[wxqc] Mercury Barometers

A.J deLange 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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