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    Re: micrometer sextants.
    From: Brad Morris
    Date: 2009 Jul 17, 15:36 -0400

    Hi Wolfgang
    Yes, the classic discussion of resolution, accuracy and repeatability.
    Resolution is where your tick marks are.  You can have as many as you want.  
    Resolution, however, is not accuracy.  Consider, I can have many tick marks, 
    but there is no statement as to the accuracy of those tick marks.  In other 
    words, you can have very fine resolution without any accuracy whatsoever.
    Now accuracy is the relationship of the index arm to the graduated arc.  When 
    the index arm points to a number, is it correct?
    Finally, the repeatability of the measurement can be considered.  Are you only 
    using the micrometer in one direction (uni-directional repeatability) or both 
    (bi-directional repeatability).  If bi-directional repeatability is 
    considered, will you examine lost motion (gear play, thrust springs, etc).  
    Even uni-directional repeatability will have errors due to the slip stick 
    behaviour of mechanisms.
    Just because a sextant has a micrometer drum, doesn't make it any better than 
    the older style vernier, in terms of resolution, accuracy or repeatability.  
    It is "easier to read" if a person can't be bothered to learn how to read a 
    vernier. It looks "high tech" and as such will appeal to the target market.
    The vernier is superior because there is no gear train and hence no lost motion.
    Best Regards
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