GW1100, Ambient (FineOffset) and Barometric Pressure calibration woes

[wil...@bubba.org]

I recently ditched my Acurite all-in-one via RTL_433 for an Ambient setup with data capture via Ecowitt GW1100B.  I couldn't be happier having access to the data without jumping through a bunch of hoops. I'm also still bitter about the Acurite SmartHub drama a few years ago.

Anyway, I've moved weewx to capture data via the GW1000 API. I am getting data, and most of that data appears to be accurate, minus the pressure. Before configuring any pressure offsets in the GW1100, I confirmed I was seeing data in Weewx (via debug output - note, I'm not sending data to Wunderground from Weewx, just using the data locally):

LOOP:   2022-07-26 17:42:17 EDT (1658871737) altimeter: 30.039101382806564, appTemp: 89.46208415470079, barometer: 30.026459802683856, cloudbase: 899.6153853914325, dateTime: 1658871737, daymaxwind: 10.2, dayRain: 0.1811023622047244, dewpoint: 75.94169230427771, heatindex: 80.65700000000001, humidex: 99.87324145788232, inDewpoint: 56.30802378104585, inHumidity: 51, inTemp: 75.74000000000001, luminosity: 16097.0, maxSolarRad: None, monthRain: 0.7716535433070867, outHumidity: 91, outTemp: 78.80000000000001, pressure: 29.766227400000002, radiation: 127.0481452249408, rain: None, rainRate: 0.0, relbarometer: 1008.0, stormRain: 0.1811023622047244, usUnits: 1, UV: 1, uvradiation: 20.0, weekRain: 17.6, wh65_batt: 0, wh65_sig: 4, windchill: 78.80000000000001, windDir: 316, windGust: 1.1184709259696521, windSpeed: 0.4473883703878609, yearRain: 0.7716535433070867

Weewx shows the correct barometer: 30.038 inHg (matching my local WX station almost perfect - 30.04), however WXView/GW1100 shows 29.76 (as does Wunderground which is connected to GW1100, not Weewx). 

In order to get the correct pressure in WXView/Wunderground, I used the Pressure Calibration utility linked on WSForum.  Based on my altitude (250ft/76.2m) and the pressure reported from WSView without any offsets configured, the reported pressure is 29.76 - note, this is what relbarometer from Weewx above directly translates to. My local WX station currently reports 30.04. Using the calibration calculator,  this gives me a relative offset of +8.7 hPa and ABS offset of  -8.4 hPa. 

When I do this, WSView (and Wunderground) report Relative as 30.07 and Abs 21.37.  The relative pressure is close to 30.04.  When I go back to look at Weewx, altimeter and barometer are None! 

LOOP:   2022-07-26 17:47:40 EDT (1658872060) altimeter: None, appTemp: 89.69719196303436, barometer: None, cloudbase: 750.1857129645515, dateTime: 1658872060, daymaxwind: 10.2, dayRain: 0.1811023622047244, dewpoint: 76.41918286295598, heatindex: 80.553, humidex: 100.2012467210435, inDewpoint: 57.01182055322032, inHumidity: 52, inTemp: 75.92, luminosity: 15741.0, maxSolarRad: None, monthRain: 0.7716535433070867, outHumidity: 93, outTemp: 78.62, pressure: None, radiation: 124.23835832675611, rain: 0.0, rainRate: 0.0, relbarometer: 1018.3, stormRain: 0.1811023622047244, usUnits: 1, UV: 1, uvradiation: 19.2, weekRain: 17.6, wh65_batt: 0, wh65_sig: 4, windchill: 78.62, windDir: 316, windGust: 1.1184709259696521, windSpeed: 0.22369418519393044, yearRain: 0.7716535433070867

My issue is that the pressure data in Weewx doesn't match the calibrated data coming from GW1100. Given that I believe barometer value is calculated in Weewx, I'm at a loss for why these are showing as None? I even lowered the MinMax values, but still None.  Here is my configuration if that would help: weewx.conf. I appreciate any guidance or direction on this; I've been pulling my hair out for a few days.  

Thanks!

[gjr80]

This question was raised as an issue on the Ecowitt gateway device driver GitHub site. The short answer is that the Ecowitt gateway devices provide two pressure values; one is absolute pressure and the other, known as relative pressure, is a simple fixed offset from absolute pressure. Ecowitt devices use the relative pressure value when publishing to WU (WU expects barometer). On the other hand the WeeWX calculated barometer value is altitude and temperature corrected pressure.

There are two approaches to dealing with this issue:

- if you want to have WeeWX, Ecowitt and WU all use the same pressure values you should add a field map extension to map the driver relabarometer field to the WeeWX barometer field, eg in weewx.conf:

[GW1000]

    ....

    [[field_map_extensions]]

        barometer = relbarometer

- if you want to use a properly corrected barometer value then you need do nothing to the driver/WeeWX. If you wish WU to use the WeeWX calculated barometer value you should disable uploading to WU by the Ecowitt gateway device and enable WeeWX uploading to WU. The Ecowitt apps/ecowitt.net will still display the Ecowitt pressure values; this cannot be changed.

Gary