EP2972129A1

EP2972129A1 - Thermal flow sensor for determining a gas or the composition of a gas mixture and the flow speed of the gas or the gas mixture

Info

Publication number: EP2972129A1
Application number: EP14706560.1A
Authority: EP
Inventors: Christoph Hepp; Florian Krogmann
Original assignee: Innovative Sensor Technology IST AG
Current assignee: Innovative Sensor Technology IST AG
Priority date: 2013-03-11
Filing date: 2014-02-24
Publication date: 2016-01-20
Also published as: DE102013102398A1; WO2014139786A1; DE102013102398B4; US20160025660A1; US10101285B2

Abstract

The invention relates to a thermal flow sensor (1) for determining a gas or the composition of a gas mixture (2) and the flow speed of the gas or the gas mixture, comprising: - a substrate (3) onto which at least one first dielectric layer (6) is applied; - at least one heating structure (8) which is applied onto the first dielectric layer (6) and which is used to heat the gas or the gas mixture (2); - at least one first temperature sensor element (9) which is applied onto the first dielectric layer (6) at a distance from the heating structure (8) and which detects the temperature of the gas or the gas mixture (2) heated on the heating structure (8); - a regulating device (11) which regulates the heating structure (8) in a first operating state such that the heating structure has a specified temperature and which regulates the heating structure (8) in a second operating state such that a power supplied to the heating structure (8) corresponds to a specified power; and - an analyzing unit (12) which ascertains at least one physical property of the present gas or the gas mixture (2) using the operating states and which determines the present gas or the composition of the gas mixture (2) and the flow speed of the gas or the gas mixture using said physical property.

Description

Thermal flow sensor for determining a gas or the composition of a gas mixture, and its flow velocity

The invention relates to a thermal flow sensor for determining a gas or the composition of a gas mixture, as well as its

Flow velocity is used.

Thermal flow sensors are well known in the art.

Such sensors are used, for example, to determine a

Flow rate of a gas used. Since thermal flow sensors physically due to the flow velocity and thermal

Properties of the gas to react by being used is an accurate or accurate determination of the flow rate as the thermal changes

Properties of the gas are no longer possible. Such changes in the thermal properties can occur in the measuring operation of a flow sensor, if, for example, the composition of a gas mixture or the gas itself changes.

Remedy this disadvantage is made by the used thermal

Flow sensor is recalibrated. Typically, a further sensor is used for this purpose, which instead of the flow velocity determines the present gas or the composition of the gas mixture so as to determine the thermal properties of the gas or gas mixture on which the determination of the flow velocity is based. However, as already mentioned, this requires a further sensor without which recalibration would not be feasible.

Alternatively, a recalibration can be carried out by means of special software. For this purpose, the present gas or the composition of the gas mixture must be entered into the software. The disadvantage is thus that a further step on the part of the operator, namely the manual input, must be performed.

It is therefore an object of the invention to propose a thermal flow sensor which has an increased user-friendliness. The object is achieved by a thermal flow sensor and a flow meter.

With regard to the thermal flow sensor, the object is achieved in that the thermal flow sensor for determining a gas or the Composition of a gas mixture, and its flow rate comprises:

a substrate on which at least one first dielectric layer is applied;

- At least one heating structure which is applied to the first dielectric layer and serves to heat the gas or the gas mixture;

at least one first temperature sensor element, which is applied to the first dielectric layer at a distance from the heating structure and detects the temperature of the gas or gas mixture heated on the heating structure;

a control device which controls the heating structure in a first operating state such that the heating structure has a predetermined temperature and in a second operating state controls the heating structure in such a way that a power supplied to the heating structure corresponds to a predetermined power;

- An evaluation unit, which determines by means of the operating conditions, at least one physical property of the present gas or the gas mixture (2) and determines based on this physical property, the present gas or the composition of the gas mixture (2) and its flow velocity.

According to the invention the object is achieved in that the thermal

Flow sensor determines the present gas or the composition of the gas mixture and at the same time the flow rate depending on the gas present or the composition of the gas mixture. For this purpose, the thermal flow sensor determines a physical property, preferably the thermal conductivity, of the gas or gas mixture and determines by means of which the present gas or gas mixture and its flow velocity.

