METHOD FOR DETERMINING THE VOLUMETRIC FLOW RATE OF A FLUID MEDIUM THROUGH A MEASURING SECTION AND ASSOCIATED MEASURING DEVICE

Abstract

A method for determining the volumetric flow rate of a fluid medium through a measuring section in a substantially gas-type-independent manner, includes heating the medium in a pulsed manner by using a heating element, detecting a first point in time at which a temperature maximum occurs at a first temperature sensor, the first temperature sensor being disposed adjacently upstream or downstream of the heating element, detecting a second point in time at which a temperature maximum occurs at a second temperature sensor, the second temperature sensor being disposed downstream of the heating element, the second temperature sensor being further away from the heating element than the first temperature sensor, and ascertaining a time difference between the first and second points in time. The volumetric flow rate is determined in dependence on the time difference. A device for carrying out the method is also provided.

Claims

1. A method for a substantially gas-type-independent determination of a volumetric flow rate of a fluid medium passing through a measuring section, the method comprising the following steps: using a heating element to heat the medium in a pulsed manner; providing a first temperature sensor adjacent the heating element and upstream or downstream of the heating element in a medium flow direction; detecting a first point in time at which a temperature maximum occurs at the first temperature sensor; providing a second temperature sensor downstream of the heating element in the medium flow direction and further away from the heating element than the first temperature sensor; detecting a second point in time at which a temperature maximum occurs at the second temperature sensor; ascertaining a time difference between the first and second points in time; and determining a volumetric flow rate depending on the time difference.

2. The method according to claim 1, which further comprises placing the first temperature sensor at a distance of less than 100 m from the heating element.

3. The method according to claim 1, which further comprises placing the first temperature sensor at a distance of between 15 m and 50 m from the heating element.

4. The method according to claim 1, which further comprises placing the first temperature sensor at a distance of between 20 m and 30 m from the heating element.

5. The method according to claim 1, which further comprises using a predefined calibration curve for determining the volumetric flow rate from the time difference.

6. The method according to claim 5, which further comprises using an identical calibration curve for gases having different thermal diffusivities.

7. The method according to claim 5, which further comprises using a gas mixture as the medium, and using an identical calibration curve for gas mixtures having different proportions of hydrogen.

8. The method according to claim 5, which further comprises using an identical calibration curve for at least one of a plurality of different gases or a plurality of gas mixtures.

9. The method according to claim 1, which further comprises determining a gas parameter in dependence on a time interval between a point in time of heating and the detected first point in time.

10. The method according to claim 9, which further comprises determining a further gas parameter in dependence on a temperature value at a temperature maximum of the first temperature sensor and a temperature value at a temperature maximum of the second temperature sensor.

11. A measuring device for ascertaining a gas-type-independent volumetric flow rate of a fluid medium, the measuring device comprising: a measuring section having a heating element for heating the medium in a pulsed manner; a first temperature sensor disposed adjacent said heating element and upstream or downstream of said heating element in a medium flow direction for detecting a first point in time at which a temperature maximum occurs at said first temperature sensor; a second temperature sensor disposed downstream of said heating element in said medium flow direction for detecting a second point in time at which a temperature maximum occurs at said second temperature sensor; said second temperature sensor being disposed further away from said heating element than said first temperature sensor; and a control device for ascertaining a time difference between the first and second points in time and determining a volumetric flow rate depending on the time difference.

12. The measuring device according to claim 11, wherein said heating element and said first temperature sensor are spaced apart by a distance of less than 100 m.

13. The measuring device according to claim 11, wherein said heating element and said first temperature sensor are spaced apart by a distance of between 15 m and 50 m.

14. The measuring device according to claim 11, wherein said heating element and said first temperature sensor are spaced apart by a distance of between 20 m and 30 m.

15. The measuring device according to claim 11, wherein said first and second temperature sensors are spaced apart by a distance of at least 100 m.

16. The measuring device according to claim 11, wherein said first and second temperature sensors are spaced apart by a distance of between 150 m and 550 m.

17. The measuring device according to claim 11, wherein said first and second temperature sensors are spaced apart by a distance of between 150 m and 350 m.

18. The measuring device according to claim 11, wherein said first temperature sensor and said second temperature sensor are: formed by wires or thin films running through said measuring channel in an exposed manner, or disposed on a membrane situated in said measuring channel, or embedded in a membrane situated in said measuring channel.

