Device and method for mixing combustible gas and combustion air, hot water installation provided therewith, corresponding thermal mass flow sensor and method for measuring a mass flow rate of a gas flow

Abstract

A device for mixing combustible gas and combustion air, hot water installation, and corresponding thermal mass flow sensor and method. The device includes an air line, a gas line with a control valve, a first measuring line connecting the air and gas lines, and a second measuring line connecting the first measuring line to the gas and/or air line, forming a three-way intersection. A thermal mass flow sensor includes first and second temperature sensors in the first measuring line, positioned respectively in a gas flow between the three-way intersection and the air line and in a gas flow between the three-way intersection and the gas line. A controller controls the control valve based on a difference, measured by the flow sensor, between mass flow rate of gas between the three-way intersection and the air line and mass flow rate of gas between the three-way intersection and the gas line.

Claims

1. A device for mixing a combustible gas and combustion air for supplying to a burner, comprising: an air line for the supply of combustion air; a gas line for the supply of a combustible gas which is provided with a control valve; a first measuring line with a first outer end which is connected to the air line and a second outer end which is connected to the gas line; a second measuring line with a first outer end which is connected to the first measuring line at a point between the first and second outer ends of the first measuring line, thus forming a three-way intersection, and with a second outer end which is connected to the gas line and/or the air line; a thermal mass flow sensor, comprising: a first temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the air line during use; and a second temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the gas line during use; a controller connected to the thermal mass flow sensor and the control valve and configured to control the control valve on the basis of a difference, measured by the thermal mass flow sensor, between the mass flow rate of the gas flow between the three-way intersection and the air line and the mass flow rate of the gas flow between the three-way intersection and the gas line.

2. The device as claimed in claim 1, wherein the first temperature sensor and second temperature sensor are each provided in a circuit for constant temperature anemometry.

3. The device as claimed in claim 1, wherein the thermal mass flow sensor further comprises a heating element arranged in the first measuring line and is situated at or close to the three-way intersection, wherein the first temperature sensor is situated upstream of the heating element and the second temperature sensor is situated downstream of the heating element, as seen in a direction from the air line to the gas line, wherein the controller is configured to control the control valve on the basis of a difference between the temperature measured by the first temperature sensor and the temperature measured by the second temperature sensor.

4. The device as claimed in claim 3, wherein the thermal mass flow sensor comprises a third temperature sensor situated on a side of the heating element facing toward the second measuring line, and wherein the processing unit is configured to determine the mass flow rate through the second measuring line on the basis of the values output by the first, second and third temperature sensors.

5. The device as claimed in claim 1, further comprising a processing unit which is operatively connected to the thermal mass flow sensor and is configured to determine a mass flow rate through the second measuring line on the basis of the values output by the first temperature sensor and the second temperature sensor.

6. The device as claimed in claim 1, wherein the second outer end of the second measuring line is connected to the gas line at a point situated upstream relative to the point at which the first measuring line is connected to the gas line, as seen in the direction of flow of the combustible gas through the gas line.

7. The device as claimed in claim 6, wherein the gas line comprises a flow restriction between the point at which the first measuring line is connected to the gas line and the point at which the second measuring line is connected to the gas line.

8. The device as claimed in claim 1, wherein the second outer end of the second measuring line is connected to the gas line at a point situated downstream relative to the point at which the first measuring line is connected to the gas line, as seen in the direction of flow of the combustible gas through the gas line.

9. The device as claimed in claim 1, wherein the second outer end of the second measuring line is connected to the air line at a point situated upstream relative to the point at which the first measuring line is connected to the air line, as seen in the direction of flow of the combustion air through the air line.

10. The device as claimed in claim 9, wherein the air line comprises a flow restriction between the point at which the first measuring line is connected to the air line and the point at which the second measuring line is connected to the air line.

11. The device as claimed in claim 1, wherein the second outer end of the second measuring line is connected to the air line at a point situated downstream relative to the point at which the first measuring line is connected to the air line, as seen in the direction of flow of the combustion air through the air line.

