Sunday, December 2, 2012
Why Doesn't A Compass Freeze?
No matter how many electronic gadgets I accumulate, every pack I own ... from day packs to backpacks ... have a plain, old, liquid-filled compass handy. No batteries to die, no electronics to crash, just a little magnet with the North magnetic pole marked (usually in red) and the earth's magnetic field.
I've hiked and backpacked in a lot of sub-zero temperatures and never had a compass freeze on me. So, now that I'm checking my gear for an end of December course in winter mountaineering in the high Sierra, I started to wonder why.
First of all, it's important to understand that the liquid inside the compass is there to stabilize the needle, and to dampen wild movements and vibrations ... to help it settle down, as it were.
My research today indicates that a mixture of alcohol and distilled water is among the most common fluids inside a compass. Pure rubbing alcohol (isopropyl) will freeze at -117F. Other alcohols will free at similar sub-Antarctic temps.
Compasses will be filled with a mixture of alcohol and distilled water which raises the freezing point.
For comparison purposes: pure ethanol (the alcohol you can imbibe) freezes at -173.2F. This means that my Jameson's -- or your Jack Daniels or other distilled adult beverage which average 80 to 86-proof (40 to 43% ethanol) will freeze around -25F or -30F. Note to self: that bottle of 100-proof Wild Turkey in my cabinet should be fine no matter what Antarctica could throw at it.
A WORD TO THE WISE: If you are headed into extreme cold, you should contact your compass manufacturer to make sure the compass can survive -- and continue to function properly -- n whatever temperatures you can expect.
Compasses (usually those used on boats) have been filled with a variety of other fluids including kerosene and mineral oil. Kerosene is flammable and mineral oil -- with a freezing point around -22F -- is viscous and will get too thick to allow the needle to swing freely when temps get much below freezing.
ANOTHER WORD TO THE WISE: A shot of Jack's will make you feel warmer when it's cold out. But that feeling is actually a dilation of blood vessels under the skin ... something that actually makes you lose heat faster.