Perhaps I can help as I own both an Ambient and Ecowitt weather station.
I recently did a major rewrite of an older Ecowitt barometer calibration procedure and the new procedure should work with any Fine Offset manufactured device with a barometric sensor. Some basics regarding this equipment: - there are no algorithms in firmware to calculate (M)SLP or Altimeter. We know that WeeWX has the algorithms we need to calculate those values therefore you need an accurate station pressure to send to WeeWX. - Fine Offset defines everything in terms of pressure so if you wish to enter your elevation you need to know your standard atmosphere pressure for your elevation(actually what you want to calculate is the pressure difference between your location and sea level). - Fine Offset weather stations employ a fixed offset system to estimate/calculate Altimeter(QNH) or MSLP (QFF) - unfortunately, MSLP can not be reliably be calculated using a simple fixed offset. In reality, the MSLP offset is actually variable - unless you live close to or at sea level. Therefore, your best bet for calibration purposes is to match the REL (relative pressure) with a close by airport's Altimeter reading. In Europe it might be better to match your government weather service to obtain decimal point QNH values. Otherwise, you are stuck with integer value QNH in the METAR reports. The procedure is too lengthy to post here so here's the link to read: https://www.wxforum.net/index.php?topic=45601.msg462080#msg462080 Instead of calculating the REL offset manually, I have suggested an alternative calculator in order to calculate the REL offset (elevation offset) directly. The shortest and perhaps easiest way to calibrate is to use the REL offset calculator and then adjust your ABS value until ABS matches Altimeter or QNH. Once you are calibrated, you will have/should have an accurate station pressure(ABS value) to use for WeeWX. No need to employ correction offsets in weewx.conf. Personally, I calibrate at least yearly with my own reference barometer that I made with a Bosch BMP390 and a raspberry pi. The Fine Offset barometer sensors are getting long in tooth and are particularly "drifty" (long term drift). PS. At 550m elevation your REL offset (REL minus ABS) should be 64.35 hPa. Your display console should show that your REL reading on the display is 64.35 (64.3 rounded) is ALWAYS higher than your current live ABS value. Once you’ve done that step, you adjust the ABS value until your REL value matches the Altimeter/ QNH of the station you are comparing to. On Friday, September 1, 2023 at 9:48:39 AM UTC-4 Tomasz Lewicki wrote: > W dniu 31.08.2023 o 16:55, 'Rainer Lang' via weewx-user pisze: > > to calibrate your WS-1001 (Ambient version of the FineOffset/EcowittWH24 > > outdoor array and the WH1080 console) you could just follow the > > instructions in https://www.wxforum.net/index.php?topic=40730.0 chapter > 6 > > determine the offset and enter it accordingly (either an offest of > > change the value, whatever the console wants --> manual). > > > > METAR, airport etc. are secondary approaches for refinements which you > > may or may not want to follow. There are lengthy and complex treatises > > on that topic in the same forum, from simple to highly complex. > > > > [by the way, if you have a European or international "version" of the > > Ambient WS-1001 and call this a clone version, then that's very strange > > wording as Ambient is the clone and not Fine Offset/Ecowitt = the > > manufacturer] > > > > but true, weewx should come to a proper result - and unless your station > > also reports wrong absolute pressure (= local pressure) - or you display > > the wrong variable/observation in your skin, then something is wrong > > with weewx. Rather unlikely. > > Did you read the barometer topic in the weewx online documentation ? > > I'm back and starting my reply from Rainer's last question. Yes, I've > read this topic: > https://github.com/weewx/weewx/wiki/Barometer,-pressure,-and-altimeter > on the very beginning. I use only 'pressure' and 'barometer' terms > below, according to definitions from wiki. > > So, I'm even more confused than before. I've made the calculations with > the calculator listed in https://www.wxforum.net/index.php?topic=40730.0 > (direct link: https://keisan.casio.com/exec/system/1224575267 - BTW they > will close at September 20) with my current conditions (pressure 944.5 > hPa, temperature 20*C, height of sensor 560 m AMSL) and got result > 1007.8 hPa - this is perfectly equal to what I see on my webpage - > 1007.7 hPa calculated by Weewx algorithm (difference of 1/10th of hPa is > neglible). I recall that I use 'software' option in weewx.conf for > 'barometer' but 'prefer_hardware' for 'pressure'. > > But very accurate numerical forecast for my location *and* METAR for > EPWR airport shows 1015 hPa. So I need to "collect" additional ~7 hPa > (1015 from METAR minus 1007.8 from Weewx calculations). Does it mean I > should: > > a) add whole calculated offset - ~70 hPa (1015 from METAR minus ~945 of > absolute pressure) - in my PWS console > > or > > b) add smaller offset - 63.3 hPa (1007.8 calculated by Weewx minus 944.5 > read by sensor) PLUS use relevant section in weewx.conf: > > # This section can adjust data using calibration expressions. > > [StdCalibrate] > > [[Corrections]] > # For each type, an arbitrary calibration expression can be given. > # It should be in the units defined in the StdConvert section. > # Example: > foo = foo + 0.2 > > to incorporate "rest" of offest (METAR value minus barometer value > calculated by Weewx): > > [StdCalibrate] > [[Corrections]] > barometer = barometer + 7.2 > > I only need confirmation that I'm going in right direction - or opposite. > > -- 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]. 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