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    Re: Bubble Sextant
    From: Douglas Denny
    Date: 2009 Jun 6, 14:15 -0700

    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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