There is a difference between sea level pressure (SLP) and altimiter setting (QNH). Specifically, SLP includes consideration of the actual temperature, while QNH does not. The QNH value is what you would dial into the altimeter in an airplane to make it read correctly on the ground at your location. Be aware that pressure readings you get from airports are QNH -- not SLP. This is because they need to tell pilots where to set the altimeter.
I have not been able to determine whether the WMR100 is displaying SLP or QNH (but I suspect it is intended to be SLP).
There is also station pressure (QFE) which is the actual barometric pressure at your location.
Anyway, I've got a freeware package called Weather Station Data Logger (WSDL) on SourceForge that offers connectivity to Weather Underground and CWOP.
Weather Underground data uses the WMR100's reported SLP value (or whatever it is). You can tweak what is reported by changing the altitude on the WMR100 console; then a different value goes to Weather Underground.
The data sent to CWOP by WSDL is computed from the WMR100's reported station pressure
using an altitude value entered by the user in an options dialog. This will allow you to tweak the barometer to give the proper reading by adjusting the altitude value slightly. Each 10 feet of altitude corresponds to roughly 0.01" Hg.
Right now, WSDL displays the WMR100's (SLP or QNH, whatever it is) but I've been thinking about a modification that would allow the user to display a computed QNH value instead. This would allow user tweaks via an altitude setting to make the barometer reading be spot on. Please take a look at the software package and post something on the SourceForge forum or this group if you'd like to see this change in the WSDL program.
Here's the URL for WSDL:http://wmrx00.sourceforge.net
The SourceForge forum can be found here:http://sourceforge.net/projects/wmrx00
If you want more information about computing SLP and QNH from station pressure, look at these excellent documents:http://www.srh.noaa.gov/epz/wxcalc/wxcalc.shtml http://wahiduddin.net/calc/refs/ASOS_Pressure.htm