12. The device as claimed in claim 1, wherein the second outer end of the second measuring line is connected to both the gas line and the air line, downstream of the point at which the first measuring line is connected to the air line and downstream of the point at which the first measuring line is connected to the gas line.

13. The device as claimed in claim 12, wherein the air line comprises a first flow restriction situated downstream of the point at which the air line is connected to the first measuring line, as seen in the direction of flow of combustion air through the air line, and wherein the gas line comprises a second flow restriction situated downstream of the point at which the gas line is connected to the first measuring line, as seen in the direction of flow of combustible gas through the gas line.

14. The device as claimed in claim 1, wherein the gas line comprises a shut-off valve upstream of the control valve, and the second outer end of the second measuring line is connected to the gas line at a point situated between the control valve and the shut-off valve.

15. A hot water installation comprising the device as claimed in claim 1.

16. A thermal mass flow sensor, comprising: a heating element; and a first, a second and a third temperature sensor, wherein the first and second temperature sensors are arranged on either side of the heating element and are arranged in one line with the heating element in a first direction, and wherein the third temperature sensor is arranged in one line with the heating element in a second direction, which differs from the first direction that lies substantially transversely of the second direction.

17. The thermal mass flow sensor as claimed in claim 16, comprising at least two temperature sensors arranged adjacently of each other on the same side of the heating element and at substantially the same distance from the heating element.

18. The thermal mass flow sensor as claimed in claim 16, wherein the heating element and the temperature sensors are arranged in a grid.

19. A method for controlling a device for mixing a combustible gas and combustion air for supplying to a burner, the device comprising: an air line for the supply of combustion air; a gas line for the supply of a combustible gas provided with a control valve; a first measuring line with a first outer end which is connected to the air line and a second outer end which is connected to the gas line; and a second measuring line with a first outer end which is connected to the first measuring line at a point between the first and second outer end of the first measuring line, thus forming a three-way intersection, and with a second outer end which is connected to the gas line and/or the air line, the method comprising: measuring a mass flow rate of a gas flow between the three-way intersection and the air line; measuring a mass flow rate of a gas flow between the three-way intersection and the gas line; opening the control valve if the mass flow rate of the gas flow between the three-way intersection and the gas line is a predetermined first threshold value smaller than the gas flow between the three-way intersection and the air line; and closing the control valve if the mass flow rate of the gas flow between the three-way intersection and the gas line is a predetermined second threshold value greater than the gas flow between the three-way intersection and the air line.

20. A method for measuring a mass flow rate of a gas flow in a device for mixing combustible gas and combustion air for supplying to a burner, the device comprising: an air line for the supply of combustion air; a gas line for the supply of combustible gas provided with a control valve; a first measuring line with a first outer end which is connected to the air line and a second outer end which is connected to the gas line; a second measuring line with a first outer end which is connected to the first measuring line at a point between the first and second outer end of the first measuring line, thus forming a three-way intersection, and with a second outer end which is connected to the gas line and/or the air line; a thermal mass flow sensor, comprising: a first temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the air line during use; and a second temperature sensor arranged in the first measuring line and positioned such that it is situated in a gas flow between the three-way intersection and the gas line during use, the method comprising: determining the mass flow rate through the second measuring line on the basis of the value output by the first temperature sensor and the value output by the second temperature sensor.