Such thermal flow sensors have the advantage that incorrect measurements of the flow velocity due to the change of the gas or the

Composition of the gas mixture can be avoided, since a change in the gas or the composition of the gas mixture also leads to a change in the thermal conductivity by which the present gas or gas mixture is determined.

For this purpose, the thermal flow sensor to a control device which is switchable at least between two operating states, wherein in the second

Operating state, the power supplied to the heating structure is constantly controlled to a predetermined power, so as to determine the present gas or the composition of a gas mixture and in the first operating state based on the determined present gas or the determined composition of a gas mixture regulates the temperature of the heating structure such that the Heating structure a predetermined Temperature, so that the gas to be heated or gas mixture corresponds at least in the area around the heating structure of a predetermined gas temperature.

In the first operating state, the regulating device regulates the temperature of the heating structure, for example by means of a DC voltage signal or alternatively an AC voltage signal,

such that the heating structure has a predetermined temperature. At a

DC voltage signal can be set the predetermined temperature directly by the DC signal, whereas in an AC signal, the predetermined temperature can be set on the average value of the AC signal.

By inferring the present gas or the gas composition, the thermal flow sensor is thus self-calibrating. In this way, a false measurement of the flow velocity can be avoided when the

Has changed gas composition.

An advantageous embodiment provides that the evaluation unit the

Composition of the gas mixture on the basis of the measured thermal conductivity determined in terms of concentration and that for the concentration determination of the

Composition of the gas mixture, the individual components of the gas mixture of the evaluation must be known. By concentration determination of the composition of the gas mixture to 5% accuracy, the

Flow rate can be determined to 10% of the measured value, preferably 5% of the measured value.

The possibility of concentration determination of the present gas mixture and the simultaneous determination of its flow velocity opens up further fields of application:

• Thus, such thermal flow sensors, for example, in

Biogasanalgen be used to determine there in addition to the flow rate and the volume fraction of methane, so that it can be concluded on the calorific value of the gas.

• Simultaneous measurement of the flow velocity and the

Composition of a gas mixture is also advantageous in the field of medical technology, for example in spirometers for checking lung function and vital capacity. • In the area of air conditioning technology, it is also conceivable to measure the amount of air and the composition (CO content) of the air flowing out of the room at the same time, in order to allow an optimal admixing of fresh air.

A further advantageous embodiment provides that the substrate has a recess at least in a first region, such that the first dielectric layer forms a membrane on the substrate at least in the first region and that the at least one heating structure in the first region acts as a membrane formed first dielectric layer is arranged. By creating a membrane and the arrangement of the heating structure on this membrane, the heating structure is better thermally decoupled from the rest of the sensor, so as to be able to perform the most sensitive, fast and accurate measurement. A further advantageous embodiment provides that a second

Temperature sensor element is applied to the first dielectric layer.

In particular, the embodiment provides that the heating structure along a

Flow direction of the gas or the gas mixture between the first and second temperature sensor elements is arranged. Furthermore, the embodiment provides that the second temperature sensor element is formed from a material which has a

Resistance temperature coefficient in the range of 1000-1 1000 ppm / Kelvin, preferably in the range of 2000-11000 ppm / Kelvin, more preferably in the range of 3000-1 1000 ppm / Kelvin. A further advantageous embodiment provides that the heating structure and the first temperature sensor element are each formed from a material having a resistance temperature coefficient in the range of 1000-1 1000 ppm / Kelvin, preferably in the range of 2000-11000 ppm / Kelvin, particularly preferred in the range of 3000-1000 ppm / Kelvin. In particular, the embodiment provides that the

Heating structure made of nickel or platinum is formed. By the formation of the heating structure and the first temperature sensor element each having a material with a quasi-linear or with a steadily increasing resistance temperature coefficient, the measurement of the temperature can be saved directly to the heating structure, as this temperature on the ohmic resistance of the heating structure and the can calculate known value of the resistance-temperature coefficient and thus can be determined. Typically, the heating structure and the first temperature sensor element is made of one and the same material, preferably platinum. It is also conceivable, however, for the heating structure made of platinum and the first temperature sensor element made of nickel to be made of different materials. A further advantageous embodiment provides that in a third operating state, the control device keeps the ratio between the supplied power and the predetermined temperature substantially constant. In this way, the measurement result disturbing influences, such as. Temperature fluctuations,

Pressure fluctuations in the determination of the flow rate can be reduced.

