Barometer Programming Guide
1-Wire Indoor Sensor/Interface Kit

Conversion Formula

where nVal is the decimal value read from the ADC.


MPX4115AP Pressure Transducer

Range: 15 ~ 115 kPa kPa = 100 kPa = 1000 mb (total span)
  200 mV ~ 4800 mV Vout = 4600 mV 4.6mV/mb

Known Global Barometric Pressure Records

Low: 870 mb    
High: 1082 mb   (high end will be compromised to 1070 mb since low end more important)

   200mb (required range is 1/5 total span)


DS2438 1-Wire A/D

Input range: 1750 mV ~ 9750 mV V = 8000 mV (10 mV steps)

  800 steps
Note: extremes of range not used to avoid possible compromise of ADC.

(200 mb) / (800 steps) =
    0.25 mb/step
  ← sufficient accuracy for 0.01” Hg resolution

Transducer Vout @ 870 mb (25.69”) = 200 mV + (4.6)(870-150) = 3512 mV
Transducer Vout @ 1070 mb (31.60”) = 200 mV + (4.6)(1070-150) = 4432 mV

Note: The value of 150 represents the bottom end of available ADC range.

Transducer Vout =
    4432 mV - 3512 mV = 920 mV
  ← this is ADC Vin range

Av Of Scaling Circuit

Gain scales transducer output to specified range for ADC.

Av(required) = Vout / Vin = 8000 mV / 920 mV = 8.696

  Stage 1 (Instrumentation Amplifier) Acl = 2.0 (fixed)
    Stage 2 Acl = 4.348 (variable to set during calibration)

V Offset

Sets expected bottom end of sensor output to 1750 mV. Once calibration is set, offset is used to compensate for altitude ASL.

Vout range = 1750 mV -9750 mV
    (1750 mV = 870mb, 9750 mV = 1070 mb)

Vin required to get 1750 mV output with Acl of 8.696:
    1750 mV / 8.696 = 201 mV

 Voffset = 3512 mV - 201 mV = 3311 mV
    3512mV is transducer Vout @ 870 mb