Description

(1) FIG. 1A shows schematically a first exemplary embodiment of a device according to the invention;

(2) FIG. 1B shows in detail the thermal mass flow sensor for the device of FIG. 1A;

(3) FIGS. 2A-C illustrate the measuring of different fluid flows along the sensor of FIG. 1B;

(4) FIG. 3A shows schematically a second embodiment of a thermal mass flow sensor for the device of FIG. 1A;

(5) FIG. 3B shows schematically a third embodiment of a thermal mass flow sensor for the device of FIG. 1A;

(6) FIG. 3C shows a circuit for a temperature sensor of the mass flow sensor of FIG. 3B;

(7) FIG. 3D shows schematically a fourth embodiment of a thermal mass flow sensor for the device of FIG. 1A;

(8) FIG. 4 shows schematically a second exemplary embodiment of a device according to the invention;

(9) FIG. 5 shows schematically a third exemplary embodiment of a device according to the invention;

(10) FIG. 6 shows schematically a fourth exemplary embodiment of a device according to the invention;

(11) FIG. 7 shows schematically a fifth exemplary embodiment of a device according to the invention;

(12) FIG. 8 shows schematically a sixth exemplary embodiment of a device according to the invention;

(13) FIGS. 9A-B show schematically a second and third exemplary embodiment of a thermal mass flow sensor according to the invention; and

(14) FIGS. 9C-D show schematically a fourth and fifth exemplary embodiment of a thermal mass flow sensor according to the invention, wherein the sensor is embodied as a matrix of heating elements and temperature sensors.

(15) Referring to the FIGS., device 2, 102, 202, 302, 402, 502 comprises an air line 4, 104, 204, 304, 404, 504 and a gas line 6, 106, 206, 306, 406, 506. Air is drawn in via air line 4, 104, 204, 304, 404, 504 by means of fan 8, 108, 208, 308, 508. Natural gas is moreover supplied via gas line 6, 106, 206, 306, 406, 506. Gas line 6, 106, 206, 306, 406, 506 is provided with a control valve 12, 112, 212, 312, 412, 512. Gas line 6, 106, 206, 306, 406, 506 debouches into air line 4, 104, 204, 304, 404, 504 for the purpose of mixing the gas with the air. Gas line 6, 106, 206, 306, 406, 506 optionally comprises at the outer end a nozzle 10, 110, 210, 310, 510 which debouches into air line 4, 104, 204, 304, 404, 504. A flow restriction 19, 119, 319, such as a narrowing, may be arranged in the air line or gas line. For example, referring to FIG. 1A, device 2 comprises an air line 4 and a gas line 6. Air is drawn in via air line 4 by means of fan 8. Natural gas is moreover supplied via gas line 6. Gas line 6 is provided on the infeed side with a control valve 12. Gas line 6 debouches into air line 4 for the purpose of mixing the gas with the air. Gas line 6 optionally comprises at the outer end a nozzle 10 which debouches into air line 4.

(16) Air line 4 is connected to gas line 6 via a first measuring line 14. The one outer end a of measuring line 14 is connected for this purpose to air line 4, while the other outer end b is connected to gas line 6. A second measuring line 16 is connected with its one outer end c to measuring line 14, while its other outer end d is connected to gas line 6. The outer end d of measuring line 16 is situated upstream of outer end b of measuring line 14, as seen in the direction of flow of the gas through gas line 6.

(17) Because the outer end c of measuring line 16 is coupled to measuring line 14 at a point between the outer ends a and b of measuring line 14, a three-way intersection (also referred to as T-junction) is formed. A thermal mass flow sensor 18 is positioned at the three-way intersection (FIG. 1B).

(18) A flow restriction 19, such as a narrowing, is arranged in the gas line between outer end b of measuring line 14 and outer end d of measuring line 16.

(19) Control valve 12 is controlled by a controller (not shown) on the basis of the fluid flows measured by sensor 18. Sensor 18 measures the magnitude of fluid flow .sub.1 which flows from the three-way intersection to air line 4. Sensor 18 moreover measures the magnitude of the fluid flow .sub.2 which flows from the three-way intersection to gas line 4. The magnitude of the fluid flow is determined by sensor 18 in the form of a mass flow rate of the flow, for instance expressed in kg/s. The output of sensor 18 is for instance a voltage or current which is indicative of the mass flow rate of the relevant gas flow .sub.1, .sub.2.

