The best way to calibrate is to compare your station pressures with a calibrated barometer of known accuracy. This will tell us if all barometers are reporting true pressure. A good low cost, accurate barometer like the Starpath USB baro would be suitable for this purpose. There's a review on wxforum.net.
On Monday, January 6, 2025 at 4:21:08 AM UTC-5 Guido Cioni wrote: > Indeed. > I believe it may still be better to read the absolute pressure and let > weewx make the adjustment because this way I can control everything..not > sure what I would do if the sensor is already delivering the relative > pressure (nor how do i make the Bme do so). > > To be fair I know tha the absolute values of pressure have little meaning > because the rate of change is the most important value when comparing > different stations. But in my case I'm often comparing these 3 stations to > diagnose foehn events so it's easier to have the 3 lines close to each > other to better see.phase shifts. that's why I'm trying to correct the one > that is far away 😁 > > On Mon, Jan 6, 2025, 10:03 '[email protected]' via weewx-user < > [email protected]> wrote: > >> The BME's itself measures temperature, relative humidity and pressure, it >> can also be configured to deliver dewpoint and rel. pressure (barometer). >> If I understand correctly, you use the BME's pressure reading and let >> weewx calculate baromter, using outTemp and outHumidity from the Davis >> Sensors. >> If so, I'd just use stdCalibrate to adapt the BME's pressure reading and >> try to get the best match. Consider that the pressure can be significantly >> different on different locations, even if they are not too far away from >> each other, so make sure you calibrate the sensor when having a flat >> pressure distribution in the atmosphere. >> >> Guido Cioni schrieb am Montag, 6. Januar 2025 um 09:08:45 UTC+1: >> >>> Well I already checked the obvious before writing here: >>> >>> - altitudes are correct >>> - temperature, humidity are correct >>> >>> Not sure if comparing the raw data would be a viable option because, as >>> you said, the stations are not located at the same altitude. >>> >>> As the curves look basically the same, with just an offset difference, I >>> was wondering if anyone specifically with the BME280 sensor had to apply an >>> offset and how they did it. >>> >>> From what I understand the BME280 sensor exposes only pressure ( >>> https://gitlab.com/wjcarpenter/bme280wx), so from the guide >>> https://www.weewx.com/docs/5.1/usersguide/troubleshooting/meteo/?h=pressure#the-pressure-reported-by-weewx-does-not-match-the-pressure-on-the-console >>> >>> it seems that the better approach would be to apply an offset directly to >>> the pressure variable in the StdCalibrate section and call it a day. >>> >>> The only thing I don't understand is why barometer is then computed >>> using the outside temperature if I also have an internal temperature >>> directly on the BME280 sensor, but it shouldn't really matter as my >>> internal and outdoor temperatures are almost the same. >>> >>> >>> >>> On Sunday, January 5, 2025 at 7:57:50 PM UTC+1 vince wrote: >>> >>>> It's a bit difficult to tell from the graph but I'm guessing it's >>>> altitude correction related. See >>>> https://www.weewx.com/docs/5.1/usersguide/troubleshooting/meteo/?h=pressure#the-pressure-reported-by-weewx-does-not-match-the-pressure-on-the-console >>>> >>>> and >>>> https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter >>>> for what might be going on. >>>> >>>> Definitely make sure you have altitude set correctly for each station. >>>> >>>> Davis's calculations are in >>>> https://cdn.shopify.com/s/files/1/0515/5992/3873/files/App_Note_28_Rev_G_Derived_Variables.pdf?v=1718824378 >>>> >>>> if you want to see what those stations are doing. It's a bit of a tough >>>> read. >>>> >>>> WeatherFlow cuts to the chase with just their formula, FWIW - see >>>> https://weatherflow.github.io/Tempest/api/derived-metric-formulas.html#sea-level-pressure >>>> >>>> On Sunday, January 5, 2025 at 10:45:41 AM UTC-8 [email protected] >>>> wrote: >>>> >>>>> >>>>> First of all: compare the raw pressure readings, but therefore all >>>>> sensors have to be at the same location. >>>>> >>>>> The calculated "barometer" value depends on other measurements such as >>>>> altitude, temperature and humidity. So, if any of the other values is >>>>> off, >>>>> the calculated result will be off, even when all sensors show the same >>>>> absolute pressure. >>>>> Guido Cioni schrieb am Sonntag, 5. Januar 2025 um 18:18:42 UTC+1: >>>>> >>>>>> I have 3 stations located close to each other (maximum distance is >>>>>> about 12 km). >>>>>> They all use the same hardware (Davis Vantage Vue). >>>>>> The only difference between them is that one of the stations (which >>>>>> is also the one that I'm using with WeeWx) does not have the console, so >>>>>> I'm using an external BME280 sensor to get the pressure reading. >>>>>> >>>>>> When comparing the barometer variable (which If I understand >>>>>> correctly should already be corrected with altitude and temperature) I >>>>>> get >>>>>> indeed a similar behaviour. >>>>>> [image: Screenshot 2025-01-05 at 18.11.45.png] >>>>>> >>>>>> However, the station with the BME280 sensor (green curve) is way off >>>>>> with respect to the other ones. >>>>>> Obviously, small discrepancies in the 'real' altitude could explain >>>>>> part of the difference, for example the one that is seen between the >>>>>> blue >>>>>> and red curves. However, I feel like the green line is way off possibly >>>>>> because of: >>>>>> - wrong calibration in the BME280 sensor >>>>>> - something else going wrong when computing barometer? >>>>>> >>>>>> Has anyone seen this behaviour before? Should I just use an >>>>>> additional line in StdCalibrate to correct the value of the pressure >>>>>> read >>>>>> from the BME280 sensor (I could use as offset the difference with the >>>>>> neighboring stations) or is there a cleaner way to correct this? >>>>>> >>>>>> Thanks >>>>>> >>>>> -- >> You received this message because you are subscribed to a topic in the >> Google Groups "weewx-user" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/weewx-user/aPUKLDkJwBM/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> [email protected]. >> To view this discussion visit >> https://groups.google.com/d/msgid/weewx-user/c73cf543-efad-4ddc-ae79-c631d3d6e402n%40googlegroups.com >> >> <https://groups.google.com/d/msgid/weewx-user/c73cf543-efad-4ddc-ae79-c631d3d6e402n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/weewx-user/8b7c7977-f389-4449-92a9-e78efbede4a2n%40googlegroups.com.
