I'm attempting to create my own hot wire liquid water content sensor, the control circuitry of which is similar to many hot wire anemometers:
RS, the sensor wire, is a 5 cm long, 125 micron diameter platinum wire with a resistance of 0.5 ohms. RT, the temperature compensation wire, is an 8 cm long, 25 micron diameter platinum wire with a resistance of 17 ohms. My understanding of how hot wire anemometers are supposed to work is this: as air flows over the electrically heated wire RS, its resistance goes down, making the Wheatstone bridge unbalanced. This increases the difference between the two inputs of the op amp, which then outputs a higher voltage, increasing the temperature and resistance of RS until the bridge is balanced again.
The issue I'm running into is that the op amp is unable to provide enough power to heat the sensor wire, which needs to be about 50°C. Im using a LM358P op amp. I think that part of the issue may be that because the resistances in the bridge are so low, the op amp needs to deliver more current than it is capable of to supply the output voltage expected. I've been supplying the op amp with between 3-32v single supply, and the output voltage and current have been around 30 mV, 30 mA respectively.
I study mechanical engineering so the workings of op amps is something I've been trying to teach myself, any suggestions for increasing the power through the sensor wire to heat it properly would be greatly appreciated!