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    Re: Sextant frames
    From: Brad Morris
    Date: 2009 Apr 16, 10:51 -0400

    Hi Jeremy
    As indicated in the Sextant Handbook, you should allow the sextant to reach
    stable temperature before use.  The reason is exactly that, the thermal coefficient of expansion.
    The sextant should not be warmed in the sun, as one side will be warmer than the other.
    This refers to the linear expansion of the material, typically in parts per million per
    degree Celsius (PPM/C).  So if we had a one meter long bar of aluminum and changed the
    temperature by 1 degree C, it would change in length by about 23 microns (~1/40th of a
    millimeter).  That's .000023 PPM/C
    The first issue that you encounter is that the parts of the sextant will not change
    uniformly during the transition to stable temperature.  Thicker parts will remain cool while
    other thinner parts will quickly warm.  This may distort your reading.  The effect
    is exaggerated in plastics, which have a large coefficient of expansion relative to aluminium.
    When comparing aluminium to bronze, we encounter a subsequent issue.  Since bronze is an
    alloy, the thermal coefficient of expansion is a function of the precise materials which
    form the alloy.  You will be hard pressed to have manufacturers tell you the composition
    of that alloy and hence, we can only guess at the coefficient.
    In general, however, bronze has a lower thermal coefficient of expansion than aluminium.
    As such, the effects of temperature are reduced.
    Based upon the values, I would indicate that this issue is dramatically overblown.  From
    my perspective, the effect is minor.  If you were to change the temperature by 100 degrees
    C, you might have issues, until the sextant temperature stabilizes .  But that is freezing
    to boiling of water.  I don't think you keep the bridge that cold.  I doubt the bridge wing
    is that hot.
    Best Regards
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Anabasis
    Sent: Thursday, April 16, 2009 9:45 AM
    To: NavList
    Subject: [NavList 7968] Re: Sextant frames
    That certainly is interesting reading (and should be copied to the
    sextant group on yahoo).  I have used sextants of all natures at sea
    (excluding plastic which I've only played with on land).  I haven't
    noticed much difference in the accuracy of my sights, but there
    certainly is a cosmetic and weight difference.  I found that my
    selection of scopes increased my accuracy, but the material of the
    sextant seemed irrelevant.
    One question however.  Do the differing co-efficients of expansions
    make any difference in the consitancy of the sight?  In other words,
    when I bring a sextant out from the AC of the bridge to the hot bridge
    wing, will a bronze or aluminum sextant tend to warp at all, and if
    so, to what degree?  I would hope that that frame shape would take
    care of this issue and by the time the fog cleared on the mirrors, I
    could be confident of the sight.
    When I bought my sextant last year, I went with the bronze Cassens &
    Plath, not because it was bronze, but because it had the highest
    claimed accuracy of sextants available new, and because of the
    polarized shade option.  The C. Plath Navistar is not bronze, but some
    sort of aluminum looking alloy.  It is quite light, and has a very
    fine claimed accuracy.  My issues with that sextant extend beyond the
    material and more to the ergonomics and usability (very poor shading).
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