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    Re: Sextant frames
    From: Bill Morris
    Date: 2009 May 12, 20:20 -0700

    In post 7981 I said I would investigate the behaviour of sextant frames left 
    in the sun. It has taken until today for my free time and the sunshine to 
    coincide.
    
    I compared the front and rear temperatures of two alloy-framed sextants and 
    the rate of temperature rise in midday sunshine, when the sun's altitude was 
    about 38 degrees. I arranged the plane of the frames to be approximately 
    normal to the sun's rays. I also compared the rate of temperature rise 
    between an alloy and a bronze-framed instrument.
    
    I measured the temperatures using thermocouples held in contact with the 
    thickest part of the frames via zinc oxide paste. The thermocouples 
    themselves were shaded from direct sun by the plastic clamp used to hold them 
    in place. The thermometer readout's resolution was to 0.1 Celsius.
    
    There was no difference in the rate of temperature rise greater than 0.5 
    Celsius between a SNO-T, a Tamaya 632D and a Brandis Vernier sextant (the 
    latter with a bronze frame). Starting at about 16.5 degrees, all took about 
    12 minutes to reach an equilibrium temperature of about 29 degrees. During 
    this rise, the temperature difference between front and back never exceeded 
    0.9 Celsius.
    
    I think we may conclude that frame flexure due to exposing a sextant to midday 
    sunshine in Autumn in 35 degrees S is likely to be negligible. If members 
    would like to suggest further experiments about the thermal behavious of 
    sextant frames I would be happy to try to carry them out.
    
    Bill Morris
    Pukenui
    New Zealand
    
    
    
    
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