Senior Member
Joined: 17/11/2011
Location: AustraliaPosts: 231
| Posted: 12:59am 14 Mar 2012 |
I read the Sparkfun and Adafruit tutorials on the BMP085 and they left out some important points.
<BMP085 Q&A>
1. Why doesn't the RAW BMP085 Barometric pressure reading match the local weather station?
Note: In most cases the local weather station will read higher (lower elevation) than the RAW pressure BMP085 reading. In my case, 32 millibars lower for 273.1 meters in elevation for my location.
Note: The BMP085 is good for +- 2 millibars.
All weather stations in the US and other countries? uses sea level as the pressure reference. This means, what you see from a weather station is without the change in pressure due to elevation changes.You can go from one weather station to the next and they keep the same pressure reference without changes elevation pressure.
Note: The pressure at sea level is 101325 Pa and the pressure goes down when there is an increase in elevation.(Air pressure is lighter [less pressure] at higher elevations)
To use the BMP085 as a true weather indicator you will need to add a calibration offset to the raw BMP085 pressure reading. By doing so, you are removing "your offset" due to elevation height. This way it will track other area professional weather station readings.
If you don't have a weather station close by to calibrate the BMP085 then use this website link:
http://www.csgnetwork.com/barcorrecthcalc.html
You need to know your exact elevation (in feet or meters) to add their factor for your BMP085 pressure calibration. Do not use a GPS to determine your elevation!
Please note: Sparkfun used the raw (uncalibrated) BMP085 pressure for weather prediction calculations. This is fine and will work but beware it will not match other area professional weather station readings without this calibration offset added.
2. Why doesn't the BMP085 / Sparkfun elevation calculation match the elevation at your location? If it does, consider yourself extremely lucky. This means the weather conditions, temperature, humidity at your location and at sea level is perfect!
The Sparkfun altitude calculation uses a fixed sea level pressure (101325 Pa). This is fine but the weather, temperature and humidity at sea level and at your location is not always perfect. For better precision you need to find out the pressure at sea level for your location "area". Website link:
http://aviationweather.gov/adds/metars/
select Metars
enter your 4 letter airport station iden. example "KOSU"
select translated - get the exact sea level, for the area.(Used by pilots)
This pressure is due to weather changes at sea level
Note: Australians need to find a similiar website that calculates the sea level pressure to put in the altitude calculation (for your country)
Enjoy
