SparkFun Forums

[iw2fvo]

good day to all in this forum,
I am very new !
I loaded on the Arduino the sfe_bmp180 library that includes wire.h and bmp180.h
Everything goes well but i do have a problem with the sea level pressure calculated by Arduino.
My home is located at 350 meters and I put this value on the .ino sketck and got this result:

provided altitude: 350 meters, 1148 feet
temperature: 27.48 deg C, 81.46 deg F
absolute pressure: 979.60 mb, 28.93 inHg
relative (sea-level) pressure: 1021.26 mb, 30.16 inHg
computed altitude: 350 meters, 1148 feet

All the stations around here and the closest airport give around 1017 mB of pressure instead of 1021 mB.

Any help/clarification please ?

Thanks in advance
iw2fvo, Ambrogio
North Italy

[jremington]

The value you report is just barely within specifications for the BMP180 (+2/-4 HPa), but the method of sea level correction in the code may not be very accurate.

See the BMP180 data sheet, "Absolute accuracy" section.

You could try correcting the reported values with a multiplicative scale factor of 1017./1021. = 0.9961, but a better approach would be a linear (y = a*x + b) calibration derived from a series of measurements at different pressures.

See this general overview for calibration: https://learn.adafruit.com/calibrating- ... -calibrate

[iw2fvo]

Thanks for reply,
I did some measurements today at my home location but I would like to repeat them on the Varese lake surface to eliminate any error due to the site elevation.
I still found differences of about 4 mB : here is one sample .
Thanks again fo the assistance,
Regards,
Ambrogio

19th august 2017: 11:23 UTC

SFE bmp189 sketch:

provided altitude: 350 meters, 1148 feet
temperature: 27.31 deg C, 81.16 deg F
absolute pressure: 974.33 mb, 28.78 inHg
relative (sea-level) pressure: 1015.77 mb, 30.00 inHg
computed altitude: 350 meters, 1148 feet

Other reference:

_Meteo stations around Varese ITA: 1012 mB
_LIMC Metar ( Milano Malpensa airport): 1013 mB QNH ( Updated 3 hrs ago )
_http://www.hochwarth.com/misc/AviationCalculator.html:1015.7 mB QNH ( entered: 350 m and 974.33 mB QFE )
_EM_406 GPS module: $GPGGA,112222.000,4547.1319,N,00849.3723,E,1,09,1.1,350.7,M,48.0,M,,0000*50
_mobile phone GPS: 347 meters +/- 6 meters
_Free map tools ( internet ) :350.9 m or 1151.1 feet @ Location 45.78557,8.82268

Notes:

BMP180 sketch is always different from other meteo stations by around 4 mB.
BMP180 sketch always agree with the AviationCalculator.html ( it is perfect ! ).
EM406 GPS module altitude "jumps "significantly" : even more than +/- 20 meters over the time ...
Mobile phone GPS altitude jumps over the time ...

[jremington]

I still found differences of about 4 mB

Of course you did. That is expected.

The specification of absolute accuracy = +2/-4 HPa means that if the sensor reports 1020 HPa, the true pressure is most likely in the range 1016 to 1022 HPa.

You can calculate a correction, which should be constant. See my other reply.