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 'michael.k...@gmx.at' via weewx-user < weewx-user@googlegroups.com> 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 michael.k...@gmx.at >>> 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 > weewx-user+unsubscr...@googlegroups.com. > 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. 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