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    Re: Sisteco Primatic Handheld Compass
    From: Brooke Clarke
    Date: 2004 Mar 17, 16:10 -0800

    Hi George:
    I've seen some articles that say the U.S. military is working on
    an all plastic battery, but until it becomes available, all other
    battery technologies I know have ferrous content.  My Suunto wrist
    watch, barometer, thermometer, compass has a calibration procedure
    you need to do each time you change the battery.  That  seems to
    be saying that even though I put back the same type of battery
    each one may be different.
    Have Fun,
    Brooke Clarke, N6GCE
    George Huxtable wrote:
    > 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
    > 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@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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