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    Re: Artificial Horizons and Mercury.
    From: Fred Hebard
    Date: 2003 Jul 15, 14:19 -0400

    George,
    
    To address your questions, in part.
    
    While taking a shot, I am almost never within less than 2-3 feet from
    my artificial horizon, and more frequently 5-15 feet away, so I don't
    think that exposure to the mercury vapor _while_ shooting would be a
    significant component of the risk.
    
    A more significant component, in my opinion, would be accumulation of
    vapor under the roof while the shot is being taken, to which one would
    then be exposed when removing the roof.  Care could reduce markedly
    this exposure.  Likewise, having to handle the mercury, moving it back
    and forth from storage container to horizon vessel, and having to clean
    it might also be significant components of exposure, along with spills.
    
    Unfortunately, and irritatingly, I am unable to locate my copy of the
    Handbook of Chemsitry and Physics to answer your vapor pressure
    question.
    
    A final comment is that the great drawback of reflective horizons is
    their limit to altitudes less than 1/2 of the sextant's maximum
    measurement.  I can't watch the sun arc through its meridian at this
    time of year in my location.
    
    Fred
    
    On Tuesday, Jul 15, 2003, at 10:48 US/Eastern, George Huxtable wrote:
    
    > 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.
    >
    >
    > ================================================================
    > 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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