A further advantageous embodiment provides that the control device controls the heating structure with an excitation signal and the evaluation unit the

Temperature of the at the first temperature sensor element and second

Temperature sensor element bypassing gas by means of at least a first response signal and a second response signal detects that the first response signal originates from the first temperature sensor element and the second response signal from the second temperature sensor element. In particular, the embodiment provides that the evaluation unit for determining the flow velocity compares the first and / or second response signal with first reference values. It is further provided that the evaluation unit for determining the present gas or the composition of the gas mixture compares the first and / or second response signal with second reference values.

A further advantageous embodiment provides that the excitation signal

Represents AC signal. This alternating voltage signal is used to generate a phase shift between the excitation signal and the first and / or second response signal that the evaluation unit determines the phase shift between the excitation signal and the first and / or second response signal. In particular, the evaluation unit performs a verification or verification of the previously determined gas or the previously determined composition of the gas mixture and its flow rate. As an alternative to the just described verification or verification, it may be provided that the evaluation unit is based on the

Phase shift determines a further physical property of the present gas or gas mixture. In particular, the further physical property represents the thermal diffusivity of the present gas or gas mixture. As an alternative to the thermal diffusivity, the thermal conductivity, the specific thermal conductivity, can also be used

Heat capacity, the density and the dynamic or kinematic viscosity are determined.

A further advantageous embodiment provides that the first dielectric layer and / or the second dielectric layer have a layer thickness of less than 100 micrometers, preferably less than 50 micrometers, more preferably less than 15 micrometers exhibit. The layer thickness both in the first dielectric layer and in the second dielectric layer represents a compromise between mechanical stability and the heat transfer through these layers. A further advantageous embodiment provides that the first and the second dielectric layer are formed from the same material. Preferably, the first and second dielectric layers are formed of a polymer.

With regard to the flow meter, the object is achieved by a flow meter with a thermal flow sensor according to at least one of the preceding embodiment.

The invention will be explained in more detail with reference to the following drawings. FIG. 1 shows a cross section of an embodiment of the invention. FIG

Flow sensors,

2 shows an electrical circuit diagram of the control device and a voltage divider which serves to detect a response signal,

FIG. 3: a first measuring curve comprises the first reference values and

4 shows a second measurement curve comprising the second reference values.

FIG. 1 shows a cross section of an embodiment of the invention

Flow sensor 1. Here, the thermal flow sensor 1, a substrate 3, on which a first dielectric layer 6 is applied. Furthermore, the substrate 3 has a recess 5 in a first region 4, so that the first dielectric layer in the first region 4 forms a membrane 7 on the substrate 3. On this membrane 7, a heating structure 8 is applied such that it extends between a first and a second temperature sensor element 9, 10 along the flow direction of

Gas or the gas mixture 2 is located and serves to heat the gas or gas mixture 2. The two temperature sensor elements 9, 10 are likewise applied to the first dielectric layer 6 and preferably arranged such that they lie in the first region 4. By means of these two temperature sensor elements 9, 10, the temperature of the heated gas to the heating structure 8 or gas mixture 2 is detected. In order to protect the heating structure 8 and the two temperature sensor elements 9, 10, a second dielectric layer 16 is applied to the first dielectric layer 6. To determine a gas or the composition of a gas mixture 2, the the side facing away from the substrate 3 of the second dielectric layer 18 exposed to the flowing gas or gas mixture 2.

The thermal flow sensor 1 further comprises a control device 1 1, which controls the heating structure 8 with an excitation signal 12 and in a first

Operating state controls such that the heating structure has a predetermined temperature and the heating structure 8 controls in a second operating state such that a power supplied to the heating structure 8 is controlled substantially to a predetermined power. The control device thus controls by means of the heating structure in the first operating state, the gas temperature of the gas or gas mixture to the predetermined temperature. In the second operating state, the control device essentially regulates the power supplied to the heating structure to the predetermined power, which is typically constant at least in the mean value. The excitation signal 13 represents an alternating voltage signal which, for example, has an excitation frequency of 1 Hz. It has proved to be advantageous if the predetermined temperature in the first operating state represents a constant excess temperature of the gas or gas mixture relative to the ambient temperature in the range of 120 ° C and if the predetermined power in the second operating state has a peak-to-peak value of 20 mW (milliwatts). The excitation frequency, the predetermined temperature and the peak-to-peak value of the alternating voltage signal can vary depending on the gas or gas mixture 2 present. In addition to the first and the second operating state, the control device 1 1 holds in a third operating state, the ratio between the supplied power and the predetermined temperature substantially constant.

