For what its worth, float switches can be fit into small places.
A compact float switch involves two weights on a cable, hung
over a pulley (actually in contact with the pulley for 450 degtrees
so it doesnt slip).  One of the weights is buoyant and will rise
with the water level, thus turning the shaft.  The shaft is then
connected to a potentiometer for an analog output, or a shaft
encoder for digital.

But the practical way to do this is with a common pressure
sensor.  In the water industry, most tanks are monitored via
pressure sensor.  And each tank has an overflow level float switch
as a backup.  Pressure sensors dont fail often, but avoiding
accidental overflows is a high priority.



----------
> From: Clyde Smith-Stubbs <clyde@HTSOFT.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Sensor questions (maybe [OT])
> Date: Tuesday, April 07, 1998 7:47 PM
>
> I need some suggestions on a couple of sensors;
>
> 1) I need to measure water level (and thus contents) in a concrete tank.
> The tank is about 2m deep, so a float is probably not practical. My
> thoughts are either an ultrasonic setup to detect the water surface,
> or a pressure sensor. The ultrasonic setup will be complicated, the
pressure
> sensor would either have to be at the bottom of the tank, or above it
with
> a hose to the bottom. The hose idea would rely critically on having no
air leaks
> in the hose, difficult to guarantee for a long time. Submerging the
pressure
> sensor would leave it liable to getting blocked by stuff on the bottom of
the
>  tank.
>
> 2) I need a "light beam" sensor to detect vehicles or people moving down
> a driveway - much like a door minder used in shops, but to operate
outdoors
> spanning a distance of about 20m. It should preferably use infra-red so
as not
> to be visible (this is not critical, but if it uses visible light it
should not
> be too bright). The electronics side of this is reasonably
straightforward - I'd
> modulate the beam at some frequency, recover the modulation and
differentiate it
> to detect sudden interruptions. This should make it immune to external
lighting,
> fog etc. The hard part is what kind of light source, detector and lenses
to use,
> especially for infrared.
>
> All suggestions from this esteemed collection of minds will be gratefully
>  received.
>
> Clyde
>
> --
> Clyde Smith-Stubbs               |            HI-TECH Software
> Email: clyde@htsoft.com          |          Phone            Fax
> WWW:   http://www.htsoft.com/    | USA: (408) 490 2885  (408) 490 2885
> PGP:   finger clyde@htsoft.com   | AUS: +61 7 3354 2411 +61 7 3354 2422
>
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> HI-TECH C: compiling the real world.