In my experience, ( downhole oil well tools), stewart and stevenson blowout
preventer controls, potted electronics used in wet locations with no
pressure...), potting sucks.   Why not just
create a pressure vessel fo it?   Potting with silicone rubbery stuff
doesn't stop pressures like 250 PSI, and seawater is death to circuit
boards.  Short of making an openable pressure vessel like a cigar tube with
big O rings that just slides open and shut, you might get away with potting
the whole in hard hard epoxy  with glass fiber added and an empty chamber
designed in around the batteries, then wrap the battery area with hard
rubber and tighten down on that zone with band clamps and let it build
pressure from some batteries that are "sealable".  Like the lead acid
suggestion.

Assuming that any old potting approach will solve this will probably get you
a circuit that lasts as long as your batteries.

With the unknowns you face, a reusable pressure vessel is what I recommend.
Just think cheap materials, simple cylinder and sphere shapes, band clamps,
O rings, pipe, stamped and drawn metal shapes like the Zero Halliburton Co.
makes, and design one.  Glass tubing will take compression.  You could
design feedthroughs out of the system, and have the display read through the
glass tube wall, and the ends seal with multiple O rings....

John G Austin TX

> -----Original Message-----
> Hey guys,
>
> I'm working on a PIC application that is meant to be exposed
> to fairly high ambient pressures - something on the lines
> of up to 250 PSI (it's for undersea use - a deep diving decompression
> computer for diving with Helium).