /(c.sub.p), also referred to as temperature conductivities (ordinate). L-gases above the H/L-gas separation line typically have higher thermal diffusivities than H-gases with the same thermal conductivity below the separation line (double arrow at x1.024). Since the gas property factor S/v.sub.x=c.sub.p.Math./ is essentially equivalent to the reciprocal value of the thermal diffusivity of the gas mixture, it is thus possible to make the distinction between H-gas and L-gas with the aid of the additionally measured thermal conductivity . All values are shown at standard conditions (0 C., 1013.25 mbar). A typical H-gas was used as reference gas (dashed line for the coordinate (1.00,1.00)).

(81) The measuring apparatus described above for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures belongs to a new category, namely Thermal conductivity and flow measurement with the aid of a microthermal sensor, cumulative adding of the flow values and a comparison of the released volume from a reference volume. Thereafter, classification of natural gases as H-gas or L-gas. The components used are inexpensive, which makes it possible to develop new markets, where currently no gas quality sensors are being used for cost reasons. From an accuracy perspective, only a few limitations compared to more expensive, commercially available devices are to be expected, since this apparatus uses only two instead of three independent measured variables for the correlation.