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Rebecca took this photo of our ship’s compass the other day. Looking at the bubble in it reminded me that I wanted to try to refill that compass. The questions is, how?

After some research, I found the following instructions on a sailing forum:

Compass Oil

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.

Dissolved Air

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 a lot 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.

Another Method

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.

Most other links seem to offer similar advice. Anyone reading this have any first-hand experience with this task?

8 Comments

  1. Shrinkage. You know about shrinkage, right?

  2. I topped mine off with compass fluid. I didn’t worry about the dissolved air. I do see a bubble when it is cold out, but that just indicates when I should be heading south.

    Eric

  3. I have, but not well. Inverted and submerged the compass in a bucket. But since we are subject to such wide temps swings, bubble reappeared. Also, not sure that removing the seal doesn’t lead to the same problem. So, new compass for me.

  4. I just topped mine off and I knew none of this. Where were you then with your fancy Internet research? At any rate, I put as much fluid in as I could (took 3 tries and I made a mess!) I have not noticed a bubble return on cold days. I wonder how much of a difference this makes (how big is the bubble on cold days?) and does the cold day bubble affect accuracy in anyway? It doesn’t sound like the Cold Day Bubble knows about Costanza shrinkage.

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