This is similar to a scuba computer that I did recently. The vessel will
deform and change volumetrically at fairly shallow depths - this is
something one learns
during the earliest scuba cert training. I wouldn't want to even try to
calibrate a system
with a differential sensor inside the vessel.
What max depth will your sub run at? for shallow depths (<100 ft.)I think
the most convenient,
reliable, and simplest solution for this may be a
Motorola gauge type pressure sensor. According to the data sheets they are
only for use in air,
but since they have a fluorosilicone gel covering the chip, I have found
that they have a
 pretty long life in fresh water, even in chlorinated pool water. These are
very small devices,
approx 20mm round by 6 thick. 5V analog output models are very easily
scaled. The device can be
inside the sub with it's barbed fitting through the outer hull with a small
o-ring.
They are fairly linear. The hardest part of working with them is
themperature compensation.
f your water temp is pretty regular, say within =/- 10C, it will be no
problem getting accurate readings
to within about 1/2 ft, down to about 100 ft.
You might want to check out the MPS5700 family.

Chris

Alternately

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Donovan Parks
Sent: Saturday, February 02, 2002 1:13 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [OT]: Measuring Depth of Submarine using Differential Pressure


Hello,

Here is the setup.  I have a submarine with a differential pressure sensor.
The high pressure end of the sensor will be placed in a 'ballon' that is in
the water and the low pressure end will be in the interior of the sub.  For
technical reasons I can't run a tub up to the surface of the water.

How do I can this differential pressure into a measurement of the subs
depth?  Since the submarnie was sealed at the water surface pressure (i.e. 1
atm) and is rigid I am assuming the pressure in the submarnie will remain 1
atm regardless of the depth of the submarnie.  Now, the pressure of the
'ballon' will change will change will depth as it is non-rigid, but what is
the conversion factor from differential pressure to depth?  I have a book
(which I don't have access to right now) that gives the relationship, but I
don't really trust the source and it is to 0 decimal places of accuracy!  I
will be using the sub in FRESH water.  (A search of the Internet has been
unfruitful - likely because I am not sure what I should be searching for.)

Regards,
Donovan

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