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Manual barometer calibration

sylr
over 5 years ago

Hi,

As a pilot I need to calibrate the barometer each time I fly in order to have the most accurate data.

In the Barometer widget when I do a calibration I'm asked the elevation and the sea level pressure.

No problem for the elevation but for the sea level pressure I'm confused.

To me the sea level pressure is the QNH, i.e., Local pressure (in hPa) plus known altitude of current location / 27.3 (because in standard atmosphere you lose 1 hPa every 27.3 feet).

So if local pressure is 1010 hPa and my current altitude is 250ft:

Sea level pressure = 1010 + (250 / 27,3) = 1019,15 hPa

If I input 1019,15 hPa as Sea level pressure it becomes the current pressure.

So I'm wondering if the input calibration menu should say "Current ambient pressure" instead of "Sea level pressure" ?

Regards.

  • 0 over 5 years ago

    The pressure displayed in the Barometer widget is Sea Level Pressure.

    So, if you want the correct Sea Level Pressure displayed in the Barometer widget, you need to calibrate it with the current Elevation and the current Sea Level Pressure.

    The calibration menu is correct.

    HTH

  • 0 over 5 years ago in reply to gaijin

    What is the meaning of "Sea Level Pressure" ?

  • 0 over 5 years ago in reply to sylr

    It ineeds the qfe but since that isn't a straightforward value it asks for qnh and altitude. 

    I'm using weather underground for qnh since the airport is 10 nm away and the metar isn't a correct value for may location.

  • +1 over 5 years ago

    The QNH passed by the tower is the effective sea level pressure. Use this to calibrate the barometer. Airfield elevation doesn't matter. 

    Arguably it's a bit pointless to do this for aviation use as the static pressure sensed by your watch will be uncorrected in flight. 

  • 0 over 5 years ago in reply to sylr
    sylr said:
    What is the meaning of "Sea Level Pressure" ?

    Barometric Pressure at MSL (Mean Sea Level).

    As a pilot, what else could it mean? (Just curious)

  • 0 over 5 years ago

    Aviation refers to standard atmospheric pressure  which is 1013.5 hPa

  • 0 over 4 years ago in reply to bluefish

    It's not correct, you're using local QNH until reach a certain altitude (i.e. transition level, e.g. 5000 ft for Czechia), where you'll set 1013.5 hPa. Reasons are obvious - terrain avoidance... :)

  • 0 over 4 years ago

    Sylr,

    This is a L-A-T-E response, however I had bought my Fenix 6X just a couple of months ago & I have been playing around with the Baro / Alt settings. I suggest the following:

    - Get the current QFE from your local airport. QFE is just the direct readout from the accurate & calibrated baro in the Met department of the airport.

    - Pull out the approach charts - if you have a Jeppesen chart, then open say the VOR or ILS chart of the airport  - The fourth row in small font gives the runway elevation in terms of pressure. For example, where I live, my airport chart states runway elevation as 102 hPa (Hectopascals). 

    - So I have to add 102 to the current QFE to to get the current QNH. So, if the current QFE is say 907.7 hPa, I add 102 to get 1009.7 hPa as current QNH. Remember, the runway elevation in pressure is a constant (that is obviously different for different airports, depending on respective elevation) that you have to add to the QFE to get the QNH.

    - Next check your airport taxy or approach chart to check elevation of the airport's ARP (Aerodrome Reference Point). Enter this value as your current elevation when calibrating the baro in the Fenix.

    Its more useful to have the Baro / Alt in the Baro watch mode for pilots - that way you know the current QNH even before contacting the tower on RT.

    To summarise, the steps are:

    - Select Alt / Baro to Baro mode.

    - Set current elevation as ARP's value.

    - Set the current QNH as calculated, as explained above.

    - The Fenix baro (in baro watch mode) now will show the QNH of the airport.

    However, there is a catch - the darned thing re-adjusts the elevation to the last GPS elevation value, when any activity (such as run or walk, etc) is used. GPS elevation is not accurate enough, so, the Baro needs to be re-calibrated again by re-entering the ARP elevation value (without changing the indicated pressure value) to get the correct QNH again.

    - I tried setting the elevation to Zero - the baro now indicates the QFE. However when an activity is started using the GPS, the darned thing again uses the GPS not so-accurate elevation value to re-set!