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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sisteco Primatic Handheld Compass
From: Brooke Clarke
Date: 2004 Mar 17, 16:10 -0800
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 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 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. > ================================================================ > >