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Barometer/Altimeter discrepency

NotSoAthletic
over 2 years ago

Hello,

Owning an Instinct 2 for a few months, I quite don't understand the barometer and altimeter functions.

Traditionally, a barometer should be used always in the same place, showing the ambiant pressure.

I calibrate the barometer as the following : reading my altitude on a GPS app on my smartphone, then entering it in the Barometer Calibration settings as well as the correct pressure at this time. Then letting the watch "do its life".

It works correctly for a few days, pressure and altitude readings are correct.

Today I moved in a totally different place (like anyone who moves... does). When waking up, altimeter showed wrong altitude (50 meters wrong), and the pressure was way off (10 mbar off).

What is the point of altimeter and barometer... Can it be used just to monitor "current pressure" at a given place and how can I set it to always show the correct "ambiant" pressure?

Thanks.

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  • 0 over 2 years ago
    NotSoAthletic said:
    Can it be used just to monitor "current pressure" at a given place and how can I set it to always show the correct "ambiant" pressure?

    Yes, if you do not care about pressure changes caused by elevation changes, you can change the pressure sensor to the "barometer" mode. It will then lock the altitude and only show the barometric pressure changes not trying to distinguish whether they were caused by a change of elevation, or by a change of the atmospheric pressure.

    See also the article Understanding How the Barometric Altimeter Sensor Works on a Garmin Outdoor Watch | Garmin Customer Support

  • 0 over 2 years ago in reply to trux
    NotSoAthletic said:
    can I set it to always show the correct "ambiant" pressure?

    It would certainly be important to point out that the built-in barometer widget does NOT display the ambient pressure, but the pressure calibrated to sea level (MSL-Pressure / QNH). This is also the value given in the weather reports.

  • 0 over 2 years ago in reply to bluefish
    bluefish said:
    It would certainly be important to point out that the built-in barometer widget does NOT display the ambient pressure, but the pressure calibrated to sea level (MSL-Pressure / QNH).

    Yes that's true, but if you enable the manual "barometer" mode, and calibrate the pressure to the ambient level instead of MSLP, it will then continue showing the ambient pressure (changed both by the elevation and the atmospheric factors), instead of MSLP. At least until the next calibration. Not sure what happens at the nightly auto-calibration - if the altitude is the same, it should continue working in the same way, but if the altitude changed, it would shift the pressure reading too, and a manual recalibration would be needed.

  • 0 over 2 years ago in reply to trux
    trux said:
    you enable the manual "barometer" mode, and calibrate the pressure to the ambient level instead of MSLP, it will then continue showing the ambient pressure (changed both by the elevation and the atmospheric factors), instead of MSLP

    Yes, that is also true. In theory. In practice, the ambient pressure at different altitudes is different because of the different air density. And this difference is not linear, but a curve. And this results in a shift.

    The air temperature also plays a role here. While the respective ambient pressure has its own temperature, the QNH refers to a fixed value (see standard atmosphere)

    See Barometric formula

    I.e. here : https://en.m.wikipedia.org/wiki/Barometric_formula 

  • 0 over 2 years ago in reply to trux

    Yes, even if you lock the watch in Barometer mode, it will be OK for the place you're in at the moment of the calibration, but it will inevitably shift when you move, not reflecting the calibrated sea level pressure, of the place B anymore, and showing variations even if the pressure stricly speaking is not changing a lot.

    So there is no way to know what your watch "pressure" refers to.

    In my opinion it should only calculate altitude then show the absolute pressure... So it will always be correct...

  • 0 over 2 years ago in reply to NotSoAthletic
    NotSoAthletic said:
    In my opinion it should only calculate altitude then show the absolute pressure

    And how should the pressure sensor know what caused the pressure change (weather change or elevation change)? Well, the auto-mode tries to guess where the change may be coming from, but first of all, it can be never perfect, and second, when both changes happen simultaneously, there is no way to tell how many mbars came due to the elevation change, and how many of them due to the weather.