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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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