Oil filled compass

[Stryker]

I have a large oil filled compass on my boat. It has a large air bubble in it and I need to refill the oil.

I have used mineral oil in the past. Is that the best choice of oils?

Is it better to replace all the oil or just top it off?

Is there any technique, I read somewhere I should chill the oil in the refrigerator first. The theory was that the oil will expand as it warms forcing a positive pressure that would prevent any air from getting in.

What do you think??

[Threemuch]

There is such a thing as compass oil. Budget Marine sells it online for 6 bucks a cup or 50 bucks a gallon.

West marine also sells it online for 9 bucks a pint or 13 bucks a quart. Sounds like a better deal.

I would use something like this and not fool around with mineral oils, although that's probably what this is. Viscosity should be pretty low, so it swings fast, but not too low that it won't settle.

Most fluids are pretty incompressible, and oils are particularly so, that's why they are good for hydraulics, so expansion and contraction should be limited. However, cold oil will be more viscous and probably keep gas bubbles from escaping. I would heat the oil first, to release diffused gases, then at room temperature, pour it as smoothly as possible to avoid getting too many air bubbles trapped. Let it sit to allow any new air bubbles to vent, then top it off.

KB

[Onokai]

I have filled up an old military repeating compass with the west marine oil. It was different from the oil which was in the compass. That made for a stratifacation line, but since this is a yard art piece I did not care. I would add a little and see if its compatable with your exsiting oil.If not drain and refill with all new. Mark

ONOKAI