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
wris****ch 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

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" schrieb im Newsbeitrag
...

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
wris****ch 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