Worsel,

Thanks for the reply.  I think what was particularly bothering me was the large temperature dependence in the correction equation that I was using to get from barometric pressure to absolute pressure.  I finally gave up and ordered an MGBox V2 to measure the pressure directly.  I hope that this will improve the accuracy of my APPM models.  Previously I was using a THUM device for temperature and humidity, and a fixed value of 737 mbar for pressure, that should correspond to a high pressure, clear day at my altitude.  

When I tried to put together an APPM model, it got about half way through and then froze up for some unknown reason.  I will try again with the MGBox V2 that hopefully will make it more accurate and worth the effort.  Since I am working in a dome, it takes some extra effort to keep swinging the dome around to get a view of the sky.  I did use the search pattern laid out for a dome that helped a bit.