How do I, using common household tools and objects, determine the (variable) RPM of a fan?
To set the scene, we recently put in a woodstove that we're using to (mostly) heat the house. This stove combines two wonderful things: 1) obsessive experimentation with a multitude of variables that affect its efficiency, and 2) playing with fire. So this is but one of the small facets of its operation upon which I'm nerding out.
The fan atop my woodstove is powered by a Stirling engine
. I move it around to optimize its speed and direct more warm air in specific directions. The fan speed is determined by the temperature differential between heat conducted from the stovetop and cooler air passing over the fan's cooling vanes. I can determine the hottest point on the top of the stove with great precision, but because the fan speed is also dependent on the flow of cooler air reaching the cooling vanes, it's difficult to determine which placement is yielding the highest RPM. The fan is spinning too fast to judge its speed.
I would like to determine the approximate RPM of this spinning fan under different conditions. I thought about taking photos of the turning blades with different shutter speeds on a dSLR, and comparing the blur, but the pre-programmed shutter speeds seem too blunt an instrument for the measurement I'd like to take. I thought about using something that flickers at a defined rate, but I don't have a CRT and can't think of anything similar.
Home engineering types, can you cobble together a means to take an approximate measurement and further enable my stove tinkering?