atmospheric pressure taken for a weather observation is measured in
millibars and respresents the barometric pressure at ground level at
that location. If the instrument is located somewhere besides ground
level it will be corrected to ground level.
The value given in inches of mercury is that barometric pressure
corrected to sea level and converted to inches of mercury. This is the
altimeter setting and will be used by aircraft to adjust their altimeter
so that when on the ground the altimeter will read the height of the
aircraft above sea level, in other words the ground elevation on which
the aircraft is sitting. This means there is no direct comparison
between the pressure given in millibars and that given in inches of
mercury (altimeter setting) unless the altimeter setting is corrected to
the same height above sea level at which the atmospheric pressure was
measured.
The Altimeter setting in Europe is not the same. There the altimeter
setting is such that the aircraft's altimeter reads ZERO when on the ground.
Two miles away is pretty close but can make a difference as can a
difference in ground elevation but it sounds like you are taking ground
elevation into account already.
cheers, Graham ve3gtc
On 2014-05-25 23:06, Mark Sims wrote:
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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