Furthermore, the control device 1 1 is designed to be switchable, so that it can be switched between the individual operating states.

In addition to the control device 1 1, the thermal flow sensor 1 in addition to an evaluation unit 12, by means of the individual operating conditions, at least the present gas or the composition of the gas mixture 2, and the

Flow rate of the gas or gas mixture determined. For this purpose, the thermal flow sensor measures the thermal conductivity of the gas or

Gas composition and determines based on the thermal conductivity of the present gas or the composition of the gas mixture. For this purpose, the evaluation unit 12 detects the temperature of the gas or gas mixture 2 flowing past the first temperature sensor element 9 by means of a first response signal 14 and the temperature of the gas or gas mixture 2 flowing past the second temperature sensor element 10 by means of a second response signal 15

Flow rate compares the evaluation unit 12 either the first Response signal 14 with first reference values 16 or the second response signal 15 with the first reference values 16 individually or else the first and the second

Response signal 14, 15 together with the first reference values 16. For determining the present gas or the composition of the gas mixture 2, the evaluation unit 12 compares either the first response signal 14 with second reference values 17 or the second response signal 15 with the second reference values 17 or the first and the second response signal 14, 15 with the second reference values 17, the first reference values 16 differing from the second reference values 17. To determine the composition of the gas mixture 2, which is preferably a binary gas mixture, such as, for example, an argon-helium mixture, the must

Evaluation unit 12, the individual components, so in the case of helium and argon, be known so that the concentration composition of the (binary)

Gas mixture is determinable. Fig. 2a) shows an electrical circuit diagram of the control device 1 1 and Fig. 2b) a voltage divider which serves to detect the response signals 14, 15. The

Control device 1 1 represents in the simplest case, a voltage divider, wherein the ohmic heating resistor of the heating structure 8 is shown with R _H and the heating resistor is a series resistor RVH upstream of the heating structure 8. By means of

Total voltage for the heating structure U _H can thus generate the excitation signal 13, which as already mentioned represents an AC signal having a peak-peak value of 20 mW. As excitation signal 13 comes both a sine wave voltage, a square wave voltage as well as any other form of a

AC signal into consideration.

Fig. 2b) shows a voltage divider which serves to detect a response signal 14, 15. For the sake of simplicity, only the voltage divider serving to detect the first response signal 14 of the first temperature sensor element 9 is shown in FIG. 2 b). In the simplest case, the evaluation unit 12 thus comprises at least one voltage divider for each temperature sensor element 9, 10. FIG. 2b) shows the ohmic resistance of the temperature sensor element with R _T. Furthermore, the voltage divider comprises a series resistor RVT and is operated with a total voltage UT, wherein the total voltage U _{T represents} a DC voltage. The first response signal 14 is tapped off over the resistance of the temperature sensor element RT.

3 shows a first measurement curve, which comprises the first reference values 16 required for determining the flow velocity of the gas or of the gas mixture 2. 4 shows a second measuring curve on the basis of which the determination of the present gas or the composition of the gas mixture 2 can be carried out.

LIST OF REFERENCE NUMBERS

1 thermal flow sensor

2 gas or gas mixture

3 substrate

4 First area

5 recess

6 First dielectric layer

7 membrane

8 heating structure

9 First temperature sensor element

10 second temperature sensor element

1 1 control device

12 evaluation unit

13 excitation signal

14 First response signal

15 second response signal

16 first reference values

17 Second reference values

18 Second dielectric layer

RH heating resistor

RT resistance of a temperature sensor element

RvH series resistor heater

RVT series resistor temperature sensor element

U _H Total voltage for heating structure

U _T total voltage for temperature sensor element

Claims

claims

1. Thermal flow sensor (1) for determining a gas or

Composition of a gas mixture (2), and its flow velocity comprising:

- A substrate (3) on which at least a first dielectric layer (6) is applied;

