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    Re: Artificial horizons and mercury
    From: George Huxtable
    Date: 2003 Jul 21, 07:57 +0100

    Thanks to Peter Ifland and Paul Hirose for between them uncovering the
    needed information about mercury.
    Peter has provided the reference-
    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
    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,
    "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
    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.
    contact George Huxtable by email at george@huxtable.u-net.com, 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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