[I posted this question but removed it after seeing some web sites;
however, since Dr.-Ing. Weyer has replied anyway, I have re-posted it.]

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
wpressure, 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