goldmine <www.goldmine-elec.com> or <www.allelectronics> are selling an
ultrasonic level detector/pump controller surplus, and cheaply as i
recall.  alternately, motorola is making an  e-field/capacitance sensor
chip that can easily be used for non-contact level sensing of conductive
or nonconductive fluids.  you putt a couple of electrodes down the
inside walls of a length of pvc pipe and look at the capacitance change.
 alternately, for and non-continous output you can just put 2 sensors at
the level you care about and look at the large change in value as the
fluid covers/uncovers that part of the pipe.  the capacitance approach
should be highly durable, easily cleaned and relatively insensitive to
dirt/oil etc. and will work with oils or other insulators as well as
water (though the values will be different depending on the fluid
involved), and you are only passing a very slight low frequency field
through the fluid with no dc component, no electrode contact, etc.

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