Air vs. Oil/ Liquid fills

I've been shopping around for gauges and came across different models that say that they're either filled with air, oil or "liquid" (the ambiguity makes me laugh). Are there differences between these types of gauges if say they're all rated for the same depth? Is there a durability factor that either oil works longer maybe or if it leaks is broken? I can't seem to find any answers through Google so I turn to you!

Oil filled gauges are used to damping the needle movement. That said I have not seen an oil filled gauge in a long time. I am talking SPG here. Compasses on the other hand are almost always liquid filled which helps with leveling.

If you have a choice, you have some thinking to do.

Air is compressible. So if you go deep enough, any gauge that is filled with air will eventually collapse. Gear with non-moving parts can be "Potted" by filling up all the internal air space with Wax or Resin to help depth proof the device.

Guages cannot be potted because they have to move, so filling them with a fluid is a good way to depth proof the gauge. Oil helps dampen springy movements while water or alcohol are less viscous.

So why don't we fill all our gauges with a fluid to depth proof them?

Because no closure is perfect. Anyone who has ever put a dive compass in checked luggage knows what I mean. When you put a fluid filled compass in an airplane hold, for some reason, it will almost always emerge with a bubble of air in the compass. And once you have a bubble, you have the worst of both worlds. Now you have a bubble and the possibility of compression, and you have a visual obstruction in your gauge.

So, when I'm confronted with a choice, I would choose fluid filled gauges and potted electronics if I were going very deep. If not, I would avoid the complexity and expense and go with air filled gauges (and a fluid filled compass.)

Right on, that helps a lot! Thanks guys

@Scared Silly, love your lengthened explanation of the idiom.