19. The measuring device according to claim 11, wherein at least one of said heating element or said first temperature sensor or said second temperature sensor is formed of a metal, a metallic alloy or a semiconductor material.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] FIG. 1 is a diagrammatic, top-plan view of one exemplary embodiment of a measuring device according to the invention;

[0030] FIG. 2 is a perspective view of the measuring device shown in FIG. 1;

[0031] FIG. 3 is a flow diagram of one exemplary embodiment of a method according to the invention;

[0032] FIG. 4 is a diagram showing a temporal temperature profile at the heating element, the first temperature sensor and the second temperature sensor in the exemplary embodiment of the method according to the invention;

[0033] FIG. 5 is a diagram showing a relationship between a volumetric flow rate and the time interval between the point in time of heating and the detected first point in time for three different fluid media;

[0034] FIG. 6 is a diagram showing a relationship between the volumetric flow and the time interval between the point in time of heating and the second point in time for the three different fluid media; and

[0035] FIG. 7 is a diagram showing a relationship between the volumetric flow rate and the time difference between the first and second points in time for the three different fluid media.

DETAILED DESCRIPTION OF THE INVENTION

[0036] Referring now to the figures of the drawings in detail and first, particularly, to FIGS. 1 and 2 thereof, there is seen a measuring device 1 for ascertaining a gas-type-independent volumetric flow rate of a fluid medium. In this case, FIG. 1 shows a diagrammatic illustration from above and FIG. 2 shows a perspective view of the measuring device 1. A fluid medium 2, which is illustrated schematically as arrows in FIGS. 1 and 2, flows through a measuring section of the measuring device 1. In this case, the fluid medium 2 is guided in a laminar fashion in a non-illustrated measuring channel formed by a tube having a substantially rectangular cross section. In this case, the fluid medium 2 passes over a heating element 4, a first temperature sensor 5 disposed downstream of the heating element 4, and a second temperature sensor 6 spaced apart from the heating element 4 at a greater distance than the first temperature sensor 5. In this case, the heating element 4 and the temperature sensors 5, 6 are embodied as wires extending through the measuring channel between two substrates 3 in an exposed manner. In an alternative embodiment of the measuring device 1, it would also be possible for the temperature sensors 5, 6 and the heating element 4 to be embodied as thin films likewise running through the measuring channel in an exposed manner. It would likewise be possible alternatively to place the temperature sensors 5, 6 and the heating element 4 as wires or thin films on a thin membrane composed of a material having a low thermal diffusivity.

[0037] The heating element 4 and the first temperature sensor 5 are disposed at a distance of less than 50 m from one another. That distance is indicated by a double-headed arrow 7. A distance between the second temperature sensor 6 and the heating element 4, which distance is indicated by an arrow 8, is significantly greater than the distance between the first temperature sensor 5 and the heating element, namely e.g. 450 m.

[0038] In order to measure a volumetric flow rate, a control device 10 energizes the heating element 4 with current pulses spaced apart in time, as a result of which the temperature at the heating element 4 is raised virtually in a pulsed manner for a short time period of less than 100 s. After each heating pulse, the control device detects the temporal profiles of the temperatures at the first temperature sensor 5 and at the second temperature sensor 6. Due to the small distance between the temperature sensor 5 and the heating element 4, the temporal temperature profile at the temperature sensor 5 is virtually independent of the flow velocity or the volumetric flow rate of the fluid medium 2. Since the second temperature sensor 6 is significantly further away from the heating element 4, the temporal profile at the second temperature sensor 6 is greatly influenced by the flow velocity of the fluid medium and thus by the volumetric flow rate. As explained in even greater detail below with reference to FIG. 3, it is thus possible to determine the volumetric flow of the fluid medium in a gas-type-independent manner from the time difference between a first point in time, at which a temperature maximum occurs at the first temperature sensor 5, and a second point in time, at which a temperature maximum occurs at the second temperature sensor 6.

[0039] FIG. 1 additionally shows an alternative position for the first temperature sensor 5 as a dash-dotted line 9. Since the first temperature sensor 5 is disposed very close to the heating element 4, it is unimportant whether it is disposed upstream or downstream in terms of the gas flow.

[0040] FIG. 3 schematically shows a flow diagram of a method for determining the volumetric flow rate of a fluid medium through a measuring section in a substantially gas-type-independent manner. In a step S1, a control device 10 energizes a heating element 4 with a short current pulse of less than 100 s, whereby the temperature at the heating element changes virtually in a pulsed manner.

