Sensor temperature control in a thermal anemometer

1. A thermal anemometer including:

an ambient sensor;

a velocity sensor;

a sample and hold circuit;

a switched heating voltage driver;

a combined error amplifier and sample and hold circuit;

a current source;

a first switch electrically connected between said current source and said sample and hold circuit and between said current source and said ambient sensor;

a second switch electrically connected between said current source and said switched heated voltage driver;

a third switch electrically connected between said combined error amplifier and sample and hold circuit and between said combined error amplifier and sample and hold circuit and said velocity sensor; and

said combined error amplifier and sample and hold circuit electrically connected between said sample and hold circuit and said switched voltage driver.

2. The thermal anemometer of claim 1 , where said sample and hold circuit includes a switch and a capacitor.

3. The thermal anemometer of claim 2 , wherein said voltage is removed across said velocity sensor and said current source is selectively connected to said ambient sensor.

4. The thermal anemometer of claim 3 , where said current source is selectively connected to said velocity sensor.

5. The thermal anemometer of claim 3 , wherein the ambient sensor includes a resistance, said resistance creating a voltage across the resistance when current from the current source is applied to the resistance, said voltage being equal to a voltage across the velocity sensor, when the same amount of current is applied to the velocity sensor.

6. The thermal anemometer of claim 1 , where voltage is selectively applied across said velocity sensor.

7. The thermal anemometer of claim 1 where said sample and hold circuit obtains a voltage value of said ambient sensors and said combined error amplifier and sample and hold circuit obtains a voltage value from said velocity sensor.

8. A method of operating a thermal anemometer including:

providing a current source;

selectively applying current to an ambient sensor;

measuring the voltage produced across the ambient sensor by the current;

selectively applying current to a velocity sensor;

measuring the voltage produced across the velocity sensor;

comparing the voltage produced across the velocity sensor with the voltage across the ambient sensor to obtain a compared voltage; and

providing a voltage source selectively connected to said velocity sensor, and adjusting the voltage in response to the compared voltage.

9. The method of claim 8 , further including storing the value of the measured voltage across the velocity sensor and storing the value of the measured voltage across the ambient sensor.

10. The method of claim 9 , further including, providing a third switch, said third switch interposed between said voltage source and said velocity sensor, said third switch being open when said first switch is closed.

11. The method of claim 8 further including:

removing the current source from the velocity sensor;

storing the value of the voltage across the ambient sensor;

removing the current source from the ambient sensor;

applying the current source to the velocity sensor; and

storing the value of the voltage across the voltage sensor.

12. The method of claim 8 further including:

providing a first switch to selectively apply current to the ambient sensor, opening said first switch when applying current to said velocity sensor;

providing a second switch to selectively apply current to the velocity sensor, opening said second switch when applying current to said ambient sensor.

13. The method of claim 8 , wherein the current applied to the ambient sensor and the current applied to the velocity sensor come from the same current source.

14. A method of operating a thermal anemometer including:

providing a current source;

selectively applying current from the current source to an ambient sensor, thereby producing a voltage across the sensor;

selectively applying current from the current source to a velocity sensor, thereby producing a voltage across the sensor; new line selectively applying a voltage from a voltage source across the velocity sensor;

providing a digital to analog converter to measure the voltage produced across the ambient sensor and the velocity sensor; and

providing a microprocessor to process the voltage values and determine ambient temperature and desired velocity sensor temperature.

15. The method of claim 14 , further including providing a reference resistance, and selectively applying current from the current source to produce a voltage across the reference resistance.

16. The method of claim 15 , further including providing the microprocessor with the voltage across the reference resistance.

17. A method of controlling the temperature of a velocity sensing element in a thermal anemometer, including:

providing a velocity sensor selectively electrically connected to a power source;

providing an ambient sensor selectively electrically connected to a power source;

obtaining the voltage across the ambient sensor;

obtaining the voltage across the velocity sensor;

comparing the voltage across the ambient sensor with the voltage across the velocity sensor; and

changing the power dissipated in the velocity sensor in response to the value obtained by comparing the voltage across the sensors.

18. The method of claim 17 , further including providing a microprocessor to compare the voltages across the sensors.

19. The method of claim 18 further including:

providing a reference voltage to said microprocessor;

providing a reference resistance; and

determining the velocity sensor resistance from the measured voltages and the reference resistance.