NavList:

Chris Maness, you wrote:  “I think I need to re-build the chambers because the Xylene is dissipating rather quickly."

Chris.  Be sure you’ve got a ball bearing under the grub screw in your bubble chambers.  This might account for your leaky chambers.

Now, I’ve been warning people for years not to lose the ball bearing, but last week I examined the official A12 Manual in detail.  Not only do they give you the ball’s part number, but they also give you the diameter.  It's 1/16".  I checked on Amazon and I see I can buy 1000 for £4.89 two-day delivery, i.e. 1/2p each.

You also wrote: “But may acquire spares so that I can use a bubble chamber with my Tamaya.”

Be careful here.  Normally one would say that the radius of curvature of the top undersurface of bubble chamber should equal the focal length of the telescope system so that the celestial body and the bubble move in unison if the sextant is tilted.  I’m not sure this would be a good idea with a marine sextant, because it would take zero on the altitude scale out of the equation.  You might be better with fiduciary marks on the top of the chamber as is done in the Coutinho attachment.  I did try to put a fiduciary mark on My A12 by sticking a 5mm black circle on top of the chamber, but I made a horrible smeary mess of it.  A dot from a spirit-based pen might have been easier to apply and rub out if it didn’t work.  DaveP