# Air Speed Measurement Sensor

Hi
I am working on a project where I am required to measure the speed of air inside a normal room with an AC or a ceiling fan. Obviously this speed will be low. Is anyone aware of a cheap, small, low cost, low energy sensor for such an application. Any help in this regards will be highly appreciated
thanks

You can measure deflection with a piezoelectric crystal.
You could also measure static discharge to the gate of a transistor (generate a steady electromagnetic field on the end of the material, and put an antenna on the gate, so as the field nears the antenna, a greater amount of energy is discharged to the antenna. Make sure to sink the antenna with a resistor (100-megaohm), because you will need fairly high voltages to have noticeable effects over air)

Or you could put an optical encoder on a pinwheel.
Or measure the pull on a strip of silk. Actually, you could might be able to follow the currents with this, too.

It sounds like you need measurements down to 50mm per second. Unless there’s something that responds on an exponential scale, I think you’re going to have trouble calibrating it. Accuracy is a very difficult thing.

(Considering it’s around easter, you could probably tie tufts of easter grass to floss instead of using silk)

How much work do you want to do?

You can get free samples of pressure transducers from Freescale, and you can build a pitot tube and a static pressure sensor with them. (There are even differential models with two inputs.) You could probably hook those up to any microprocessor with analogue inputs, and translate pressures into velocity using a bit of math (Bernoulli’s principle, essentially). See here, for example.

I’ve seen these work well enough to read back airspeed on model aircraft (for an autopilot), so you can probably expect decently good results. (I nearly built one of these systems myself, but we managed instead to salvage the sensors from an old version—I just built the pitot-static tube itself.)

On the other hand, for simplicity, try to get your hands on a commercial hot wire anemometer. I don’t know whether any exist that can read low speeds reliably, but if you’re planning to run the ceiling fan at full speed, I don’t doubt that it would pick it up.

By the way, may I assume that you’re not aware that this forum is generally used for discussions among members of the FIRST Robotics Competition community? We’ll probably be able to give you general advice, but you’re going to find that a lot of answers tend to be centred around the competition challenge itself. Feel free to ask, however.

Besides a pitot tube, you could also use a turbine (fan).

Pinwheel with encoder sounds like the lowest hanging fruit to me.