Hi Jim,
Any idea of the max rpm it will display? I used a cycle computer (without an rpm function) on my old mill, making it believe it was on a 1667mm circumference wheel and reading out in kph. Multiplying the display by 10 then gave the rpm (e.g. 45.6kph = 456rpm). But it didn't expect anyone to pedal faster than 99.9 kph, limiting it to 999 rpm.
Wilkinsons sell one with "AS 11G" on the front for £8 which has an rpm function. I've got one of those now, and despite what its Chinglish instructions say, it will go up to at least 3000rpm.
One note of caution: the sensor heads contain a tiny glass encapsulated reed switches. Very fragile, as I found out the hard way.
Andy