Fan Speed Measurement - Part I

I wanted to determine the speed in RPMs of a computer fan that I had. Thus, using the CBL data-recording device with the light probe, I built a photogate of sorts to time the spinning of the fan blades. I used:

• A CBL (Calculator Based Laboratory) with light probe
• TI-83 Plus for data collection
• IR LED (940 nm wavelength)
• The fan
• AC adapter (9-12 VDC) for power

This was very simple to set up. I placed the light probe resting on a plastic strut in front of the fan, and lined up the LED on the other side so that there was a clear sight when the fan blades were not blocking them. Then I set up the CBL and calculator to record the light level every .0001 seconds, for 512 samples. From the data table I have graphed a portion of it:

My resulting calculations were t /hat from .0029 seconds to .0355 seconds, 13 blades passed through the measurement point. Since there are 7 blades on the fan, the difference in the time divided by the blade count multiplied by the number of blades in one revolution gives a time of .01755 seconds for one revolution of the fan. To calculate RPM, divide 60 by that value and the answer is that this fan has a speed of ~3418 RPM. This value does seem accurate, because from 2500-4000 RPM is the range of most computer fans.

Fan Speed Measurement - Part II

The next logical step was to measure the speed of my ceiling fan in its three settings: low, medium, and high. To accomplish this I taped the light probe to the ceiling, using nearby shelves to sit the CBL and 83+. I held a small flashlight under the fan blades pointed at the probe, and recorded the following data:

This time I will spare the calculation details. The results seem to indicate accurate measurements.