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    Re: Bubble sextant construction
    From: Paul Brewer
    Date: 2013 Mar 21, 16:35 -0700
    In the case of the RAF Mk IX series of sextants the diaphragm was used to increase the volume of the chamber assembly. There is a reservoir connected to the bubble chamber and eventually this is emptied and air is fed to the chamber. The size of bubble could be controlled by the deflection of the diaphragm. In that type of arrangement there would be essentially no pressure difference from no bubble to full bubble. Some of the American chambers did increase or decrease pressure on a sealed system to change the size of bubble.

    As to bubble size, too small a bubble  can make it slow to react. The official guides for the MkIX suggest a bubble size but within a fairly large range. I personally try to match my bubble to my target when I am calibrating but I have little experience with taking sights in anger.

    Paul.

    On 21 March 2013 14:38, Randall Morrow <randall.f.morrow{at}kp.org> wrote:

    Can antone tell me how the older sextants change bubble size? Was it with application of pressure or by addding-subtracting volume of fluid? Some old drawing I have seen suggest a bellows for pressure. Also, how much of a difference in accuracy is gained by matching bubble size to the body observed? If it is significant, why do the post WWII versions not have bubble adjustments?
    Thanks.
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