MPL3115A2 Pressure Sensor Hookup Guide
Nate 
Pressure vs Altimeter Setting
If you grabbed a few pressure readings and became confused when you checked your local weather conditions, you're not alone. The absolute pressure that the MPL3115A2 pressure sensor outputs is not the same as what weather stations refer to as pressure. Weather stations report pressure in lots of different units:
- millimeters Mercury (mmHg)
- inches Mercury (inHg)
- millibars or hectopascals (hPa)
- pounds per square inch
- atmospheres (Atm)
- kilogram per centimeter
- inches of water
In barometer mode, the MPL3115A2 outputs pressure readings in Pascals. This is most closely related to millibars or hectopascals. But, why does the sensor not agree with the station around the corner? This is because many stations report pressure in a few different formats. Have a look at all these numbers for the Boulder/Denver area. The key is that your local weather station is probably reporting the Altimeter setting.
Thank you National Oceanic and Atmospheric Administration (NOAA)! Did you know they're headquartered here in Boulder, CO?
- Station pressure - This is the pressure that is observed at a specific elevation and is the true barometric pressure of a location.
- Altimeter setting - This is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude.
- Mean sea level pressure - This is the pressure reading most commonly used by meteorologists to track weather systems at the surface. Like the altimeter setting, it is a "reduced" pressure, which uses observed conditions rather than "standard" conditions to remove the effects of elevation from pressure readings.
The calculation to get from Pascals to 'Altimeter setting' is a bit gnarly:
Grab the full formula here and give this great Altimeter setting calculator a try. This formula relies on two things: knowing the current pressure in milibars and knowing the height above sea level that the pressure was read. We recommend you capture altitude using a local survey point or a GPS receiver.
If you installed the MPL3115A2 library, you should also have the BarometricHgInch example sketch under the Examples->MPL3115A2_Pressure menu under the Arduino IDE. We didn't build this calculation into the library because it could potentially chew up a lot of RAM and code space calculating all the floating point math. But, if you're doing home weather station calculations, this should get you started.