So, I decided to replace the dome on my compass with a shiny new one, the old one was scratched and has seen better days. It's a Ritchie flush mount. Got a kit with dome, seals, and diaphragm and added new liquid. It was a slight challenge getting all the air out, but I got there eventually. Put it back on the yacht and it all looked like new.

A couple of days later I went back on the yacht and noticed an air bubble the size of a 5 cent piece. I couldn't find a leak anywhere and assumed that there must have been air trapped in somewhere and bubbled up or the liquid leaked and evaporated. I left it to be dealt with another day.

Another couple of days later I was back on the yacht and, to my surprise, there was no more bubble. Nothing at all. And no, the liquid didn't leak out, it was full. An air bubble at the top of a dome can not escape, the yacht hasn't moved healed or anything that could have pushed the bubble into a cavity within the compass.

I have been told by Ritchie to use low odour Paraffin Spirit to refill the compass (not Paraffin oil). I could only find Paraffin oil, so I decided to go with low odour Kerosene spirit, which is a less refined version of Paraffin.

Does anyone know what's going on there?

I only topped mine up to remove a bubble but got the same advice Jolene mentioned above re cooling. My research suggested put the compass and top up fluid in the freezer for 24 hours prior.

I presumed the idea was I'd get some contraction when chilled and then expansion as it returned to ambient temp when back on the boat.

My reading suggested mineral oil (in my case baby oil) rather than parrafin though.

Like I said on my post when I did it, I don't profess to be an expert.

GKandCC said..
Jev7337, some gauges etc purposefully have a bubble which allows the dampening fluid to expand and contract with changes in the ambient temperature without blowing the seals. If there was not this bubble a big rise in temp would force the liquid out through the rubber seals, possibly damaging them.

No, my compass has a diaphragm to compensate for expansion of volume due to temperature changes, that got replaced as well.
The temperature has been all over the place in Melbourne lately (as always) but have not seen any sign of a bubble over the last 2 weeks.