[0041] Afterward, the control device 10 simultaneously detects the temperature profile at the first temperature sensor 5 in a step S2 and the temperature profile at the second temperature sensor 6 in a step S3. The change in the temperatures at the temperature sensors 5, 6 are influenced on one hand by processes which also take place in the stationary medium, for example by diffusion, and on the other hand by the movement of the fluid medium over the heating element 4 in the direction of the second temperature sensor 6. The temperature profile at the heating element 4 and the measurement values detected by the control device 10 for the temperature sensor 5 and the temperature sensor 6 are shown schematically in FIG. 4 for a flow velocity of a fluid medium. In this case, the solid line shows the pulse-like temperature change at the heating element 4. The dashed line shows the measured temperature profile at the first temperature sensor 5 and the dash-dotted line shows the temperature profile at the second temperature sensor 6. A reduction of the maximum detected temperature and a widening of the temperature maximum can in each case be discerned therein between the temperature profile at the heating element 4, the temperature profile at the first temperature sensor 5 and the temperature profile at the second temperature sensor 6.

[0042] A temporal spacing between the first point in time at which the temperature distribution has a maximum and the beginning of the heating pulse, that is to say the start of the pulse of the solid line in FIG. 4, is determined in a step S4 from the temporal profile of the temperature at the first temperature sensor 5, that is to say for example from the dashed line from FIG. 4.

[0043] FIG. 5 shows by way of example the relationship between a volumetric flow rate and a time interval between the point in time of heating and the detected first point in time for three different gases. The measurement values for nitrogen are shown as rhombi, the measurement values for methane are shown as crosses, and the measurement values for a further natural gas are shown as circles. It can be discerned in this case that the time interval between the point in time of heating and the first detected point in time is substantially independent of the volumetric flow rate of the gas.

[0044] A step S5 involves ascertaining a second point in time, at which the temperature profile at the second temperature sensor 6, that is to say for example the dash-dotted line in FIG. 4, has a maximum.

[0045] FIG. 6 shows the time intervals between the point in time of heating and the second point in time, once again for the three different gases shown in FIG. 5. It can be discerned in this case that the time difference shown in FIG. 6 is primarily dependent on the volumetric flow rate of the gases, but the time intervals, depending on the gas, have a deviation of up to approximately 20% for the same volumetric flow rate. A determination of the volumetric flow rate with a common calibration curve for the gas types shown would thus lead to relatively large measurement errors.

[0046] A step S6 involves calculating the time difference between the first and second points in time. This corresponds to subtracting the measurement values shown in FIG. 5 from the measurement values shown in FIG. 6. The result of this calculation is shown in FIG. 7 once again for the three gases and for various volumetric flow rates. The time differences for the different gases shown therein are virtually identical in the case of each volumetric flow rate. Therefore, in a step S7 it is possible to use a common calibration curve which is dependent exclusively on properties of the measuring device 1 and of the surrounding measuring channel and which is stored in the control device 10, in order to convert the time difference calculated in step S6 into a volumetric flow rate.

[0047] In order to ascertain further parameters of the fluid medium 2 besides the volumetric flow rate, in a step S8 a second calibration curve stored in the control device 10 is used to ascertain a first gas parameter, namely a thermal diffusivity, from the time intervaldetermined in step S4between the point in time of heating and the first detected point in time. In this case, advantageously, thermal diffusivities for a plurality of heating intervals are calculated and averaged in order to minimize measurement errors.

[0048] In addition, the temperature value at the temperature maximum of the first temperature sensor, that is to say the maximum of the dashed curve in FIG. 4, is ascertained in a step S9 and the temperature value at the temperature maximum of the second temperature sensor 6, that is to say the maximum of the dash-dotted line in FIG. 4, is ascertained in a step S10. A step S11 involves determining a further gas parameter, namely the thermal conductivity, and additionally ascertaining what gas or what gas mixture forms the fluid medium, from these two temperature values and the gas parameter determined in step S8. In particular, multidimensional calibration curves or tables of values can be used for this purpose. In particular, it is possible, however, to determine a thermal conductivity from the temperature values calculated in step S9 and step S10 and to determine a gas type or the composition of a gas mixture from the thermal diffusivity determined in step S8 and the thermal conductivity determined. In particular, a proportion of hydrogen can be determined in this case.