I ran across this very issue when trying to calibrate my barometer chip against
the NWS station located less than two miles away. Their numbers for millibars
and inches of mercury do not agree. I sent them an email and asked what was
going on. They said their instruments read out in millibars (to three decimal
places) The reported value is converted to sea level pressure and reported to
two decimal places. They are also converted to inches of mercury for their
reports. Only problem is their conversion constant is NOT the proper value.
They consistently report around 0.02" too high. I reported this back to them,
but have received no further responses.
Note that the conversion between true pressure readings and sea level pressure
involves an equation with about a fifth power/root (depending upon the
direction of the conversion) so it can be quite sensitive to true chip
calibration. The pressure chip that I am using (MP5611) is factory calibrated
and has calibration constants stored on-chip (the Bosch BMP085 and BMP180 chips
also do this), but the soldering process can affect the chip so you need to do
some final calibration. The MP5611 can detect the air pressure change seen by
raising the chip less than 6 inches...
Relevance of temperature/humidity/pressure sensors to time-nuttery? We all
know the comparatively massive effects of temperature on our equipment. But
humidity and air pressure also affect them in many subtle and not-so-subtle
ways. I'll post some recommendations/observations on various sensor chips in a
while.
---------
One funny thing about weather measurements is that the data that NOAA reports
is not what it would seem. The standard ASOS data (which is what NOAA reports
in its local current conditions) includes barometric pressure in inches of
mercury and in hectoPascals. It turns out that neither is the actual barometric
pressure.
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