FatSebastian wrote:How well does it workl? Also, does it work continuously like an analog tachometer? I would like to be able to measure, say, the decay rate of spin down. (For example, how much time does it take to go from X RPM to X/2 RPM?)
There are definite limitations with this device; one might have to pay more than $13 for a more versatile instrument. It updates the display about twice a second, and afterwards can report highest, lowest, and last reading. I don't think one could measure decay times to sub-second accuracy.
Still early days with this, so still tinkering. I ran a few trials the other night, but that's all so far. Trials with the single (1.5 oz.) weight gave max-RPM readings of 1700, 1664, 1655, and 1668, and there's some chance of operator error on the first reading. (Using two hands to hold the tach and the spin ring, respectively, I may have lost aim briefly. I may try to construct a jig that holds the tachometer and pin vise in the correct relative positions.) The two-weight (3 oz.) readings were 1868, 1820, and 1875 rpm.
I'm not sure if this would be reliable enough to detect friction differences at the level we're interested in. Re-reading the original description, a 1/4 ounce weight is suggested; perhaps the lower forces would help bring smaller differences more readily to light. I've got to read up more on angular momentum, something I'm certain was not covered in my high school physics class.