(20) Sensor 18 comprises in this example a heating element 20, for instance in the form of a resistor or hot wire. Three temperature sensors 22, 24, 26 are arranged around heating element 20. Temperature sensors 22, 24 are situated on either side of heating element 20. Temperature sensor 22, heating element 20 and temperature sensor 24 thus lie in one line, at least substantially in one line, in measuring line 14. The third temperature sensor 26 is positioned on the side of second measuring line 16 relative to heating element 20. It is recommended to place temperature sensors 22, 24, 26 at substantially the same distance from heating element 20, as shown in the example. If they are alternatively placed at different distances, this has to be corrected for in the processing of the values measured by sensors 22, 24, 26.

(21) Temperature sensors 22, 24, 26 can for instance the thermistors, such as PTC or NTC thermistors.

(22) Heating element 20 is supplied with a predetermined power, preferably a constant power. The heat produced by heating element 20 is in this way known. If no flow of gas takes place, the heat is distributed evenly and temperature sensors 22, 24, 26 measure the same temperature. If flow of a gas (natural gas, air or a natural gas-air mixture) however takes place through measuring lines 14, 16, the heat is distributed by that gas and temperature differences result. The mass flow rate and the direction of fluid flow through the three-way intersection can therefore be determined on the basis of the temperature measured by sensors 22, 24, 26.

(23) The mass flow rate of the gas flow .sub.1 in the direction of air line 4 is proportional to the difference in temperature measured by sensor 22 and sensor 26 (FIG. 2A). If use is for instance made of a constant heating power of heating element 20, it is the case in a linear approximation that: .sub.1=C*(T.sub.22T.sub.26), with C representing a constant. Similarly, it is the case that .sub.2=C*(T.sub.24T.sub.26) (see FIG. 2B). To calculate the total fluid flow .sub.3 flowing into the three-way intersection it is the case that: .sub.3=.sub.1+.sub.2=C*(T.sub.22+T.sub.242*T.sub.26).

(24) By controlling control valve 12 with the controller, the pressure P.sub.air (FIG. 1A) and pressure P.sub.gas_out (FIG. 1A) are kept equal to each other. In the case of equal pressure it is the case that .sub.1=.sub.2. Whether the pressure is indeed equal is measured by determining the difference in temperature between temperature sensor 22 and temperature sensor 24. FIG. 2C illustrates a situation in which the pressure is unequal, so that the gas flows in the direction from sensor 24 to sensor 22. Sensor 22 will then detect a higher temperature than sensor 24. The controller therefore controls control valve 12 on the basis of the difference in temperature between sensor 22 and sensor 24.

(25) The gas flow .sub.3 through the second measuring line 16 is correlated to the flow through gas line 6 and depends among other things on the pressure drop P.sub.gas_inP.sub.gas_out. The gas inflow can therefore be determined on the basis of .sub.3.

(26) For calculating the gas flow through gas line 6 on the basis of .sub.3 use is optionally made of calibration data and/or data about the sensor characteristic.

(27) Instead of a mass flow sensor 18 with three temperature sensors 22, 24, 26 it is alternatively possible to use a conventional mass flow sensor 18 (FIG. 3A) with two temperature sensors 22, 24 and a heating element 20. In similar manner as described above, control valve 12 is controlled on the basis of a difference in the temperature measured by sensor 22 and sensor 24. This temperature difference is a measure of .sub.1.sub.2. The total flow rate of the gas flow .sub.3 can be determined on the basis of the temperature measured by sensor 22 and/or sensor 24. This is because heating element 20 heats the gas with a constant power. If the gas flow .sub.3 increases, the temperature measured by .sub.1 and .sub.2 will decrease. Because the gas flow is controlled such that .sub.1=.sub.2, it is the case that .sub.3=2*.sub.1=2*.sub.2. Expressed as a function of temperature of sensors 22 and 24, this results in .sub.3=R*2*T.sub.22=R*2*T.sub.24, wherein R is a constant which depends among other things on the constant, predetermined power of heating element 20. .sub.3 is preferably determined on the basis of the sum of the temperatures measured by the sensors, as according to .sub.3=R*(T.sub.22+T.sub.24), so that the determined flow rate |.sub.3| is less susceptible to measuring inaccuracies.

