[wxqc] Barometer Accuracy and Standard Deviation of ASOS Stations?

[Geoffrey Dick]

Does anybody on this list know the targeted, or expected accuracy
for Barometric Pressure of ASOS calibrated stations?  Once calibrated,
how much standard deviation is tolerated?

For several months, I have been tracking barometric pressure of 3 
airports that are within 12 miles of my location.  The stations
are within a few meters in elevation of one another.
Using the daily Gladstone summary, I see them wandering slowly,
over several days, +/-0.8 millibars many times in opposite 
directions from one another.  Could temperature, weather station
power supplies, or some other environmental component be a factor? 

Geoffrey Dick
WA4IKQ, AS140