Re: Aneroid barometer questions
From: Dr.-Ing. Juergen Weyer (weyer_at_orion.hrz.tu-freiberg.de)
Date: 02/07/05
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Date: Mon, 7 Feb 2005 08:57:21 +0100
Hi,
if you really want to measure the "real" absolute pressure, no adjustment is
necessary IF the barometer was calibrated for 0 m height.
If it was "calibrated" for 600 ft height, it would show you NOT the "real"
absolute pressure but the absolute pressure that would have the place if it
would be at 0 m height.
Explanation with simple words:
In the news the forecasters give an air pressure, which is calculated back
to 0 m altitude. That is, because they do not now, at which altitude you
live and - of course - they cannot give pressures for all locations with
different altitudes all over the country.
So if you want to measure the "real" absolute pressure, you have to have an
instrument, which is calibrated at 0 m altitude (or for 0 m altitude in a
lab). If you go then to 600 ft or 1000 ft or whatever, you will have the
right absolute pressure at your instrument.
So if you live, lets say, at 1000 ft altitude then for the most people it
would be a bit confusing to hear in the news, that the air pressure is 1013
hPa and when they look to the barometer it shows 977.8 hPa. This is the
reason, why barometers are "calibrated" to the altitude. That means. in this
case they would adjust it to 1013 hPa despite the real pressure is 977.8.
There is also a formula for the re-calculation. But the pressure is not only
dependent on the altitude, but on the density of the air and consequently
dependent on the temperature and humidity. For this reason the formula was
established with measurements which were carried out over years to get an
average value:
B = B(0) * [ 1 - (0.6459/T(0)) * 10 ^-2 * (H - H(0)) ] ^5.255
[mbar = hPa]
B(0) and T(0) are barometric pressure and temperature (in k !) for 0 m
altitude.
(This formula include an adiabatic exponent of 1.234)
So if your instrument was really calibrated for 600 ft = 182.88 m you can
either calculate the difference to your altitude or you ask a weather
station at your altitude.
If you want to read the real absolute pressure, you also have to calculate
back to 0 m or you can go to 0 m altitude and compare it to a calibrated
instrument (once more propably at a weather station).
Best regards
Juergen
"Ian W. Douglas" <idouglas@calcna.ab.ca> schrieb im Newsbeitrag
news:Pine.A41.4.05.10502061822430.55748-100000@srv1.calcna.ab.ca...
>
> Recently I acquired a mechanical dial-type aneroid barometer as a
> birthday present, and as part of a suite of 3 instruments in a
> "decorative weather center" with real weather instruments. I took
> the barometer out of the package and the instructions said that I
> would have to adjust the instrument for altitude, by means of a small
> metal key inserted in the back of the case and turned one way or the
> other; and the owner was to compare the reading on the barometer with
> the pressure given by the nearest possible weather bureau to do this.
> The rationale given for this was that the instrument was constructed
> at an altitude of about 600 feet above sea level, and that of course
> not everyone who would buy this make and model of barometer lives at
> this altitude. I live at a considerably higher altitude, so I decided
> that such an adjustment was in order.
>
> I am wondering whether this type of adjustment is necessary from a
> purely "academic" standpoint, whether this adjustment has
> advantages and disadvantages from a practical standpoint, and also
> what those advantages and disadvantages might be.
>
> What I mean is that an aneroid barometer measures absolute pressure,
> and the dial on the instrument does not make a claim or
> representation to the contrary. Absolute pressure is absolute
> pressure, regardless of altitude.
>
> My understanding of the situation is this: if by adjusting the
> instrument I contrive matters such that pressure readings obtained
> on my instrument at my location fall between the high and low ends
> of the scale, I have a convenient apparatus for detecting changes
> in pressure; however, the numbers on the dial no longer have meaning
> from a canonical or generally accepted standpoint. The scale has
> now become an arbitrary one which I have made up to suit my location
> and altitude.
>
> My contention is that it might be necessary for me to adjust the
> barometer with the key sometimes. However, this would only be to
> compensate for small mechanical errors which make the instrument
> drift away from the correct pressure, in the same way that your
> wristwatch drifts away from the correct time, necessitating
> re-setting/re-calibration every so often.
>
> If it is necessary to adjust an aneroid barometer for altitude, then
> why is it not necessary to adjust an aneroid sphygmomanometer (blood
> pressure gauge) for altitude?
>
> Who is right: me or the manufacturer of my aneroid barometer, and
> why?
>
> Regards, Ian Douglas
>
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