Bubble in prismatic compass

[jungleisneutral]

Hi guys, first post.

My own WWII-era CKC MKIII prismatic compass developed an approximately 11mm wide air bubble after recent air travel. I came upon a very quick and easy way to get a bubble out of Silva compass capsules and thought I'd give it a go on my MkIII prismatic.

Here's what I did, step by step...

Step 1. Put the compass on a window sill in direct sunlight on a sunny day.

Step 2. Check the compass after a couple of hours and you'll see the bubble has either gone or reduced in size.

Worked for me.

Plan B was going to be to strip the compass and do a complete refill using purified kerosene (plain lamp oil). I haven't ever done this, but the idea is that you take the capsule out of the case and crack the filling screw carefully so as not to damage the o-ring. Remove the screw and submerge the compass capsule in purified kerosene, manipulating the capsule to (hopefully) manoeuvre the bubble out of the filling port. Replace the filler screw while the capsule is still submerged, wipe it clean and wipe some clear varnish over the edges of the filler screw. Then reassemble the compass. Once again, I'd like to point out that I've never performed that particular bubble removal technique so if anyone's compass explodes...

Try the window sill/sunny day method first.

That's a pretty case. A very Great War-looking piece of kit. It looks a little too big for the MKIII, but it's my understanding that they were also made for military engineering instruments such as barometers. I had one I picked up off ebay a while ago and discovered that it was too big for my compass by about 5-10 mm on all sides and that under the "leather" it was made from cardboard. I've also got an Enbeeco M88-ish prismatic compass which came with a similarly-constructed case, but with the correct dimensions, and it too was coated cardboard. I ended up buying some leather hide and I'm going to have to make my own for my MKIII and my US Verner's Pattern prismatic. For outdoors use, the MKIII rides in a Canadian issue webbing pouch.

[jungleisneutral]

I found a photo of my compass with its post-air travel bubble -



And one I just took showing the mysteriously absent bubble after the sunny window sill procedure -

[browningautorifle]

Put the compass on a window sill in direct sunlight on a sunny day.

That works great to expand the liquid and of course will eliminate the bubble temporarily. If it appears, it'll be back. It's like electronics, it won't hear it's self. It has to be fixed eventually.

[jungleisneutral]

That works great to expand the liquid and of course will eliminate the bubble temporarily. If it appears, it'll be back. It's like electronics, it won't hear it's self. It has to be fixed eventually.

I'll let you know how it goes and whether it needs to be fixed eventually or whether it healed itself.

[Peter Laidler]

BAR is absolutely correct. Your bubble is probably still there but trapped inside the......., er....., bubble trap! It's a small rim around the inner edge of the bowl.

Immersing the bowl into a bowl of kero is OK but BETTER if you can seal the bowl and degas the liquid. A simple way is to use a bolt-through tyre valve, evacuate the air, allow to stand for a few hours while the air is evacuated from the kero in the bowl. Release depression and then reseal compass under the liquid. New fibre or neoprene seals should be easy to find/replace

[browningautorifle]

In ours, the bubble would come and go depending on how warm it was. Usually if we had any wonky compass' they would appear in the cooler climates. They don't get better, they have to be fixed. First time your compass gets cold, it'll be back.