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From: George Huxtable (no email)
Date: Tue Jul 15 2003 - 10:48:27 EDT
This mailing is only indirectly related to navigation.
I am taking a different tack on this question of artificial horizons. It
seems to me that by far the most accurate method is a mercury horizon, the
main snag being its toxicity. So I am trying to assess just how dangerous
mercury really is, when used outdoors as an artificial horizon.
I am aware that the Nav-L mailing list is a great source of odd
information, because it contains experts in such diverse fields. Does
anyone know, please, (or kan someone look it up) what the equilibrium
vapour pressure is of mercury, at ambient temperatures of, say 20 deg and
40 deg Centigrade (Celsius)?
Thanks to Phil Guerra, via this list, I have a copy of
http://www.llnl.gov/es_and_h/hsm/doc_14.05/doc14-05.html#2.1
This is a recent document from Lawrence Livermore Reseach Laboratory about
safe handling of Mercury etc.
It quotes "maximum 8-hour average concentration levels permitted" of
mercury vapour, as 0.025 milligrams per cubic metre.
I am trying to compare this with the estimated level that might apply to an
observer when using a Mercury horizon outdoors, which will be a very
different environment to a draught-free laboratory.
Presumably, the maximum permitted level quoted above must be much less than
the equilibrium vapour pressure of Mercury; otherwise, it would be
impossible for that concentration to be reached in a real-world situation.
But I would like to be certain about that, which is the reason for my
enquiry above.
Let me explain my thinking so far, to see if anyone can knock holes in it.
I am presuming that when taking an altitude with an artificial horizon, the
observer has his nose and mouth about half-a-metre (20 inches, say) from
the mercury pool. Do others agree that this is a realistic figure? If not,
please suggest a better figure, and I can make a suitable adjustment.
Assume that at that distance, the observer is suffering that maximum
concentration of 0.025 milligram per square meter.
We can put the mercury pool at the centre of an imaginary cubical wire-cage
which is 1 metre each way, with our observer at the edge of the cage, so
1/2 metre from the Mercury pool. If that cage were uniformly filled with
mercury vapour at that maximum concentration, then it would contain 0.025
milligrams of mercury. Actually, it won't be uniform; the concentration
will be greater nearer the pool, so the total mercury vapour content of the
box is likely to exceed 0.025 milligrams.
We are out in the open air, so there will be a wind, or at least a draught.
Conditions will not, in general, be completely still. Let us assume a local
wind speed of force 1 on the Beaufort scale, which is 2 knots, or about 1
metre per second. Surely, the local wind speed, even inland, will seldom be
less than force 1. Do others agree that this is reasonable?
A wind-speed of 1 metre per second implies that our 1 metre cubical cage
will be swept clear by fresh air each second. To maintain the maximum
concentration, then the pool must evaporate enough mercury vapour to
replace what was lost; that is, at least 0.025 milligrams each second. To
do so, it must lose at least 0.025 milligrams of liquid mercury each
second. At that rate, the pool has to lose just over 2 grams of liquid
mercury each day. If the wind were stronger than force 1, it would have to
lose correspondingly more.
So we should be able to test whether there is a real hazard to human health
at 1/2 metre from the mercury pool. If we expose a suitable dish containing
mercury to the outdoor air then only if it loses weight by evaporation at
the rate of 2 grams per day, or greater, will there be a human hazard under
force-1 conditions. For stronger winds, the loss would have to be
correspondingly greater. This should not be a difficult matter to monitor,
as an experiment. All that's needed is a suitable amount of mercury (which
I haven't yet found how to obtain) to put into an appropriate dish in the
open air, to be weighed from time to time. It seems to me that a four-inch
diameter pool (about 10 cm) would be suitable for the purpose of an
artificial horizon. Is that reasonable? This would be about 80 square
centimeters in area, and if filled to a depth of 0.5 cm would contain 40
cubic cm of the liquid, which would weigh roughly 550 grams, rather more
than a pound (it's dense stuff).
To me, it seems unlikely that such a dish of mercury would, in fact, lose
its substance at such a high rate, even in the open air. If it did, it
would have vanished completely in about 9 months. Perhaps it does, though.
It's worth measuring, rather than speculating, unless anyone is aware of
such measurements having already been made by others.
I can see that some sort of rain-shield would be needed to prevent
contamination of the surface by raindrops, and also some fine netting to
keep out birds and insects.
To those that argue that I have made some crude approximations, I admit it.
However, it's worth pointing out that the maximum permitted exposure
applies to a worker over the whole of each 8-hour working day, and not even
the most dedicated inland navigator is going to spend all his time
measuring artificial-horizon altitudes. So there's a big additional
safety-factor built-in there.
Comment, especially critical comment, is invited, before I start blundering
up what may be a blind-alley.
By the way, my doctor in his surgery still uses an old-fashioned mercury
manometer for measuring blood pressures, which must to some extent expose
the liquid to the air outside it. Any risk will be to him rather than to
his patients. Mind you, he hasn't been very well lately...
George Huxtable.
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contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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