[wxqc] Mercury Barometers
ajdel at cox.net
Thu Dec 18 14:32:07 CST 2008
RE: Aneroid barometers are more accurate, less bulky and much less
than mercury barometers.
As the old encyclopedia Britanica says: "The aneroid requires,
however, to be repeatedly compared with a mercurial barometer, being
liable to changes from the elasticity of the metal chamber changing,
or from changes in the system of levers which work the pointer. Though
aneroids are constructed showing great accuracy in their indications,
yet none can lay any claim to the exactness of mercurial barometers.
The mechanism is liable to get fouled and otherwise go out of order,
so that they may change 0.300 in. in a few weeks, or even indicate
pressure so inaccurately and so irregularly that no confidence can be
placed in them for even a few days, if the means of comparing them
with a mercurial barometer be not at hand."
Now time has passed and technology improved but I'd bet the only thing
that needs to be changed in this quote (besides removal of the
subjunctive which has been removed from the English language as surely
as mercury has been removed from Western life) would be the 0.300 inch
number which may be somewhat smaller in a modern instrument.
As the bloke at Medford Clock says WRT to Fortin Barometers: "They are
probably the most accurate of all the barometer types."
And thanks for pointing me to Medford Clock - what I am really
concerned about is that I have a place I can get my instrument
repaired should it ever need it. I haven't looked to see if the
diaphragm is leather but that is certainly the traditional material
and eventually I would expect it to dry out and crack. I note from the
website that he has been hit by a fire and had to close the shop but
is still in business. Hope he continues.
I think perhaps precision and accuracy are being confused here.
Aneroid (and piezoelectric) instruments can be build with greater
precision (though have a look at the Descartes/Huygens design for
mercury) but must be frequently calibrated if they are to be accurate.
I have an aneroid instrument and it certainly does not track the
airports very well. That's why I got the Fortin barometer. It does.
There are advantages to the aneroid with respect to bulk. The
mechanical problems with aneroids are largely overcome by the use of
electronic (piezoelectric) sensors but, as is the case with any other
sensor (including mercury barometers) there are offset , slope and non
linearity (all of which are functions of temperature and age) to be
taken into account. While these may be quite precise accuracy is
limited to about 0.02% only when calibrated against mercury (and they
are around $2K).
More important than accuracy in the present application is
repeatability because QNH is derived from pressure readings made by
mercurial barometers as corrected by standard formulas and altimeters
are designed consistent with these. The pressure readings may, of
course, be made by other types of instrument but they will be
reported as inches of mercury (in the US) and thus the instrument
calibrations must ultimately be traceabke to a mercury column.
Toxicity I don't take seriously simply because the mercury is
contained. I played with mercury fairly frequently as a kid as did
many of my friends and colleagues (in the sciences and engineering)
both here and abroad have reported doing the same without detriment to
health. I have not, however, worked as a hatter. I don't want this to
turn into a political flame war (and feel some responsibility since I
started this thing) but my experience (nor that of most technically
oriented people I have known in my career) just doesn't jive with the
current attitudes towards this substance. Giiven that the entire
annual US production of mercury (1.3 million pounds) if converted to
gas and released to the atmosphere would comprise from 0.4 to 2% of
the amount released by the earth's crust annually I have trouble
understanding why I shouldn't be trusted with a pound or 2 in a
barometer or a few grams in a thermometer or why, when a fever
thermometer is dropped in a school the building (the same one in which
we as kids played with mercury) needs to be evacuated. It seems to me
common sense is on vacation and I believe that thinking people should
be concerned about this.
While we are doing numbers on mercury there has been some discussion
on what's in the space at the top of the tube in a mercury barometer.
It is mercury vapor but a very low pressure i.e. 0.002 mmHg at room
temperature. This pushes down on the column and thus every reading
should have 0.002 mm (0.000078") added to it. This is one of the nice
things about mercury.
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