- At least one heating structure (8) which is applied to the first dielectric layer (6) and serves to heat the gas or the gas mixture (2);

at least a first temperature sensor element (9) spaced from the

Heating structure (8) on the first dielectric layer (6) is applied and the

Temperature of the heated at the heating structure (8) heated gas or gas mixture (2) detected;

- A control device (1 1), which controls the heating structure (8) in such a way in a first operating state that the heating structure has a predetermined temperature and in a second operating state, the heating structure (8) controls such that one of the heating structure (8) supplied power corresponds to a given power;

- An evaluation unit (12), by means of the operating conditions, at least one

physical property of the present gas or the gas mixture (2) determined and based on this physical property of the present gas or

Composition of the gas mixture (2) and determines its flow rate.

2. Thermal flow sensor according to claim 1, wherein the evaluation unit (12) determines the composition of the gas mixture (2) concentration and wherein the concentration of the composition of the gas mixture (2) the individual components of the gas mixture (2) of the evaluation unit (12) to be known have to.

3. Thermal flow sensor according to claim 1 or 2, wherein the substrate (3) at least in a first region (4) has a recess (5), so that the first dielectric layer (6) at least in the first region (4) on the Substrate (3) forms a membrane (7) and wherein the at least one heating structure (8) in the first region (4) on the membrane (7) formed first dielectric layer (6) is arranged.

4. Thermal flow sensor according to one or more of the preceding claims, wherein a second temperature sensor element (10) on the first

dielectric layer (6) is applied.

5. A thermal flow sensor according to claim 4, wherein the heating structure (8) along a flow direction of the gas or the gas mixture (2) between the first and second temperature sensor elements (9, 10).

6. Thermal flow sensor according to claim 5, wherein the second

Temperature sensor element (10) is formed of a material having a resistance temperature coefficient in the range of 1000-1 1000 ppm / Kelvin, preferably in the range of 2000-11000 ppm / Kelvin, more preferably in the range of 3000-1 1000 ppm / Kelvin ,

7. A thermal flow sensor according to one or more of claims 1 to 5, wherein the heating structure (8) and the first temperature sensor element (9) are each formed of a material having a resistance temperature coefficient in the range of 1000-1 1000 ppm / Kelvin, preferably in the range of 2000-1 1000 ppm / Kelvin, more preferably in the range of 3000-1 1000 ppm / Kelvin.

8. Thermal flow sensor according to one or more of the preceding

Claims, wherein the control device (1 1) holds in a third operating state, the ratio between the predetermined power and the predetermined temperature substantially constant.

9. Thermal flow sensor according to one or more of the preceding claims, wherein the control device (1 1) the heating structure (8) with a

The control unit detects the temperature of the gas (2) flowing past the first temperature sensor element (9) and the second temperature sensor element (10) by means of at least one first response signal (14) and one second response signal (15).

10. Thermal flow sensor according to claim 9, wherein the evaluation unit (12) for determining the flow velocity compares the first and / or second response signal (14, 15) with first reference values (16).

1 1. A thermal flow sensor according to claim 9 or 10, wherein the evaluation unit (12) for determining the present gas or the composition of

Gas mixture the first and / or second response signal (14, 15) with second

Reference values (17) compares.

12. Thermal flow sensor according to one or more of the preceding claims, wherein the excitation signal (13) represents an AC signal.

13. Thermal flow sensor according to claim 12, wherein the evaluation unit (12) has a phase shift between the excitation signal (13) and the first and / or second response signal (14, 15) determined.

14. Thermal flow sensor according to claim 13, wherein the evaluation unit (12) based on the phase shift performs a check of the determination of the gas or the composition of the gas mixture (2), and its flow rate.

15. Thermal flow sensor according to claim 13, wherein the evaluation unit (12) based on the phase shift, a further physical property of

present gas or gas mixture determined.

16. Thermal flow sensor according to one or more of the preceding claims, wherein the first dielectric layer (6) and / or the second dielectric layer (18) have a layer thickness of less than 100 microns, preferably less than 50 microns, more preferably less than 15 microns.

17. A thermal flow sensor according to one or more of the preceding claims, wherein the first and second dielectric layers (6, 18) are formed of the same material.

18. A thermal flow sensor according to claim 17, wherein the first and second dielectric layers (6, 18) are formed of a polymer.

19. Flowmeter with a thermal flow sensor according to at least one of the preceding claims.