Here's a non electronic way to skin the cat discussed in the "See
Water bilge switch report" thread.

In order to have a longer ON / OFF range than provided by common level
switches, put a standard float switch in a coffer dam of the height
you want to start the pump. Put one of those valves used in anti
siphon loops in the bottom. The bilge will fill until it overflows
the cofferdam and the float switch will turn on the pump. The
cofferdam will empty out through the one way valve with the bilge and
the float switch will go off.

This would also prevent surging water from burping the pump in a
seaway. You might need a pretty clean bilge to keep the valve from
plugging up. OTOH a screen across the top of the cofferdam would
probably keep things clean enough though. It would also protect the
float switch.

Damn. I like this idea! Much simpler wiring. With our large flat
bottom sump, I've got to build the cofferdam anyway to avoid the surge
problem. Saves forty bucks of additional float switches. What's not
to like?

The one way valve is the key thing. It has to let the water drain out
through 3 to 4 inch head but seal tight as the bilge fills. I just
bought a siphon break for our head. I'll fiddle with it and see if
these easily available and replaceable valves are the answer.

--

Roger Long