How do you measure fan speed?

You can use a specialized wind speed gauge, which I did not have.

You can also use a small scrap of paper and a video camera (which I do have).

The idea is that the bit of paper will move just about as fast as the air is moving.  To measure how fast that is, you can record some video and look at one frame showing the paper moving. I used four trials and averaged the results, for each speed setting.

One standard NTSC video frame is composed of two interlaced fields, each 1/60 second in duration, so the full frame shows the paper's motion over a time interval of 1/30 second. This assumes you have your camera's shutter set to the standard video exposure of 1/60 sec, and also that your NLE exports full combined-field frames (not just a single field with the other field interpolated). Both of these are true in this case.

I put a six-inch length of masking tape on the back of a chair, to use as a reference in measuring distance. In the image below, you see the blurred trail of a scrap of paper moving from right to left just above the chair. As you can see the paper is also rotating. Since this is a combined-field frame, you see dark lines where the paper is not present during the period of the other video field. Note that the bright lines shift from odd scanlines to even lines as the paper moves from field 1 to field 2. This image is enlarged to 200% so you can see the scanlines more clearly on your screen.

In this example the paper moved a horizontal distance of about 6 inches in 1/30 of a second, which is 180 inches/sec, 15 feet/sec or 10.2 mph.

paper-motion

(The chair is a relic from the 70's, in fake-leather-textured vinyl. No animals were harmed during this experiment.)

If you are interested in measurements on fans, you might also enjoy this page .

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