malaigo wrote:
The windmeter manufacturer uses a linear ax+b formula to convert from impeller rpm to m/s wind speed. So I read impeller rpm and just use the values of the constants a and b that they provide. Doing k(ax+b) where k is a fudge/calibration factor doesn’t seem so easy to do. There is always a bit of wind outside. Seems it’d have to be done indoors (no wind), ride at a selection of fixed speeds down some long corridor. Then one might as forget k and just use the new empirically determined a’ and b’ values.
Yes, the rpm to speed formula is published by the manufacturer. I have one and have tested it a few times. Good for what it costs, but not stellar. But Let's say for a minute that number is right.
The issue is, the airSpeed measured at the windMeter is not the airSpeed if, for example, you placed the windmeter a few feet out in front. There is a correction that must be applied to the wind speed.
If there is 0 wind, and I put the meter 10feet out in front of me, and I go 36km/h I will probably see 36km/h. If I put the meter 1m in front of my torso, I will probably see 33km/h, however 36km/h is the number I want to be using in my calcs.
Windmeter calibration is not trivial. Very coarse calibration is easy, very precise calibration is not.