Air Bubbles in Compass

[ams511]

I sent my 11 year old compass back to Oceanic because it had a large air bubble in it. They sent me back a new compass with smaller air bubbles in it. The question I have is, do air bubbles affect the performance of the compass? Thanks

 

[RichLockyer]

I sent my 11 year old compass back to Oceanic because it had a large air bubble in it. They sent me back a new compass with smaller air bubbles in it. The question I have is, do air bubbles affect the performance of the compass? Thanks

If they are very small now, I'll bet that you won't be able to see them at 100ft
The only potential problem would be if there is enough air to cause the housing to compress enough to crack, or if the bubble gets UNDER the card and causes an edge to float up and hit the lens, causing it to stick.

 

[ams511]

Thanks for the reply Rich.

The bubble right now is 1/4". The one I sent back was about 1/2 - 3/4 or an inch. The next time I go to a dive store I will have to take a look at the compasses, to check for air bubbles.

 

[DeepTrip]

A little question, why they need the bubble? Why don't they fill in the compass with liquid?

 

[miketsp]

This answer was published in a recent PADI magazine.
"Yes, having a bubble in the compass could interfere with the needle being off a by few degrees. This could mean missing the target completely.
No compass should have a bubble in it.
My suggestion is that you should get a compass without a bubble."


IMHO that sounds almost right. A 1/4inch bubble doesn't seem that small. The liquid is there for 3 reasons: a) damping of the movement, b) to serve as a non compressible interior so there is no stress on the plastic casing and c) to avoid corrosion of the metal parts.
If the bubble is big enough that its compression will cause flexing of the plastic case then after a number of deep dives you will start to get stress cracks.
I have never seen a divers compass with an air bubble.

Oh yes I just remembered, in the case of larger compasses (boats or planes) - the liquid also serves as a lubricant to avoid pivot wear.

 

[RichLockyer]

I have never seen a divers compass with an air bubble.

I've seen several... all of them Oceanic or Genesis (same design, different logo on the card).
I gave mine to my wife for non-diving applications and have been using a Suunto SK-7 for years.

 

[miketsp]

I've seen several... all of them Oceanic or Genesis (same design, different logo on the card).
I gave mine to my wife for non-diving applications and have been using a Suunto SK-7 for years.

That's probably the best thing to do if the manufacturer won't exchange it.
I too use a Suunto SK-7 as do most people I know. Good device and comes in all the regional variants.

 

[Wally1716]

...
Bubbles are not meant to be there!!
..
They form when the flexible walls bend
due to pressure stress ( inward if diving,
outward if flying ). After a while or
too many "bends", the plastic might not
return to its original size reducing the internal
pressure and causing some of the dissolved
gas in the liquid to come out of solution.
Its effect on the direction is normally minimal,
but, if it is large enough, it could displace the
position of the plate or the needle giving an
erroneous heading.
...

 

[DA Aquamaster]

In my experience my compasses have all stayed relatively bubble free until my first entry into cold water on a hot day.

My theory on it is that the fluid in the compass expands slightly in the sunny warm weather and expands the warm case very slightly. The case however then hardens faster than the liquid inside contracts upon entry into the chronically cold water in this area and a bubble results inside as the pressure is reduced inside the compass.

I suspect the only way to prevent the problem would be to use a fluid with absolutely no dissolved gases at all. I suspect compass manfacturers strive for this but don't exactly knock themselves out over it.

In my experience a small bubble is not an issue, and an in some instances can even aid in keeping the compass level. But a large bubble will interfere with the leveling of the card and consequently the movement of the compass card either by getting under the card or by getting between the card and the top of the case and forcing one side of the card downward. When the bubbles get that bad the compass gets replaced.

I have thought about using a heated needle to aspirate the bubble and then use a syring to top off the case with alcohol. But the whole process and the neccesary resealing of the hole has never struck me as being entirely worth the trouble. It's one of those things I'll try if I ever get real bored.