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I have recently restored two sextants,  A fairchild A10  and  Link A12.  Both 
needed the bubble chamber re-filling.

The best fluid to use by far in my opinion is  silcone fluid.  It is 
completely inert, available in a very wide range of viscosities, water clear; 
low surface tension; low toxicity; low vapour pressure.  The only 
disadvantage is ridiculously high cost - at least here in GB it is.  As it is 
made by Dow Corning the Americans will probably obtain it half the cost than 
in England.

I used Dow Corning 200 / 1cS  polydimethylsiloxane polymer; (silicone fluid).
Cost in UK (2009)  was nearly ?50 for 400G (about 500ml).

The 1cS is one centi-stoke viscosity, the same as water. It forms a superbly 
active bubble on clean glass.

Refractive index 1.382
Density 0.82G/ml
Boiling range 148 - 158 C. 
Freexing point - data sheet says 'not determined'.  It presumably does not 
freeze but becomes more viscous.  Viscosity is still hardly changed at minus 
30 degrees C being about 2.5.

Cleaning the bubble chanber is vital to get a good bubble.  I dismantled both 
sextant bubble chambers to do this.  They use fine lead wire as a seal in the 
Faitchild A10; and a thick lead washer in the Link A12.  The Link was a 
problem and needs care. The lens is stuck in with a thin layer of what 
appears to be shellac on its edge as well as the lead washer.

Removing the lens and cleaning with degreasant; then immersing in conc. nitric 
acid for a few minutes clenaed the glass to perfection.
Re-assembly using 'O' -ring seals completed the restoration of the chamber 
ready for filling with the silicone fluid.

It's a treat to see the bubble actively dancing about now instead of being sluggish and uncertain.

Douglas Denny.

Chichester.  England.


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