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Sensors
by Mike Manes, W5VSI
Sent 02-Apr-03
Pressure
EOSS has used the Motorola MPX5100A conditioned absolute pressure sensor for many years. I was dismayed to notice that Motorola has altered their spec pressure range to bottom out at 2.3 psia lately, however. This is the barometric ambient at about 33,000 ft. But I knew the zero drift spec even before ate up the absolute accuracy at 10 mbar (0.15 psia) at 100,000 ft. However, I expect that the current devices will work OK down to a hard vacuum, provided you do a calibration run vs. GPS. The MPX5100A is available online from DigiKey and Allied for about $20.
Temperature
We use the LM335 linear temp sensor for inside and outside air temp. Altho these devices are spec'ed only down to -40C, they do register lower temps. Biggest problem, especially at altitude, is self-heating. Be sure to shield the outside sensor from the sun and bond it to a heat sink that gets some good airflow from the slipstream.
Both devices operate fine with the 0 - 5Vdc 8-bit ADC in the KPC-3+. But the Vref for that ADC is just the +/- 5% Vcc source. So we include a LM385-2.5 stable 2.5V reference on a 3rd analog channel for telemetry reference.