Ask A Scientist General Science Archive

name         Ed
status       other
grade        other
location     PA

Question -   Hi, I recently purchased a liquid filled US Navy compass
dated 1943 made by the Lionel Corp.in N.Y. I want to remove the large air
bubble inside the compass. What is the liquid inside the compass?
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I worked at Lionel and can tell you that this liquid was absolute ethyl
alcohol (the drinkable kind).

regards,
George Vitt
Sr. Scientist, Ret.
Hughes Aircraft Co.
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A very interesting purchase.  At one time the fluid used was kerosene.  If
that is the original fluid and if it has a bit of a yellow haze that could
be what you have.  Now they use a silicone fluid that is clearer and not
flammable.  I would suggest that you consider replacing all the fluid with
the modern material.  You can find compass repair materials at aircraft
supply outlets.  A web search should find a couple of sources.

Larry Krengel
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I do not know anything about that specific compass, but I am guessing the
bubble is there for leveling, and is meant to be there.

J. Elliott
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I do not know what they actually used, and I have not seen this kind of 
compass.
My main question is whether there is paint inside, or only textured metal 
surfaces.
I can describe the basic ideas.

You (and the original makers), want something clear and non-yellowing, 
moderately viscous, and harmless to metal and paint.
Mineral oil is pretty good in that regard.
100% Isopropanol would not be terrible, but it is not very viscous and 
leaks and evaporates.
Glycerin is viscous and will not corrode metal (might even dissolve small 
amounts of water leaking into the case, protecting metal from corrosion).
However it is a strong solvent and kills most paint in the not-too-long run.
It does the same thing brake fluid does to auto paint, but slower.
Fluorocarbon oils are entirely safe for paints, but it is overkill, and I 
am sure that is not what was used in 1943.
Silicone oils can be harmless too.

At this time, you want something mutually soluble with the old oil.
so you need to get some of the old oil out of the compass to try it.
Put a drop into each clear glass of:
  - water,
  - drug-store mineral oil,  (or maybe vegetable oil is quicker for you)
  - 91%-grade rubbing alcohol  (91% isopropanol, 9% water)
  - drug-store glycerin
and let them sit still a while,
and watch to see which one bends to near-invisibility first, instead of 
staying in a sharply-defined glob.
Only wiggle or shake if it takes longer than an hour to dissolve.
If shaking makes the solution cluttered or milky, that is bad, very insoluble.

My guess the mineral oil will dissolve first and smoothest.
I might keep all the old oil that can be poured out in some tiny bottle, 
and fill the compass with the new mineral oil.

The most conservative way to clean out old oil is to refill with you 
chosen new oil and shake or soak, then pour out and refill again.
If you wish to do a quicker or more thorough clean-out of old oil or new,
then 100% Isopropanol is your safe universal solvent that can evaporate to 
dryness.

Sorry if that is long-winded!  Maybe someone else will know the short answer.

Jim Swenson
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