Luftdruck/Messwerte korrigieren unter weewx

  • Unser User Florian --> Großhöbing - 15786 hatte folgende Frage

    wie korrigiere ich denn den Luftdruck? Habe eine Froggit WH1080 Wetterstation, die über weewx auf einem Raspberry Pi ausgelesen wird.


    Leider konnten wir ihn nicht eine schnelle Hilfe anbieten und so musste er sich selber helfen.

    Nun bekam ich seine Antwort.

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    Guten Morgen Udo,


    hab über Nacht noch was getestet, deswegen jetzt erst die Antwort.


    Ich hab auf der Seite von weewx (http://www.weewx.com/docs/usersguide.htm) nochmal nachgeschaut und es gibt tatsächlich einen Abschnitt in der Dokumentation, der sich mit manuellen Korrekturen beschäftigt. Ich zitiere diesen mal:




    [StdCalibrate]

    This section is for configuring the StdCalibrate service. This service offers an opportunity to correct for any calibration errors in your instruments. It is very general and flexible.

    Because this service is normally run after StdConvert, the units to be used should be the same as the target unit system chosen in StdConvert above. It is also important that this service be run before the archiving service StdArchive, so that it is the corrected data that are stored.

    In a default configuration, calibrations are applied to observations from the hardware. They are not applied to derived calculations since the StdWXCalculate service runs after StdCalibrate.


    [[Corrections]]

    In this section you list all correction expressions. For example, say that you know your outside thermometer reads high by 0.2°F. You could add the expression:

    outTemp = outTemp - 0.2

    Perhaps you need a linear correction around a reference temperature of 68°F:

    outTemp = outTemp + (outTemp-68) * 0.02

    It is even possible to do corrections involving more than one variable. Suppose you have a temperature sensitive barometer:

    barometer = barometer + (outTemp-32) * 0.0091

    All correction expressions are run in the order given.

    Both LOOP data and archive data will be corrected.

    If you are using a Davis Vantage instrument and all you require is a simple correction offset, this can also be done in the hardware. See your manual for instructions.





    Weiter unten auf der Seite beim Troubleshooting gibts dann noch einen wichtigen Hinweis, auch der sei mal zitiert:





    Meteorological problems


    The pressure reported by WeeWX does not match the pressure on the console

    Be sure that you are comparing the right values. There are three different types of pressure:

    Station Pressure The Station Pressure (SP), which is the raw, absolute pressure measured by the station. This is pressure in WeeWX packets and archive records. Sea Level Pressure The Sea Level Pressure (SLP) is obtained by correcting the Station Pressure for altitude and local temperature. This is barometer in WeeWX packets and archive records. Altimeter The Altimeter Setting (AS) is obtained by correcting the Station Pressure for altitude. This is altimeter in WeeWX packets and archive records.

    Any station might require calibration. For some hardware, this can be done at the weather station console. Alternatively, use the StdCalibrate section to apply an offset.

    If your station is significantly above (or below) sea level, be sure that the station altitude is specified properly. Also, be sure that any calibration results in a station pressure and/or barometric pressure that matches those reported by other stations in your area.


    Calibrating barometer does not change the pressure displayed by WeeWX

    Be sure that the calibration is applied to the correct quantity.

    The corrections in the StdCalibrate section apply only to raw values from the hardware; corrections are not applied to derived quantities.

    The station hardware matters. Some stations report gauge pressure (pressure) while other stations report sea-level pressure (barometer). For example, if the hardware is a Vantage station, the correction must be applied to barometer since the Vantage station reports barometer and WeeWX calculates pressure. However, if the hardware is a FineOffset station, the correction must be applied to pressure since the FineOffset stations report pressure and WeeWX calculates barometer.


    Langer Rede kurzer Sinn: Man muss in der /etc/weewx/weewx.conf im Abschnitt corrections folgenden Eintrag vornehmen (Zitat 1):


    pressure = pressure - xyz


    Dass der Wert "pressure" und nicht der Wert "barometer" korrigiert werden muss, kann man aus den Beschreibungen im Troubleshooting (Zitat 2) herauslesen, da ich ja eine FineOffsetUSB Station habe. Dabei ist es noch zu beachten, dass der Wert, um den korrigiert wird, in "inHg" und NICHT in "hPa" angegeben wird.

    Ich habe meinen Luftdruck z.B. um 3,5 hPa nach unten korrgiert, das sind 0,10335 inHg, also lautet die Zeile in der weewx.conf bei mir: pressure = pressure - 0.10335


    Der Dezimaltrenner ist hier ein Punkt und kein Komma, darauf muss man ebenfalls achten.

    Nachdem man die Zeile eingefügt hat, speichern und danach kurz mit "sudo service weewx reload" die weewx.conf neu einlesen und das wars dann auch schon.



    Diese Anleitung darfst du sehr gerne in meinem Namen im Forum veröffentlichen.



    Viele Grüße

    Florian

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    Danke Florian, das du uns und anderen User damit eine Anleitung gegeben hast.

  • Udo

    Changed the title of the thread from “Luftdruck korrigieren an einer Froggit WH1080” to “Luftdruck/Messwerte korrigieren unter weewx”.