I am building a small device that operates underwater so I am powering it from this http://www.homedepot.ca/webapp/wcs/... 10000048&N=0&Ntk=P_PartNumber&showreviews=1". It is 11W, (120V 60Hz, .19A, 180GPH). I will attach my rotating device directly to the impellor shaft of the pump (after removing the impellor). So effectively, I am using the pump as nothing more than a waterproof motor. The device I'm powering is a disc about 6" in diameter that I want to spin at about 10-15RPM, which I will get by hooking up various nylon step-down gears. What I need to do is figure out the RPM of the pond pump. Not sure how to do this. Yes, obviously a stroboscope would be perfect, but... Follow my logic. Presumably, the 60Hz AC plays a role in the RPM. If there were only 2 "brushes" (I think they're actually brushless) in the motor (at 180 degrees), that should make for one AC cycle per rotation, which would produce a 3600RPM motor. But I don't know if I can count on only one set of "brushes", there could be four or even six, making for a 1800RPM or 1200RPM motor... Any ideas about how to figure out the RPM? It doesn't have to be too accurate.