Dan:
I might have mentioned that I have made several pressure-tight enclosures
for my dive computer proto's. It's not as bad as one might think. From an
engineering standpoint the absolute pressure on the enclosure is not very
high at depth. Especially if you are a recreational diver respecting the 130
ft depth limit. The max pressure your device will see is less than 75 psi.
This is a trivial amount of pressure for a water/air seal, and as suggested,
o-rings will do this in their sleep. Most enclosure designs go wrong when
the structural shape is not analyzed to consider loading on major surfaces
and support for the seal. The Otter Box that flexed and snapped the sensor
in  Smith's computer was caused by the psi at depth x the approx. 15 square
inch box lid.
At 130 ft, that would have been around 1100 lbs of force on the lid -
obviously it never made it nearly as deep. Without any structural support at
the center of the lid, it simply flexed until  the wall reached the sensor
tube.

Think about it - if two opposing sides of the enclosure (call them top and
bottom) have to be large enough for one to allow you to see a display, use
this to your advantage. If the other 4 sides are kept low in surface area,
relative to the 2 large sides, the deeper you take the box the better it
will seal since the increasing pressure will press the top and bottom
together against the seal with more force than the pressure trying to deform
your box from the sides.

You can get away with a fairly simple enclosure using this principle, as
long as it is rigid-walled and well fitted.

Mechanically actuated switches are also a no-brainer if you actuate them
using round shafts through o-rings in the wall. You can also engineer the
shaft-to o-ring seal, but when you do, you'll find that with such low
pressures, it's not worth the effort. Just pick a ring that will fit in your
design, and be sure that you have a wall thick enough to hold it.

The major manufacturers imply that there's rocket science involved in
designing their dive computers, but as far as the case/enclosure goes, don't
be deterred by the hype. It's all 1st year engineering.

Chris

Chris

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of D Lloyd
> Sent: Thursday, March 21, 2002 8:37 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: Submersible sensors
>
>
> Hi,
>
> Thanks to all who responded to this request. I was
> particularly interested
> in the dive data logger as my application is almost exactly the same
> (although it will be a one-off and was attempting to be a
> cheapskate and
> build it out of the ever-increasing number of sample parts I
> have....!)
>
> Now all I need is a cheap, submersible enclosure with a clear
> 'lid' to see
> the LCD!
>
> Thanks again,
>
> Regards,
> Dan
>
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