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George;

There are a few other factors that you need to consider.

1) the distance the sextant is held away from the mercury pool when
taking a sight depends on how large the pool is. For example if you were
using a using a lake, as if you would! Then the sextant could be several
feet above it, but if you are using a Petri dish, say, then you would
need to have your nose very close to the surface. Much nearer than the
half a metre you propose.

2) The hazard from mercury vapour would be greatest when the air is
still. In any draught greater than about force 1,  the vapour would
disperse. But in any wind greater than F1 the surface of the liquid
would be disturbed so it would be unusable with out some kind of shield
to keep the air still!

3) Mercury exposed to the air gradually corrodes and a scum forms on the
surface. It needs cleaning from time to time. The modern way is vacuum
distillation but that is out of the reach of most us. The traditional
method is to make a purse shape out of chamois leather, to contain a
couple of ml and then to twist the bag so as to squeeze the mercury
through the leather this would be collected and after a few passes would
be clean enough not to adhere to glass. Of course there is a hazard here
to be considered.

4) Mercury can be obtained from a chemical wholesaler if you have a
go-between who will vouch for your suitability to handle it. Under the
UK  COSHH (Control of substances Hazardous to Health) Regulations all
sorts of forms will have to be filled in and inspections of your place
of storage  made! It just isn?t worth the hassle! But, barometers do
have to be repaired and you may know someone who does this work.
I have no idea what the position is in other countries.

5) If you insist in using mercury, why not consider some kind of
containment.
My artificial horizon which you have seen consists of a hollow glass 90
deg prism made of 1/4 inch float glass pieces glued together with fish
tank adhesive, to make a totally enclosed bottle with a flat bottom, Two
small holes are drilled and plugged through the parallel sides for
filling.  Sights are made through the two 45 deg faces. The whole thing
is fixed on top of a tripod and only crude levelling is necessary. I
filled this device over 30 years ago and the mercury has not diminished
in the slightest.
 The only drawback has been that the glass has tarnished with contact
with the mercury for some reason I don?t know why. Maybe the mercury is
amalgamating with the lead in the glass.

6) I am not sure if plastic is a suitable material for a AH. It has too
high a coefficient of expansion and may distort in the heat of the sun.
Also if it is not silvered its reflectivity may not be sufficient for
star sights. Maybe someone has some experience of this as I have not
tried it myself. How would it be supported without distortion? Three
point mounting would be fine if you have a table to stand it on but out
in the field a tripod would be better and a levelling head necessary.
It will be important that the levelling screws have a very smooth motion
and placed far apart for fine adjustment.

7) The precision of the AH will depend on what you want to use it for.
Latitude sights using stars need the very best equipment. For clearing a
Lunar much less precision is required and a plastic AH may well be
adequately for this.

8)In the UK I purchased a replacement spirit vial for a theodolite
repair for about ?10. The sensitivity was marked as 30"/div.
The top of the tube is engraved with a ladder of divisions equal spaced
about the centre and about 2mm apart. Two major marks are drawn just far
enough apart to bracket the bubble.
When the tube is levelled carefully with the bubble between the major
marks any subsequent tilt making the bubble move by 1 div (2mm) means
the tube is off-level by 30".  It is possible to get more sensitive
levels than this. After all theodolites are made to 1/5 sec calibration
and they have to be levelled to this degree of precision.

I found a supplier in London on the internet.
It is not necessary to buy a complete engineers level if you are going
to mount it permanently on a reflector. The theodolite vial is fixed in
an brass adjusting tube using plaster of Paris.

Clive





-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of George
Huxtable
Sent: 15 July 2003 15:48
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Artificial Horizons and Mercury.

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 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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