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Thanks to Peter Ifland and Paul Hirose for between them uncovering the
needed information about mercury.

Peter has provided the reference-

http://acc6.its.brooklyn.cuny.edu/~scintech/mercury/WhatBigDeal.htm

Peter describes the equation given in that paper, "VC =S-Q" as "fuzzy", and
I agree. It simply can't be right. This is obvious immediately by looking
at the dimensions. You can't subtract Q from S unless they have the same
dimensions.

Q is defined as "the air flow rate from the room in m^3" (cubic metres) but
a volume flow rate has to be in cubic metres per hour, or per second, or
something similar. S is given as 7 micrograms per square centimeter per
hour. There's no way one can validly subtract one of these dissimilar
quantities from the other.

C is defined as "the concentration in micrograms per hour", but I can't see
any sense in that definition either. Presumably a concentration would have
to be in micrograms per cubic metre, not per hour.

So that equation, and the definitions and units, seem both confusing and
confused, and we had best disregard them.

What is really useful, however, that I couldn't find anywhere else, is the
stated evaporation rate for mercury at 20 deg celsius, of 7 micrograms per
square centimetre per hour. Can we take that figure on trust, when all else
seems so dodgy? It's all we have for now, so we can accept it but view it
with some suspicion until some cofirmation is found.

Going back to my earlier mailing, I suggested the following-

===============================

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.

==========================

Into this, we can now plug in an evaporation rate of 7 micrograms per
square centimetre per hour. at 20 deg Celsius.

I suggested earlier a mercury pool of 80 square centimetres, so that would
imply a total mercury loss of 560 micrograms per hour (or 0.16 micrograms
per second) at 20 deg Celsius.. The crude assumption was that this
evaporation would uniformly occupy an imaginary cube of 1 cubic-meter
volume, with the observer at 1/2 metre distance, but be blown away, by a
force-1 wind, every second.

The amount of mercury in that cubic metre, assuming it was uniformly
dispersed, would be the same as the amount of mercury lost by the pool each
second .

The resulting mercury concentration would then be 0.16 micrograms per cubic
metre.  How dangerous is that level of exposure?

Peter quoted from that website,
http://acc6.its.brooklyn.cuny.edu/~scintech/mercury/WhatBigDeal.htm-

"The Agency for Toxic Substances and Disease Registry
(ASTDR)'s minimal risk level (MRL) for mercury vapor
inhalation is .3 ug/m^3. This is an estimate of the daily
human exposure that will most likely not result in risk. The
occupational exposure limit set by the U.S. National
Institute for Occupational Safety and Health is 50 ug/m^3."

Presumably the lower figure  of 0.3 micrograms per cubic metre is the level
to which whole populations might be continuously exposed with acceptably
low risk, while the much higher level of 50 micrograms per cubic metre is
considered acceptable for the smaller population of laboratory workers over
their working day.

My crude, but generally coservative, model implies that at 20 deg Celsius
under force-1 conditions, an observer, while using a mercury horizon at 1/2
metre distance, would be receiving about half the mercury exposure that's
continuously acceptable for a population, and only 1/300 of the limit for a
lab. worker. However, he will spend only a tiny fraction of his day doing
so.

Mercury exposure would increase if the temperature exceeded 20 deg Celsius
or on the rare occasions when wind speed fell below force 1, by an amount
that I um unable to estimate without more data. When an observer was
directly downwind of the mercury pool his dose would inrease above the
figure stated above, and with other wind directions would be
correspondingly less.

===============================

I conclude from the above evidence that using a mercury horizon in the open
air is a perfectly safe activity.

This is in contrast to the situation when mercury vapour is used indoors,
when dangerous levels can indeed build up if the ventilation is poor.

Even in the open air, sensible precautions should be taken if vapour can be
confined: for example, when removing a cloche from over the liquid, the
observer should stand well to windward for a few seconds.

Any comments or criticisms on this crude analysis of the situation would be
most welcome. If any errors are found, I would be very keen to have them
brought to my notice.

George Huxtable.




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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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