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Please note I am not asking 'in what conditions some compasses can develop a bubble', what I am asking is, why can't manufacturers make liquid filled compasses that will not, ever, develop bubbles. If we take atmospheric pressure (i.e. 'altitude') as a cause of bubbles, I fail to see why a compass could not tolerate the pressure difference. At the top of Mt Everest atmospheric pressure is ~1/3 of sea level. That's not a lot, especially if we consider that we can manufacture equipment for diving that survives without a problem to much greater pressure changes. Making a case/liquid that would not bubble in the cold should also not be such trial.

So why do we still have compasses that develop bubbles in 2018? I accept that people now use GPS units and such, so the actual R&D for boring compasses is probably little, but I am surprised nonetheless.

  • It turns out to be surprisingly hard to keep air out of things perfectly (old TVs had lots of getter materials to suck up what made it into the vacuum tube for example). And, once air can get in, bubbles may occur. – Jon Custer Mar 15 '18 at 13:55
  • If air can get it the liquid should get out, no? they one would see the housing is not sealed anymore (assuming it was). In any case I can buy a watch that keeps 30 atmosphere worth of water pressure out so it seems a trivial issue to solve mechanically (proper housing with screw closure?) – user1256923 Mar 15 '18 at 14:06
  • Liquids have surface tension, air molecules don’t. And, you can lose an air bubble worth of liquid and not notice it - the density ratio is ~1000:1. – Jon Custer Mar 15 '18 at 14:17
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    So called ‘hermetic seals’, for high reliability applications, are defined in Mil-Std-883. A device with a real, measurable leak rate of Helium can be considered hermetically sealed. Those of us who try to make components and devices that do not leak (for real or just measurably) can attest that it is a decidedly non-trivial task. – Jon Custer Mar 15 '18 at 18:05
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    Billions of car tires leak way more than a compass bubble. Get Mil-STD-883 (available on line) and see. High reliability microelectronic packaging is hard. – Jon Custer Mar 16 '18 at 1:25
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Either small leaks or changes in atmospheric pressure. However if the bubble is less than 1/4 inch across it won't affect accuracy.

Bubbles can form from high elevation or exposure to cold temperatures, which cause the liquid to contract, leaving an air bubble behind. Usually, the bubble will disappear when the compass is returned to sea level and/or room temperature. If it doesn't, place it in a warm spot-like a sunny windowsill-so the liquid can heat, expand, and return to it's normal volume.

Compass Bubbles

A compass is usually filled with water or compass oil, which is a mixture of water and mineral spirits. Bubbles usually emerge in compasses as a result of drastic atmospheric pressure changes or a slight leak in the housing material.

Source

Generally, magnetic compasses are designed to not have a bubble; if you see a bubble, the chances of a leak is high. This may seem unlikely to you, because you may not have noticed any fluid escaping from the compass.

...

Your compass has most likely developed a very small leak, which has allowed some fluid to escape. When you observe the bubble in the morning, your compass is cool and the missing fluid has caused a bubble to form. As the temperature rises, the fluid expands, thereby taking up volume and making the bubble disappear.

Source

There are a number of patents on compasses with efforts to solve this problem.

A method of preventing air bubbles in a compass equipped with fluid expansion capability, wherein a compass unit is assembled with a fluid expansion diaphragm; the compass unit is filled with fluid expanding the diaphragm, and sealed while maintaining the diaphragm in its expanded state for preventing air bubbles in reduced pressure and/or reduced temperature conditions, without significantly changing the basic structure of the compass.

...

Description

FIELD OF INVENTION

This invention relates to an air bubble free compass.

BACKGROUND OF INVENTION

Some compasses are designed with a housing which includes a neoprene expansion diaphragm that allows for expansion of the damping oil when the compass is subjected to heat which can occur when the compass is in the sun on the bridge of a ship. The diaphragm expands and contracts maintaining proper fluid pressure in the compass unit.

Air bubble free compass and method of manufacturing same

A compass housing capable of withstanding high external pressure such as encountered in deep submergence applications. The housing includes a pair of hemispherical portions interconnected to form a sealed enclosure and at least one portion being formed of a resilient material capable of expansion and contraction in the presence of varying differential pressure conditions. The housing is completely filled with a damping liquid under pressure to provide an internal pressure higher than atmospheric pressure such that in the presence of higher external pressures, the inner and outer pressures tend to equalize, thereby minimizing the differential pressure on the housing itself.

High pressure compass housing

According to the invention, the rst-mentioned object is realized by the liquid being enclosed within a chamber formed of a cover portion of transparent material and -a. thin resilient bottom portion secured thereto, this bottom portion being adapted to adjust itself to the changes of volume of the enclosed liquid caused by variations in temperature.

The latter object is fullled, according to `the invention, by the compass needle being arranged on a pin Whichis carried by an intermediate partition or bridge, a frame or the like arranged between the transparent cover and the resilient bottom, the said partition or the like being provided with openings and being so arranged that the distance of the needle from -.the cover remains substantially consta-nt in spite of the move-v ments on the resilient bottom.

Fluid compass

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    wouldn't it be simpler to make a rigid housing that does not expand at ~ 1/3 sea level pressure and does not let air in? – user1256923 Mar 15 '18 at 15:02
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    If you can invent a material that does then I think you will be a very rich man. All materials expand and contract with heat/pressure. – user2766 Mar 15 '18 at 16:50
  • @user1256923: the cold-temperature bubbles appear with rigid housing, i.e. volume in the housing changing much less than the density of the liquid, in first approximation leaving you with lower volume of liquid + a little vaccum bubble. Of course, the bubble is not really vacuum (2nd approximation): it is filled with evaporated compass liquid according to vapor pressure at the given temperature. – cbeleites supports Monica Mar 15 '18 at 16:52
  • @cbeleites what is stopping compass manufacturers from filling the capsule with cold dampening liquid, which will expand but not suffer from contraction in those occasions when temps dip? or use a capsule/liquid combo with similar expansion and contraction? – user1256923 Mar 15 '18 at 17:37
  • @user1256923: well, if you fill in the volume cold, and the liquid expands in warmer temperatures, that added volume needs to go somewhere. Maybe a not very rigid case (with added bubbles in high altitude because of low pressure), or the diaphragm mentioned in the answer. If the case is rigid and filled by liquid, it will crack at high temperature. Or you go for a rather cheap but very robust technique that can buffer such volume changes: a gas bubble which as a gas is compressible. Aaaaha. – cbeleites supports Monica Mar 15 '18 at 21:17

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