I recently tested my altimeter. I took two aircraft altimeters, one recently calibrated, along with my project unit over the Cascade Mountains from about sea level. The road has several altitude markers and two summits with altitudes close to 5,000 feet. My project altimeter was within 10 feet of the calibrated altimeter over the trip. The uncalibrated unit was within 25 feet.
My project altimeter used an MPX4115 sensor feeding the ADS1100 A to D on the inverting input. The non inverting input was fed through a 10 turn pot. It was used to adjust the base altitude to surface level.
No op amp was employed. The PIC 16F676 read the ADS 1100 and performed a table lookup using a piece wise linear interpolation as described in Microchip Application Note AN942.
A 2x20 LCD is used for readout. My next step is to save a stack of recent history to calculate a rate of climb. I've already included the traces on the board to include a temperature gauge using a KTY-81 sensor, which I have tested in a separate unit.
When complete the unit will be used as a hot air balloon instrument package.
**broken link removed**