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I know this is an old thread but it seems like as good a place as any to add what I have learned on the subject to the collective knowledge base.
A hundred years ago, alcohol was a common compass dampening fluid but modern compasses use either Odorless Mineral Sprits or an Exxon Mobil product called Isopar L. The difference between the two is that the flash point is lower in the Isopar L product and that is apparently safer in a fire. I don't know about you, but the compass is the last of my worries in a fire. My compass is about 25 years old and so I added Odorless Mineral Spirits from Home Depot with good success. Watch out for the "green" mineral spirits; it is milky white - probably not what you want.
The manufacturer and compass shops will tell you that you must put the mineral spirits in a pot and pull a 28"Hg vacuum on it for 15 minutes to remove dissolved air. If you don't do so, you will get a bubble on cold mornings. If you have a paint pot I suppose you could use it together with the manifold vacuum from an idling car engine for this purpose, but there is another way. After you fill your compass with Odorless Mineral Spirits (being careful to leave no bubble), put it in the freezer overnight. In the morning you will find a bubble. That bubble is the dissolved air that the vacuum pump would have removed. If you remove that air bubble, you are good to go. I can think of two ways to get rid of the air bubble. Either apply light pressure to the bellows to force the bubble out or add more mineral spirits.
Some Shrinkage Will Occur
If you remove the plug to add more mineral spirits, you will find that a lot more air is sucked into the compass as the volume of mineral spirits has shrunk due to the change in temperature. No problem, just top it up. Now this is important - as the compass begins to warm up the mineral spirits will expand and you will need to allow the excess spirits to bleed out through the plug (remember, you added alot more mineral spirits that the volume of the bubble to compensate for the shrinkage). Once the compass is close to room temperature you can close the plug and go sailing.
I'm sure you're thinking: why not just put the mineral spirits in the freezer overnight. Good question. I used an epoxy syringe to fill the compass and it was all that syringe could do to suck up the spirits at room temperature. The rubber plunger seal kept coming off the plunger. I don't think it would handle the colder temperatures and greater viscosity very well. If you decide to cool the spirits in the freezer, consider that the can will deform badly as the spirits shrink. Leaving the cap loose would solve that problem but consider that the vapors are flammable and a freezer fire might melt your ice cream. I think it's just a tad better to add the spirits to the compass at room temperature.
Last edited by dcmeigs; 06-18-2010 at 12:16 PM.