Flow sensor with thermocouples

• US 20070241093A1
• Filed: 03/27/2007
• Published: 10/18/2007
• Est. Priority Date: 03/31/2006
• Status: Active Grant

• Current Assignee: Sensirion Holding AG
• Original Assignee: Sensirion Holding AG
• Inventors: Mayer, Felix, von Waldkirch, Marc, Lechner, Moritz, Hornung, Mark
• Granted Patent: US 8,011,240 B2
• Time in Patent Office: Days
• US Class Current: 219/385
• CPC Class Codes: G01F 1/6845 Micromachined devices
• CPC Class Codes: G01F 1/6888 Thermoelectric elements, e.g. thermocouples, thermopiles
• CPC Class Codes: G01F 1/696 Circuits therefor, e.g. constant-current flow meters

Abstract

The flow sensor comprises a heater arranged between two sensing thermopiles. In addition, at least one monitoring thermocouple is provided for measuring the temperature of the heater. The signal from the monitoring thermocouple can be used to improve the accuracy of the device in several ways. In particular, the signal from the monitoring thermocouple depends on changes of the Seeback constant and other inherent properties of the thermopiles in the same way as the signal from the sensing thermopiles, which allows a compensation of such effects. The signal from the monitoring thermocouple can also be used as a reference voltage to an A/D converter for converting the signals from the sensing thermopiles.

Claims

1. A device for measuring the flow of a fluid comprising

a substrate,

a heater arranged on said substrate,

at least one first and one second sensing thermopile for measuring a temperature difference between a location upstream and a location downstream of said heater and for generating a sensing signal, which sensing thermopiles are arranged on said substrate, wherein a junction of said sensing thermopiles closest to said heater is arranged at a first distance from said heater,

at least a first and a second monitoring thermocouple arranged upstream and downstream from said heater for generating a monitoring signal, which monitoring thermocouples are arranged on said substrate, wherein a junction of said monitoring thermocouples closest to said heater is arranged at a second distance from said heater, wherein said second distance is smaller than said first distance, and

a control circuit adapted to measure a flow of the fluid over said heater and said sensing thermopiles from a flow-dependent heating of said sensing thermopiles by said heater.

2. The device of claim 1 further comprising means for generating a signal proportional to a ratio of said sensing signal and said monitoring signal.

3. The device of claim 1 wherein said sensing signal is proportional to said temperature difference.

4. The device of claim 3 wherein the monitoring signal corresponds to a sum of the voltages from the first and the second monitoring thermocouple.

5. The device of claim 1 wherein said substrate is divided into a measuring region and a regular region, wherein a thermal conductance in said measuring region is lower than a thermal conductance in said regular region, and wherein said heater, said sensing thermopiles and said monitoring thermocouples are arranged at least partially on said measuring region.

6. The device of claim 5 wherein some junctions of each of said first and second sensing thermopiles are arranged on said regular region.

7. The device claim 5 wherein a first junction of each of said monitoring thermocouples is arranged on said measuring region and a second junction of each of said monitoring thermocouples is arranged on said regular region.

8. The device of claim 5 wherein a both junctions of each of said monitoring thermocouples are arranged on said measuring region.

9. The device of claim 5 comprising a hole or recess in said substrate and a membrane structure extending over said hole or recess, wherein said measuring region is defined by said membrane structure.

10. The device of claim 1 further comprising a control loop controlling a current through said heater in order to keep said monitoring signal constant.

11. The device of claim 1 further comprising an analog/digital converter for converting said sensing signal, wherein said analog/digital converter has a reference input and generates a digitized value of said sensing signal normalized by a signal at said reference input, wherein said monitoring signal is applied to said reference input.

12. The device of claim 1 wherein said sensing thermopiles extend parallel to said monitoring thermocouples.

13. The device of claim 1 wherein said monitoring thermocouples are arranged symmetrically in respect to a longitudinal axis of said heater.

14. The device of claim 1 wherein the junction of said monitoring thermocouples closest to said heater is adapted to measure a temperature of said heater.

15. A device for measuring the flow of a fluid comprising

a substrate, wherein said substrate is divided into a measuring region and a regular region, wherein a thermal conductance in said measuring region is lower than a thermal conductance in said regular region,

a heater arranged at least partially on said measuring region,

at least one sensing thermocouple for generating a sensing signal, which sensing thermocouple is arranged at least partially on said measuring region, wherein a junction of said sensing thermocouple closest to said heater is arranged at a first distance from said heater,

a control circuit adapted to measure a flow of the fluid over said heater and said sensing thermocouple from a flow-dependent heating of said sensing thermocouple by said heater, and

at least one monitoring thermocouple for generating a monitoring signal, which monitoring thermocouple is arranged completely on said measuring region, wherein a junction of said monitoring thermocouple closest to said heater is arranged at a second distance from said heater, wherein said second distance is smaller than said first distance.

16. A device for measuring the flow of a fluid comprising

a substrate,

a heater arranged on said substrate, wherein said heater comprises several conductors with at least one gap between said conductors,

at least one sensing thermocouple for generating a sensing signal, which sensing thermocouple is arranged on said substrate, wherein a junction of said sensing thermocouple closest to said heater is arranged at a first distance from said heater,

a control circuit adapted to measure a flow of the fluid over said heater and said sensing thermocouple from a flow-dependent heating of said sensing thermocouple by said heater, and

a plurality of monitoring thermocouples for generating a monitoring signal, which monitoring thermocouples are arranged in said gap and comprise a first bar of a first material extending from a first end of said heater to a center of said heater and a second bar of a second material connected at said center of said heater to said first bar and extending to a second end of said heater.

17. The device of claim 17 further comprising

a third bar of said first material being connected to said second bar at said second end and extending to said center and

a fourth bar of said second material being connected to said third bar at said center and extending to said first end.

18. A device for measuring the flow of a fluid comprising

a substrate,

a heater arranged on said substrate,

at least one sensing thermocouple for generating a sensing signal, which sensing thermocouple is arranged on said substrate, wherein a junction of said sensing thermocouple closest to said heater is arranged at a first distance from said heater,

a control circuit adapted to measure a flow of the fluid over said heater and said sensing thermocouple from a flow-dependent heating of said sensing thermocouple by said heater,

at least one monitoring thermocouple for generating a monitoring signal, wherein said monitoring thermocouple is arranged on said substrate, wherein a junction of said monitoring thermocouple closest to said heater is arranged at a second distance from said heater, wherein said second distance is smaller than said first distance and

an analog/digital converter for converting said sensing signal, wherein said analog/digital converter has a reference input and generates a digitized value of said sensing signal normalized by a signal at said reference input, wherein said monitoring signal is applied to said reference input.