"Bob" wrote in news:1140021985.976464.88870
@g43g2000cwa.googlegroups.com:

But the article
description mentioned some electrical magic that I do not understand.



These electronic-controlled pumps come on every few minutes to measure
motor current, which will be low if the pump is dry, but high if the pump
is underwater and DOESN'T cavitate madly. If the current is low, the
electronics shuts it back down until it times out again. Runs about 3
seconds if dry. Runs until the current shows a good drop indicating the
pump ran out of stuff to pump overboard if the initial current is high.

No magic, just an series small resistor to sense motor current and a custom
IC to monitor it, already programmed.