(28) In a further variant the mass flow sensor 1018 (FIG. 3B) is embodied without a separate heating element. Two temperature sensors 1022, 1024 are each incorporated in an electric circuit. Because a current runs through the temperature sensors, they produce heat. This heat changes the resistance of temperature sensors 1022, 1024. Positive Temperature Coefficient (PTC) thermistors are preferably applied as temperature sensors 1022, 1024. The heating of the sensors 1022, 1024 then results in an increase in the resistance. Negative Temperature Coefficient (NTC) thermistors can alternatively be applied, wherein the heating results in a decrease in the resistance. In a further alternative a hot wire is applied as temperature sensor, in similar manner as a PTC thermistor.

(29) The self-heating of temperature sensors 1022, 1024 is influenced by the passing gas. Temperature sensors 1022, 1024 are in particular cooled by the passing gas. The degree of cooling depends on the mass flow rate of the gas flow: the more gas flows past sensors 1022, 1024 per unit of time, the more sensors 1022, 1024 are cooled. In short, the gas flow influences the temperature of sensors 1022, 1024 and thereby the resistance, which resistance can be measured. For this measurement the sensors 1022, 1024 are incorporated in a measuring circuit. The circuit is for instance configured to apply a constant voltage over the relevant sensor 1022, 1024. The gas flow cools sensor 1022, 1024 so that its resistance decreases (PTC) or increases (NTC), which provides for respectively an increase or decrease of the current when the voltage remains constant. The current is measured and is a measure of the mass flow rate. The output of the circuit is for instance a voltage indicative of the current through sensor 1022, 1024 and thereby of the respective mass flow rate |.sub.1| or |.sub.2| in question.

(30) It is noted that, although the mass flow rate || of the relevant gas flow can be measured with sensors 1022, 1024, the direction of this gas flow cannot. The direction is however fixed because of the chosen configuration of the second measuring line.

(31) In another example a sensor 1022, 1024 is incorporated in a circuit configured to maintain a constant current through sensor 1022, 1024. In such a circuit the voltage over sensor 1022, 1024 is a measure of the mass flow rate of the gas flow.

(32) A circuit which is configured to keep the temperature of sensor 1022, 1024 constant is however preferably applied. An example of such a circuit is shown in FIG. 3C. The circuit comprises an amplifier, such as an op-amp, with a negative input and a positive input. The circuit further comprises a Wheatstone bridge in which a PTC thermistor R.sub.ptc is incorporated in the shown example. Point A of the Wheatstone bridge is connected to the negative input of the amplifier, while point B is connected to the positive input. If the temperature of R.sub.ptc drops because gas flows past this sensor, its resistance will decrease. The amplifier will however maintain equal voltage at its two inputs, so that V.sub.+=V.sub. and V.sub.A=V.sub.B. When the resistance decreases, the amplifier will thus inject more power into point C of the Wheatstone bridge, whereby the output voltage V.sub.out also increases. V.sub.out is therefore a measure of the mass flow rate of the gas flow: a higher mass flow rate means a higher voltage V.sub.out.

(33) FIG. 3D shows a variant of FIG. 3B wherein mass flow sensor 1018 is embodied with three temperature sensors 1022, 1024, 1026. A mass flow rate of the relevant flow can be determined with each temperature sensor 1022, 1024, 1026 in similar manner as described above for mass flow sensor 1018. In short, a measuring circuit with sensor 1022 produces an output indicative of |.sub.1|, a measuring circuit with sensor 1024 an output indicative of |.sub.2| and a measuring circuit with sensor 1026 an output indicative of |.sub.3|.

