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    Re: Aircraft Octant Optimal Bubble Size
    From: Bill Morris
    Date: 2013 Mar 7, 00:43 -0800


    I have used isopropanol in A12 bubble cells. Obviously, one cannot use it in vapour pressure chambers as one would never get a bubble to form. In these I have successfully used xylene and naphtha (otherwise known as Ronsonol cigarette lighter fluid). Xylene seems to seek a way out of the chambers with great ease and if O rings are used to reseal the chamber, it, and to a lesser extent, naphtha, will attack ordinary rings so it is wise to use Viton rings.

    Shellac is soluble in alcohol (but not to any extent in hydrocarbons) and should not be used in the British MkIX series, as the cells are sealed with shellac. One of the US ones is too, but I cannot remember offhand which one. It's probably on my web site somewhere (www.sextantbook.com).

    The refractive index of the fluid is important in those sextants where the image of the observed body is viewed through the bubble chamber. The refractive indices of the isomers of xylene are somewhere around 1.5, similar to that of glass, and if it is specified it should be used in these sextants.

    The freezing point of isopropanol is -89 C and of hexane -94 C. The different isomers of xylene freeze at higher temperatures, around -40 C except for p xylene, which is much higher.

    Those considerations apart I doubt that it much matters whether xylene, hexane, naphtha or alcohol is used, providing the cell roof is clean. I have made at least a dozen precision spirit level vials with sensitivity of 10 seconds per 2.5 mm bubble movement and they work perfectly well when filled with alcohol. The levels in bubble sextants are rather less sensitive. If they were not they would be impossible to use. The vibration from a piston engined aircraft is probably enough to keep the bubble lively.

    Bill Morris
    New Zealand
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