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    Re: Compass tilt. and other compass issues
    From: Bob Peterson
    Date: 2004 Jun 21, 11:57 -0500

    I'll use George's ?s to respond to all:
    
    1)  Sestrel does make a range of models.  The model ready for world
    traveling and several magnetic latitude zones is the "Major" (has approx
    5 1/2" diam dome).  They make a smaller (approx 4" diam) model called a
    "Minor" and a strange goose-neck mounted model called a "Moore".  Those
    I do not know about though I'm sure the "Moore" will NOT change zones.
    George, sounds like you have a "Minor".
    
    2)  Careful with the baby oil.  Without stabilizers in the fluid the
    stuff tends to jell up with time and exposure to air.  The real issue is
    pulling the entrained gases out of solution.  Need a vacuum pump for
    that though there is a heating technique in the oven.  Not recommended.
    The manufacuterer has designed the compasses for a particular damping.
    (Remember those dynamic effects?) Overdamping is not always the answer.
    Baby oil is almost for sure too viscous.  The fluids used today are
    variations of Exxon Isopar though Odorless Mineral Spirits was used in
    the past.  Still,  they all must be treated properly before filling.
    
    3)  Before filing an old compass (pre WWII) be careful to determine if
    it was oil filled or alcohol filled.  Getting this wrong will destroy a
    perfectly good instrument.  Oil has a high expansion coefficient with
    temperature.  It wants to move, so a large expansion mechanism must be
    built into the compass.  If you see a rubber expansion diaphragm or a
    bellows with many (more than two) ridges, it is probably oil filled.  If
    you see only a flat plate, you can be damn sure it is alcohol filled.
    Alcohol (plus water) doesn't require the volume range.  But here's the
    real trick.  Alcohol compasses are painted with special paint!  It's egg
    tempra paint 'cause nothing else will with stand the alcohol (It is 190
    proof with some water.  Martinis anyone?).  But this paint takes a
    witchdoctor license to mix it up and apply.  So, don't mis-fill your
    compass with the wrong fluid!  And especially don't fill it with
    anti-freeze!  The damn stuff is toxic and makes a complete mess of
    things.  I've had to throw away perfectly good compasses because someone
    filled it with anti-freeze.  Also the orings/gaskets on an alcohol
    compass must be gum-rubber.  Use buna-n and you will end up with a tarry
    mess.
    
    4)  Because most modern compasses are oil filled, it is important that
    they be removed for winter storage.  When the compass is filled (new or
    repaired) the ambient temp is approx 70 degrees.  In  Chicago, the run
    up on the high side during summer is 30 degrees (--> 100 degrees).  But
    the down side during winter is 90 degrees (--> -20 degrees) so they take
    a  much harder beating during winter.  There simply is not enough temp
    range for the expansion mechanism to accomodate and the compass is
    trying to implode.  Something will give.  Then an interesting cycle
    starts.  Usually the exp diaphragn tears to relieve the pressure which
    makes the compass happy at the low temp.  But then the temp rises on the
    next warm day and the fluid expands except it now has a place to go -->
    out.  Which it does and evaporates.  Next cold night?  Repeat cycle
    until the compass is empty.  Hmmmm?  Wonder were all the oil went?
    
    5)  Back to compass dip:  Indeed the old sea captains trick that Trevor
    referred to would work, but you have to know how much.  There are tables
    in HO226 (now absorbed into Bowditch)(BTW,  I think Ken at Celestaire
    still has some last copies of HO226) to calculate "how much"  but again
    there is nothing like a measurement (data) to build confidence.  It
    would not be the Flinders Bar (which is the vertical "soft-iron"
    corrector along with the quad sphere for horizontal soft-iron
    correction) rather the vertical field corrector. This is a vertical tube
    mounted dead center below the compass for a vertical magnet to be
    raised/lowered and thus to "rebalance" the  dip angle.  So it all comes
    full circle.  BTW, all these details were worked out by a very bright
    Brit, named Lord Kelvin who gave us lots of other physics.
    
    6)  Trevor -- If you can find the S/N on your Ritchie flattop, I can
    find out when it was built.  Ritchie has complete log books of every
    compass they ever built.  The books are hand written and go back over
    150 years showing date of build and who it was sold to and when
    rebuilt.  Amazing!  They built compasses with both quarter points and
    degress.  I have heard here in the States that the Merchant Marine
    wanted to use quarter pts and the Navy preferred degrees as a result of
    the quality of the recruits.
    
    I think that covers it.  Thanks. -- Bob Peterson
    
    George Huxtable wrote:
    
    >I can see that changing the distance between the pivot and the COG is very
    >likely to affect vulnerability to horizontal accelerations. There are
    >Sestrels and Sestrels, so I am interested to know which (if not all) models
    >he is referring to. For more than 30 years, I have used the common
    >spherical type of Sestrel with a 3-inch (or so) card, and an internally
    >gimballed cage carrying lubberlines and pivot-socket. Having used no other,
    >I am not in a position to evaluate its performance against other compasses,
    >though I have not had cause to complain: not even under rough conditions in
    >a 26-footer. I know that Sestrel have made other, very different,
    >compasses, some really big for large vessels, flat disc types with external
    >gimballing. So it would be interesting to learn whether the instability
    >problem Bob identifies applies to all Sestrel compasses or to specific
    >types, such as mine.
    >
    >
    >
    >I have had reason to refill an oil-filled compass with new oil, and have
    >found Johnson's baby oil, straight from the bottle, to be perfectly
    >satisfactory. After 5 years or so, it's just as crystal-clear as on the day
    >it went in, without a hint of a bubble. And the amount of damping seems
    >just right, to me.
    >
    >For compasses that use a spirit-water mix, I think removal of dussolved
    >gases may be more of a problem. After refilling such a compass, I put the
    >assebly into a vacuum chamber (with the filling-plug removed and its hole
    >at the top). The amount of bubbling surprised me, and I needed to top it up
    >a few times. Even after that, the compass developed a bubble later. So I
    >would be reluctant to undertake a spirit refill, but with baby-oil, there
    >seems to be no such problem.
    >
    >
    >
    --
    
    Robert S. Peterson
    31 N Alfred, Elgin IL  60123  USA
    847/697-6491
    Compass Adjusting & Repair for Lake Michigan Navigators Since 1985
    Physics @ Bartlett HS
    e-mail: rspeterson(at)wowway.com
    
    
    

       
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