# Thermistor temperature dispersion

I am designing a thermistor circuit to determine if a nozzle is becoming plugged or flowing freely. My idea is to use a low resistance thermistor and resistor to form a voltage divider that when powered would become hot, and immerse that into the stream of water. The water flowing by with the nozzle unplugged would keep the thermistor cooled to a certain temperature and when the nozzle started to become plugged the flow past the thermistor would decrease and allow the temperature to rise.

I am looking into designing the circuit and buying all of the parts but I have absolutely no heat transfer experience and don't know how sensitive the circuit needs to be. To have a low enough resistance so the circuit actually heats up I am planning on using a 33 ohm bias resistor and a 33 ohm thermistor.

So my question is how do I calculate what voltage I would need to feed into the voltage divider to heat the circuit up to say 100 degrees C, and how do I calculate say how much heat there would be dissipated if the thermistor at 100 degrees C was immersed into a 3/8" diameter PVC pipe with a flow of .5gpm?

• This is often done in air with a "hot-wire anemometer"; I don't know how well it would work in water. – pjc50 Jun 18 '13 at 22:21