Galfert,
Thanks for the post, and your suspicions are correct the issue appears to be how WeeWx uses the pressure value passed to it from my WS-2800 weather station. I figured out what seems to be the issue and a workaround. In short, I fear that WeeWx is not using pressure correctly for WS-2800 devices. Here are my learnings...
First, I tried to understand what pressure readings come from the WS-2800 device. I connected the usb device to HeavyWeather, which is a Windows app and which worked just fine before I went to WeeWx. HeavyWeather creates a currdata.lst record of current data, and in this file there are 2 types of pressures: ABSOLUTE and RELATIVE. My software showed me a relative pressure reading that matched the display on the weather station, but null values for absolute pressure:
[pressure_absolute]
hpa="---"
inHg="---"
[pressure_relative]
hpa="1006.4"
inHg="29.72"
Next. I reconnected the USB to my Pi computer and tried a couple of tests with the WeeWx.conf file. Based on Users Guide documentation, my guess was that WeeWx also tries to read two types of pressure: Station PRESSURE, which is supposedly the raw data from the device, and Sea-level Pressure (labeled BAROMETER), which the WeeWx doc claims to calibrate to the proper altitude. The config file also lets us use HARDWARE, SOFTWARE or PREF_HARDWARE settings. So here's my tests and the results:
First, I set both readings to HARDWARE and got no (i.e. null) readings in WeeWx for barometric pressure. This led me to believe that it wasn't reading the Station Pressure value correctly.
Next, I set both readings back to the default PREF_HARDWARE and set my my station ALTITUDE = 0 (i.e., Sea Level). I found that WeeWx now "calculates" barometric pressure to be the same value as the device setting. I.e., it no longer revises the station pressure to a new value.
So, by faking out the altitude, I'm now getting accurate barometric pressure settings in the WeeWx database and on my Wunderground site:
https://www.wunderground.com/dashboard/pws/KCOBAYFI8, but I had to force WeeWx to think that my device is at sea level. It seems that WeeWx is using Station Pressure as Sea Level Pressure, which is an error, unless there is something in the config file that I'm missing or setting up incorrectly. This would lead me to believe that the mapping of data for WS-2800 devices may be in error.
Should I submit this as a possible issue to the WeeWx user group?