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From: Brooke Clarke (no email)
Date: Wed Mar 17 2004 - 19:10:10 EST
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
http://www.PRC68.com
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
> 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 , 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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