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    Re: Sisteco Primatic Handheld Compass
    From: George Huxtable
    Date: 2004 Mar 17, 23:48 +0000

    Robert Eno asks about a Tritium-illuminated handheld compass.
    
    Tritium is a radioactive gas, an isotope of Hydrogen, so chemically it
    behaves just like Hydrogen. It is sealed into a tiny glass capsule, which
    has a green phosphor coating inside. The low-energy beta-particles, that it
    gives off as it decays, excite the phosphor into giving off light. It has a
    half-life of a bit over 12 years, which implies that after 12 years the
    light output is down to half its initial value, after another 12 years it's
    down to a quarter, and so on, as its initial stored energy gradually runs
    down.
    
    On the other hand, because each disintegration produces so little energy, a
    lot of disintegrations per second (measured in Curies) are required in the
    light-source, to provide sufficient light. This figure tends to frighten
    the uninformed.
    
    The beta-ray particles that Tritium emits have such low energy that they
    don't exit the glass capsule. Even if the Tritium gas were to be released,
    these betas wouldn't pass through your skin, or even a piece of paper.
    Hydrogen, as a gas, isn't readily absorbed in the body; if you breathe it
    in you will breathe it out again. If it should be burned (by passing
    through the flame of an oil cabin lamp, say) it would become Tritiated
    water-vapour, which like normal water-vapour is readily absorbed. That
    would increase the hazard greatly.
    
    As Peter Smith rightly said- "Tritium is
    pretty benign, being a low-energy beta emitter, but if you managed to break
    the capsule and swallow the stuff... well, I leave that to your lawyer's
    imagination."
    
    There was a time when literally millions of phones in the UK were fitted
    with Tritium illumination of the dial, which was not regarded as any sort
    of hazard. In the end, when these became obsolete, they were gathered
    together for scrapping in enormous numbers. And that gave rise to a great
    disposal problem.
    
    Which brings me to a situation where Tritium lighting can conceivably
    become dangerous, There was a time (and it may indeed still be the case)
    when civil passenger aircraft had to be fitted with emergency lighting to
    indicate exits and escape routes which would keep going even if all power
    failed, and Tritium lighting, in large quantities (far, far greater than
    any compass requires) was installed to provide it. In the event of an
    aircraft crash, you would come down sealed into a cabin with lots of
    Tritium-containing glassware, and perhaps with a fire to convert it into
    hazardous Tritiated water-vapour. Well, perhaps in those circumstanes you
    have more pressing matters to worry about...
    
    Another point about Tritium is that it's a component of the Hydrogen bomb,
    but you would need an awful lot of light-capsules, and a lot of other
    technology, to make anything of it. It's just another factor which excites
    a degree of official nervousness whenever the word Tritium crops up.
    
    ===================
    
    Getting back to compasses, I wonder if the solid aluminium body of Robert
    Eno's Sisteco serves a special purpose. If you surround a moving magnet
    with thick high-conductivity material (and Aluminium is second only to
    Copper in that regard) the eddy-currents that are induced in the metal will
    act to oppose any movements of the magnet, and so may provide a mechanism
    for damping of compass oscillations without need for any liquid damping.
    Does Robert Eno's compass manage to do without any liquid damping, I
    wonder?
    
    For years I have used liquid-damped compasses of the hockey-puck type,
    originally designed by the French firm Morin, sold under various labels as
    the "Opti Compas" or "Mini Compass". These are very simple, clever, and
    effective. A 10-degree wide patch of the scale engraved on the rotating
    transparent card appears, magnified, in a prism, as you look over the top
    of the puck. At night, it's lit from below, either by a Tritium light, or
    you can now buy a fluorescent version.
    
    Here comes the really clever bit. That 10-degree-wide scale that you can
    see in the prism coincides exacty with a 10-degree-wide patch of the
    horizon that you see above it. Below any object on the horizon, its
    corresponding bearing is shown on the scale. You can twist the compass
    around a bit, or move your head,and provided the object stays within that
    10-degree arc, its correct bearing is shown below it. As a result, there's
    NO NEED FOR ANY PRECISE ALIGNMENT OF THE COMPASS! So it has no sighting
    vanes, because none are needed. It may be (I don't know) that other makers
    have cottoned on now to this simple trick, but Morin was certainly the
    first to get the optics right in that way.
    
    The only complaint I have had about the Mini-compass is the gradual dimming
    of the Tritium illumination, which has combined with a similar dimming of
    my old eyes to make it nearly useless in the dark, after many years. In the
    UK, although some traders will sell me a new compass fitted with a Tritium
    light, I have found none who will supply or fit a replacement Tritium light
    to an existing compass.
    
    I have found that the light-capsule is held in place with an aluminium
    spacer, by a rubber disc which acts as a plug. One owner has told me that
    in his compass the capsule had at some time fallen out and vanished: in his
    case, then, disposing of it was not a problem. It's possible to wheedle out
    that plug with care and remove the capsule (which is fragile). This is a
    job you would do at your own risk and I can't advise about any regulations
    that might apply in your country, nor about how to dispose of the capsule.
    
    You can use a LED (light-emitting diode) instead as a light-source. From
    memory, the LED I fitted was rectangular, something like 5mm by 2.5 mm,
    which fitted nicely into the spacer and illuminated just the right area of
    the compass-card. I have found a red LED to work well. You don't need a
    lens-type; just one with a flat face. You need only a milliamp or less of
    current, so battery drain is no problem. As a power source, I've used a
    3-volt calculator battery, a resistor of a couple of thousand ohms or so to
    limit the current, and a push-button switch. You might consider fitting two
    such push-buttons, with differing resistors, to give a choice of
    illumination to match the ambient brightness. A good subject for tinkering.
    
    One difficulty is finding a suitable coin-cell that doesn't contain any
    ferrous material and so doesn't affect the magnetic field. I was unable to
    find such a battery so was driven to dangling that little calculator-cell
    on its wires, 6 inches or so below the compass, which was far enough to
    remove its influence. Not a very satisfactory solution, and if anyone knows
    of a suitable non-magnetic coin-cell I would be pleased to hear.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.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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