Altimeter

I got four pressure sensors (two MPX4250 and two MP3H6115A) from freescale sometime back. I tried the two 4250s but they didn’t work for some reason. MP3H6115A is a surface mount device (SMD) which can measure pressure from 15kPa to 115kPa. Since I have previous experience working with SMDs, (SHT11, humidity sensor) I decided to work with this. The output (pressure) is reported as a voltage ranging from 0 to 2.8 v. I used PIC16F88 since it has an internal 10-bit ADC (analog to digital converter) and 4k code space.

Pressure is given by
P = (Vout/Vs+.095)/0.009 kPa

Where Vs, supply voltage = 3v
Vout, Output voltage measured from ADC

Altitude is given by

H = -ln(P/Po)*R*T/g

Three variables have to be measured:
T - average temperature in degrees Kelvin
Po - atmospheric pressure at ‘zero’ level
P - atmospheric pressure at current level

Remaining values are fixed constants:
R – universal gas constant = 286
g – gravitational acceleration = 9.81

Assuming Po=101.325 kPa and T=303.15K (30C)
The device reported H = 52.5m

Accuracy of this value is largely depending on the resolution of the ADC.
10-bit resolution of the ADC is not even near to get a reasonable accuracy of the altitude. One unit change in the ADC measurement will change 10m of height. To obtain 1m sensitivity, I need at least 12-bit ADC. So I ordered 12-bit ADCs from Maxim-IC and waiting until they arrive for the final product.