Hi David,
There are many commercial units available made exactly for this job.
Most of them involve the vertical movement of the air-to-water
interface at the surface of the water in the tank to register level.
This range includes those units which rely on things that float,
and units which rely on things that bounce echoes,
and units which rely on conduction sensors that make contact with
the water itself.
All of these types which involve surface interaction have their own
problems ranging from pivots that sieze, floats that leak, cobwebs
that echo, damp that attacks electrics, corrosion that can wreck
electronics, insects that set up home in electrical units. Problems
too numerous to list, most of which can be overcome by frequent
maintainence.
The most reliable arrangement that i have met for this job where an
underground tank is involved, is the simple bubbler pressure sensor.
With this type of unit all the electrics and sensing gear is above
ground, and can be easily accessible if necessary.
The piece that goes into the water is just a length of tubing with an
open end. Probably tied to a stone to ensure it stays at the bottom.
Tubing probably plastic so that it wont corrode. Weight probably a
stone or a brick so that it wont corrode. Tied with maybe itself or
plastic string so that it wont corrode.
This is the most reliable and maintainence free arrangement i have
ever seen on this type of job. Just keep the air flow pretty low,
just enough for about a bubble per second roughly, as i have known
one which was set too high had a build up of grime and dust due to
the air flow. It did not stop it working, but it got a bit messy.
after cleaning and adjusting the air flow down to a sensible low flow
it was fine and stayed clean. I think faster moving air seems to stir
up dust and carry it about.
The associated electrics for this type of arrangement is very simple
generally a diaphragm is used to operate switches at the pressures
corresponding to the 'Hi' and the 'Lo' levels.
If an 'all-electronic' arrangement is wanted, there are pressure units
that can be used with electronics instead of diaphragms.
Fish-tank type pumps are cheap and very reliable, and count typical
running time in months or years.
Best of luck with your project, John