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    Re: Bubble sextant intervention needed.
    From: Brad Morris
    Date: 2012 Oct 19, 01:37 -0400

    just my two cents....

    1) You are essentially destroying the re-sale value.  Future generations will not want the one you have gooped up, they will prefer the unmolested one.  These devices are not being manufactured or replaced, so each one harmed is slowing reducing the existing population.  In other words, you don't own that A12, you just have temporary custody!

    2) Be very careful not to get paint near any knobs, dials or rubber that may get gummed up with paint.  You don't want to have to take it apart to clean it, just to get to re-assemble it.  It may be possible that that's not in your skill set.

    3) As you note, it will be butt ugly.  How do you intend to mask the surfaces to not be painted so as to get clean lines and concentric circles around dials.  It will look decidedly amateurish if you just rough it in with a free hand paintbrush, assuming that you aren't a sign maker or landscape artist.

    As more of an instrumentation enthusiast than a mathematics guru, I cringe when I hear of equipment being mashed up, rather than being used in a respectful manner.   Wouldn't it be better to reserve your A12 for evenings and cooler days?  Perhaps you can restrict the duration of use to keep it cool.

    On Oct 19, 2012 1:05 AM, "Paul Dolkas" <paul@dolkas.net> wrote:


    I have a stupid question - or perhaps I just need somebody to stop me from doing something stupid. I’ve been playing around with my A-12 bubble sextant for a few months now, and I’ve noticed an interesting phenomena. The bubble cell, unlike some of the more expensive models, has a fixed volume. So when it gets hot outside, the fluid inside the cell expands and the bubble gets smaller. On at least one occasion, it has disappeared entirely. Knowing the pressure that must exist inside the cell to do this, this can’t be good. Luckily, I haven’t had it break the cell window.

    Which brings me to my point. I know some nautical sextants are painted white to avoid getting hot and going out of alignment in the tropics. However, I have yet to see any white aircraft sextants out there. Now, this probably because models like the A-12 were made to be used in open cockpit aircraft, where freezing was more of a concern than baking. But for those of us who use them at sea level, wouldn’t it make sense to paint them white - at least on the outsides?

    So to check this out (and to embrace my inner science geek) I ran an experiment where I took 2 aluminum blocks and painted one white & the other black and left them out in the midday sun (solar elevation: 45o ). Naturally the black one ran hotter, by about 20oF. I did some calculations, and this modest temperature rise is enough to almost make a small (1/16”) bubble disappear.

    So my question is: aside from making what will probably be a truly butt-ugly sextant, is there any reason I shouldn’t paint the outside of the sextant white? (naturally, I would leave the inside black to minimize stray light.) I kinda hate to go against tradition, but there don’t seem to be any real downsides to doing this. Can anybody think of any?


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