Re: correct air pressure

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

>Date: Thu, 26 Mar 1998 11:56:23 -0600 (CST)
>Errors-To: melissa@romulus.ehs.uiuc.edu
>Reply-To: radsafe@romulus.ehs.uiuc.edu
>Originator: radsafe@romulus.ehs.uiuc.edu
>Sender: radsafe@romulus.ehs.uiuc.edu
>From: Bob Flood <bflood@SLAC.Stanford.EDU>
>To: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
>Subject: Re: correct air pressure
>X-Listprocessor-Version: 6.0c -- ListProcessor by Anastasios Kotsikonas
>X-Comment:  RADSAFE Distribution List
>
>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
>
>