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    Re: sextant for use on land
    From: Jean-Philippe Planas
    Date: 2006 Sep 10, 03:15 -0500
    I strongly suggest that you use the correct four splines Bristol wrench to be able to apply the correct torque amout without damageing the screw head as it seems to be sealed at the time. .
    At the origin pure Xylene was used as the bubble fluid. I guess it was chosen because of its physical caracteristics at high and low temperature.
    As you don't intend to take shoots from an airplane astrodome flying at 30000ft at a temperature of -55°Celcius but want to use it in your backyard, ordinary xylene should suite. I used ordinary xylene and it works fine. Some authors and manuals say that you can use alcohol in instead of xylene.
    The proper way to fill the bubble cell is to add small quantities of xylene until the desired size bubble is obtained. If too much fluid is injected, the syringe is to be be emptied, the assembly turned with the inlet to the bottom, and the chamber partially emptied by forcing air with the syringe. Then repeat the procedure for filling.
    The bubble is formed by filling the chamber with fluid and NOT by WITHDRAWING fluid or by injecting air into tha chamber. So says the manual.

    "James R. Van Zandt" <jrvz@comcast.net> wrote:

    Geoffrey Kolbe wrote:
    > The bubble is repairable. You will need to obtain a small bottle of
    > pure xylene and a (preferably glass) syringe with which to insert the
    > xylene into the bubble chamber. You will also need to fashion a
    > screw driver to undo the small "Bristol" screw on the chamber. A
    > small Phillips screw driver is the usual starting point for this
    > exercise. There is a steel ball under the Bristol screw, so when you
    > get the screw out, don't loose that ball! A bubble exactly 5mm in
    > diameter in the bubble chamber is equivalent to one degree diameter
    > in the optical system. This is twice the diameter of the sun, so is
    > good for that, and is good for star sights as well.

    Thanks! My set screw had some sort of clear coating (lacquer?) which
    came off with lacquer thinner. That let me see clearly enough to
    identify it as a .069 inch Bristol screw with four splines. A 1.8 mm
    standard screwdriver actually fits pretty well, but didn't break it
    loose with the amount of torque I wanted to apply. Presumably there
    is lacquer on the threads. So I'll look for the right wrench - I
    haven't found a local source for that yet.

    I have also determined that I can buy xylene at Home Depot - but only
    in a one gallon can. Since I only need about a tablespoon, I will
    look a bit further - maybe at a hobby shop.

    BTW I assume they picked xylene for the low melting point and low
    viscosity (both lower than water, according to
    http://www.engineersedge.com/fluid_flow/fluid_data.htm). However I
    see from the Wikipedia article that xylene has three isomers, and that
    one of them melts at 13 C. I suppose Link was careful about which
    isomer they used. I doubt that Home Depot cares, but maybe there's
    something on the label.

    I'm also wondering if rubbing alcohol wouldn't be about as good for my
    purposes :-)

    I do have a syringe, but it's plastic. I'll just have to hope it
    doesn't dissolve in xylene. Thinking ahead, it seems easiest to me to
    fill the chamber completely, then inject an air bubble of the right
    size, drop in the steel ball, insert the set screw, and finally coat
    with clear nail polish.

    - Jim Van Zandt

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