I have designed PIC-based air speed sensors. The method that I usually use is to take a PTC (positive temperature coefficient) thermistor with a transition temperature of about 120C and apply 12V across it. The thermistor acts like a heater and thermostat all in one, and will consume just enough current to maintain its temperature at the transition temp. The cooling effect of air flowing over it depends on 1. The square root of the air velocity (King's law) and 2. The ambient temperature.

When selecting the PTC thermistor, you have to be certain that the device is a very small one and has low enough resistance to achieve its transition temperature with only 12V applied across it.

I put a small resistor (4.7 ohms is a good value) in series with the PTC thermistor and amplify the voltage drop across the resistor with an op-amp. I feed the output of the op-amp into an A/D input of my PIC.

I use a normal negative-temperature coefficient thermistor in series with a resistor to measure ambient temperature.

I can then calculate the air velocity in feet-per-second.


Charles Linquist