(34) In a device 102 according to a second exemplary embodiment (FIG. 4) the second measuring line 116 is situated downstream relative to the first measuring line 114. In other words, the outer end d of measuring line 116 is situated downstream relative to outer end b of measuring line 114, as seen in the direction of flow through gas line 106. The sensor will likewise be mirrored relative to FIG. 1B, i.e. with temperature sensor 26 on the opposite side of heating element 20 (to the right instead of to the left of element 20 in FIG. 1B). Because of this alternative configuration the direction of flow of .sub.1, .sub.2 and .sub.3 is opposite to the corresponding flows in FIG. 1A. It is however likewise the case for device 102 that: .sub.3=.sub.1+.sub.2 (with the flow directions defined as in FIG. 4).

(35) Control valve 112 of device 102 is controlled such that P.sub.gas_in=P.sub.air. In that case .sub.1=.sub.2. In short, just as with device 102, the controller (not shown) controls control valve 112 on the basis of the difference between .sub.1 and .sub.2. This difference can be determined on the basis of the temperature measured by sensors 22 and 24, as according to .sub.1.sub.2=C*(T.sub.24T.sub.22). In short, if sensors 22 and 24 measure the same temperature, then it is the case that .sub.1=.sub.2.

(36) In device 102 fan 108 is placed upstream of measuring line 114 in the direction of flow of the air, instead of downstream of measuring line 14, as in FIG. 1A. It is noted that this is optional: a fan can be placed upstream or downstream of the first measuring line as desired in any embodiment of the device according to the invention.

(37) In a third embodiment device 202 has a configuration for measuring the flow rate of the supplied air instead of the flow rate of the indrawn gas (FIG. 5). In this case the second measuring line 216 is connected to air line 204 instead of to the gas line. In air line 204 a flow restriction 219 is provided between the outer end d of measuring line 216 and the outer end a of measuring line 214. For purposes of comparison, in FIG. 1A the flow restriction was situated in the gas line. It is once again the case that .sub.3=.sub.1+.sub.2. In the embodiment according to FIG. 5 .sub.3 is however a measure of the indrawn air flow instead of the indrawn gas flow. In this case .sub.3 depends among other things on the pressure difference P.sub.air_inP.sub.air_out. In other words, |.sub.3| is a measure of the flow rate of the air flow.

(38) Device 302 according to a fourth embodiment (FIG. 6) has a second measuring line 316 which, just as in the embodiment according to FIG. 5, is connected to air line 304. In contrast to FIG. 5, measuring line 316 is however situated downstream relative to measuring line 314. In this situation |.sub.3| is also a measure of the flow rate of the air flow.

(39) In a fifth embodiment (FIG. 7) the second measuring line 416 is connected with its second outer end d to both air line 404 and gas line 406 in that lines 404, 406 and 416 converge in mixing chamber 428. No fan is shown in this example, but the device comprises a fan downstream which draws in the mixture as according to arrow Z as a result of an underpressure P.sub.Z being realized relative to P.sub.air and P.sub.gas.

(40) A flow restriction 419a is provided in the air line 404 downstream of the first measuring line 414. A flow restriction 419b is provided in gas line 406 downstream of the first measuring line 414.

(41) In the fifth embodiment .sub.3=.sub.1+.sub.2 is a measure of the total flow of the gas-air mixture in the direction of the burner. Control valve 412 is once again controlled on the basis of .sub.1.sub.2, as determined by means of mass flow sensor 418. This achieves that .sub.1=.sub.2 and, as a result, P.sub.air=P.sub.gas.

(42) In a sixth embodiment (FIG. 8) the gas line is provided with a shut-off valve 530 upstream relative to control valve 512. It is noted that a shut-off valve is preferably also provided upstream of control valve 512 in the above described embodiments.

(43) Device 502 according to the sixth embodiment comprises a second measuring line 516 which is connected to gas line 506 at a point upstream of control valve 512. In the shown embodiment with optional shut-off valve 530 outer end d of measuring line 516 is situated between control valve 512 and shut-off valve 530. In the configuration according to FIG. 8 the flow .sub.3 therefore depends on the pressure difference P.sub.gas_supplyP.sub.gas. .sub.3 thereby forms a measure of this pressure difference. In the same way as described above, control valve 512 is controlled so that .sub.1=.sub.2 and thus P.sub.air=P.sub.gas.

