Donovan Parks wrote...

>I'm only looking at a depth of only 10 feet.  Right
>now I am using the Motorola MPX5050 (a differential pressure sensor) and=
 am
>*hoping* that the sub will not change volumetrically at shallow depths =
so I
>can get an accurate differential pressure.

How much the internal volume of your sub changes with depth will depend
on its geometry. =20

If it has a circular cross-section the hull material will end up
experiencing purely compressive forces, and I doubt whether you'd be
able to detect any change in the sub's internal volume (and thus its
internal air pressure).  On the other hand, if it has an elliptical
cross-section it's going to experience forces which will tend to flatten
the ellipse, and which may make it deform very significantly indeed if
the hull material isn't quite rigid.  So, shape matters.

As for temperature errors, looking at the Motorola datasheet for the
MPX5050 it appears the sensor and its integrated electronics will have a
temperature-induced error of  1% of full-scale or less from 0-60 deg. C
(i.e., +/- 0.075 PSI maximum error, equivalent to a depth error of 2"
H2O).  And you're hardly likely to encounter this much temperature
change.

A much bigger error source will be the change in air pressure inside the
hull caused by changes in the air temperature.  With a fixed mass of air
trapped in a fixed volume, the pressure will be proportional to absolute
temperature.  To compensate for this you can use a PTAT temperature
sensor like National's LM335 and measure its output with the PIC A/D,
and do the compensation in software.

=46rankly, I think it would be a whole lot easier to just use an AP =
sensor
like Motorola's MPX4200A (3-29 PSI absolute) instead; no fuss, no muss.
I'm using these things in a product and have found them to be very
stable with temperature.  Great little devices.

Hope this helps a bit...

Dave

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