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Re: correct air pressure



At 08:35 AM 3/26/98 -0600, you wrote:
>>Any comments and/or references on how to correct for altitude and
latitute on
>>a barometer calibrated at the sea level would be appreciated. I am
interested
>>(for beam calibration) in corrections required by both a mercury and non-
>>mercury barometer. I made an audit at a radiation therapy department and
there
>>was a discrepancy of 20 Hg mm between the readings of the barometer and the
>>air pressure given by the weather service. The "local" physicist says
that it
>>is because he made the "corrections" while the weather service gives the sea
>>level pressure; the weather man says that this is the pressure measured in
>>their yard (close to the hospital; so I take it as being the "real" or
>>absolute pressure, the one which I need for Cpt). How do I solve the
problem?

Well, those mis-spent years as a meteorologist finally become useful!

The local weather service determines pressure in 2 ways, each for a
different purpose.

Naturally, pressure varies dramatically with elevation. A pilot trying to
land needs to know the actual air pressure so s/he can set the altimeter
for local conditions. Thus, the pressure value must reflect actual
conditions including current weather AND altitude.

However, it is impossible to use such altimeter settings to plot on a map
and analyze for high and low pressure centers. The variations in altitude
would dominate the data rather than the weather patterns. So, weather
observations also include another measure of pressure. In this case, the
actual local pressure is corrected using a standard technique to predict
what the pressure would be a zero elevation (sea level) given the observed
pressure at the local elevation. This is the value used to analyze charts
for highs and lows.

To avoid confusion of the 2 values, they are reported in different units.
When last I worked in the weather business, sea Level Pressure was reported
in millibars, and would be a number around 1,000 (less than  900 is usually
rather ugly weather, and over 1050 would be severe clear). Standard
pressure is 1013.2 mb. The altimeter setting was reported in inches of
mercury (let's not start the SI units debate again), and standard pressure
is 29.92 inches.

Most NWS stations have two barometers, a mercurial barometer (national
standard, nuisance to use) and an aneroid barometer (direct reading,
checked against the mercurial at some regular interval and can be
adjusted). A correctly functioning mercurial barometer requires only
correction for temperature, to compensate for temperature-induced changes
in the volume of its mercury; otherwise, it is inherently accurate. In
reality, that temperature correction is too small to bother with. The
aneroid is what they make their routine hourly measurements with. Most are
graduated in millibars or mm Hg, and may have a correction applied to
account for the difference between its readings and tyhe mercurial
barometer used as a reference. The reading is adjusted for elevation to
give the sea level pressure value; the unadjusted reading is converted to
inches for the altimeter setting.

For correcting ion chamber measurements, the local pressure, i.e., the
altimeter setting, is the number to use.

Moving a mercurial is not recommended - it is not a portable instrument.
But an aneroid barometer can be moved, if you are careful. If you have an
aneroid barometer, I recommend you take it to your local NWS station, place
it next to their aneroid, and compare readings. Note any correction
necessary, and then return your site.

The 20 mm Hg difference between your local measurement and the NWS station
isn't necessarily a problem. The difference in elevation and the current
pressure gradient could account for that much difference. What is important
is to make sure you are comparing similar measurements - the altimeter
setting and sea level pressure will be the same (except for the units) ONLY
for stations at sea level. All others will show a systematic difference
between the two values. Make sure that any comparison to the weather
station data uses the correct value.


---------
Bob Flood
Dosimetry Group Leader
Stanford Linear Accelerator Center
(650) 926-3793
bflood@slac.stanford.edu