Greg I had forgot about the SDX series and that would be a better choice
over the SCX. For a model rocket altimiter, you might want to check out
Lucas NovaSensor's NPP-301-100A which is a 15 PSI absolute sensor in a SO-8
package. It's low cost and the temperature compensation typically consists
of two resistors. There is no pressure port (just a hole in the package) so
the chip will fit in a very small space and it weighs only 0.10 grams. Lucas
sent an application kit and I made a simple barometer. Though not as
accurate as the SCX/SDX or NovaSensor's NPC-1220 series, the results were
still very good. They have two related application notes:

      Basic Temperature Compensation for the NPP-301
      Designing an Altimeter using the NPP-301

   I also checked out Motorola's MPX4115 series that David Fansler
mentioned. This seems ideal if the cost is right. You give it +5V and you
get a conditioned voltage output from ~0.5V to 4.8V.

   Back to your current `woes', I'd recommend selecting the sensor you are
going to use in the altimiter, breadboard the circuit and test it with a
DVM. Once your are satisfied with the results, then go on to the A/D
interface. It sounds like you are going around in circles with the basic SX
element considering the calibration and compensation issues. As far as why
the circuit stops working with the PIC hooked up, I've been following this
but I don't have a clue at this point...

   - Tom

      http://www.novasensor.com/
      http://mot-sps.com/products/sensors/

At 09:35 AM 2/2/00 -0700, Greg Hartung wrote:
>   I am using an SX15A, which is an absolute, uncompensated.  This is just
for
>testing, I am sure I will go with a compensated DIP package like the SDX15A2
>(the SCX are way too many $).  So for now, I'm not worried about temperature
>compensation, but this is all for a rocket altimeter so it will become
important
>later.


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)