(44) Device 502 comprises in the shown embodiment an optional flow restriction 519 in air line 504, downstream of the first measuring line 514.

(45) In devices 102, 202, 302, 402 and 502 it is likewise possible to apply sensor 18, 1018 or 1018 of FIG. 3A, 3B or 3D instead of sensor 18, 118.

(46) For the purpose of measuring the mass flow rate of the various gas flows the thermal mass flow sensor according to the invention optionally comprises more than three temperature sensors and/or more than one heating element. FIG. 9A shows a sensor 618 in which two temperature sensors 622, 624 are provided on either side of heating element 620, wherein temperature sensors 622, 624 are arranged substantially in one line with heating element 620. This is similar to the embodiment according to FIG. 1B. Sensor 618 however comprises two sensors 626a, 626b instead of one sensor 26. These temperature sensors 626a, 626b are arranged at substantially the same distance from heating element 620. The mass flow rate of flow .sub.1 can now be determined on the basis of the temperature measured by sensors 622, 626a and 626b. By providing two temperature sensors 626a, 626b the accuracy of the determination of the mass flow rate of flows .sub.1 and .sub.2 is increased.

(47) On the basis of a substantially linear relation, .sub.1 is for instance calculated as follows: .sub.1=V*(T.sub.6220.5*T.sub.626a0.5*T.sub.626b), wherein V is a constant. In short, the average value of sensors 626a and 626b is used as temperature upstream of element 620 and the temperature of sensor 622 is used as temperature downstream of element 620.

(48) In a further embodiment (FIG. 9B) sensor 718 comprises two heating elements 720a and 720b. Two temperature sensors 722a, 722b are provided on a first side, while two temperature sensors 724a, 724b are also provided on an opposite side. The sensors 722a, 722b, 724a, 724b are configured to measure the flow rate of a gas flow in a first direction. Provided on another side of heating elements 720a, 720b are three temperature sensors 726a, 726b, 726c which are configured to measure the flow rate of a gas flow in a second direction lying substantially perpendicularly of the first direction.

(49) Heating elements 720a, 720b are supplied with a predetermined, constant power, so that the heat production is constant. Sensors 722a and 722b measure the heating up as a result of flow .sub.1. Sensors 724a, 724b measure the heating up as a result of flow .sub.2. Sensors 726a, 726b, 726c measure the cooling as a result of flow .sub.3. By always providing more than one sensor the accuracy of the temperature measurement is increased. The accuracy of the determined mass flow rate is therefore increased.

(50) In a further embodiment (FIG. 9C) a thermal mass flow sensor 818 is provided as a matrix sensor of heating elements H and temperature sensors R. In the example of FIG. 9C the components are arranged in a grid, wherein nine sensors R are placed around each heating element H.

(51) An alternative arrangement of sensors R and heating elements H is however likewise possible, as illustrated with thermal mass flow sensor 918 in FIG. 9D. In this example temperature sensors R and heating elements H are arranged in a checkerboard pattern.

(52) The matrix sensor is for instance embodied as a thin film sensor. The matrix sensor is for instance produced by applying a thin film to a substrate. The substrate is for instance of a material with low thermal conductivity, for instance a ceramic material. An electronic structure is then arranged on the thin film, wherein the measuring resistors and the heating resistors are formed. This can for instance be realized by etching of the thin film layer, or another known technology for chip production.

(53) The matrix sensor according to the invention can be applied not only in a device for mixing gas and air, but can also be used in other applications. The matrix sensor can for instance be applied in order to determine the flow profile in a conduit. The matrix sensor can be applied for the purpose of both measuring a gas flow and measuring a liquid flow.

(54) The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.