Immerse in water during chamber testing?

[g2]

My dive computer -- like most, if not all? -- has a little sticker on it warning that the computer should be immersed in water when chamber testing, because air pressure alone may damage the pressure sensor. I've been trying to figure out why it should make any difference, but I confess I'm baffled.

Both air and water are fluids, so it shouldn't matter so long as conductivity isn't an issue. Even then, why would the sensor be damaged? And if it's a permeability problem, what if there happens to be small air bubbles trapped against the sensor during a dive, as there probably always are?

I guess it might help if I knew how pressure sensors work. Anybody have any insight into this?

g2

 

[Greg G.]

I'm not sure I understand what you meant by "both air and water are fluids". Air condenses, water does not. Air is a gas, water is a liquid.

Originally posted by g2
My dive computer -- like most, if not all? -- has a little sticker on it warning that the computer should be immersed in water when chamber testing, because air pressure alone may damage the pressure sensor. I've been trying to figure out why it should make any difference, but I confess I'm baffled.

Both air and water are fluids, so it shouldn't matter so long as conductivity isn't an issue. Even then, why would the sensor be damaged? And if it's a permeability problem, what if there happens to be small air bubbles trapped against the sensor during a dive, as there probably always are?

I guess it might help if I knew how pressure sensors work. Anybody have any insight into this?

g2

 

[Bob3]

The reason submersible instruments need to be pressurized in water is that a gas, such as air, can slip past seals that would keep water out.
When you bring the instrument back to normal pressure, POP! They're not designed to handle internal pressure.
Some high end dive watches have a vent so they can be used safely in chambers.

 

[joewr]

Folks,

Bob and Greg have the technology down pat! However, how about a few definitions?

Gas, Liquid and Solid are the states of matter that we usually concern ourselves with: a gas flows and is compressible; a liquid flows and is not compressible; a solid neither flows, nor is compressible.

However, all three can be fluids or, more correctly, can be fluidized. This is obvious for gases and liquids, but perhaps not so obvious for solids. However, let me give you a "down home" example of a fluidized solid: the silt or sand that you kick up when you let your fins get to close to the bottom of whatever body of water you are diving. More esoteric are the fluidized beds of solids used in the petroleum and chemical industries.

Joewr...who just could not help himself...

 

[Iguana Don]

Joewr,

You took the words right outta my mouth!

ID

 

[Fishkiller]

Originally posted by joewr


Liquid and Solid are the states of matter that we usually concern ourselves with: a gas flows and is compressible; a liquid flows and is not compressible; a solid neither flows, nor is compressible.

solids flow ever seen a old glass window

 

[CheeseWhiz]

:doctor: Technically, glass is a liquid, since there is no defined phase change going from the obviously liquid form to what-appears-to-be-solid form.

As for air and water being fluids, I seem to remember from physics classes that both are defined, in narrow academic terms, as fluids. I would have to look it up, but it's probably in the sense of having weaker molecular bonds than solid-state matter, which allows for free movement of individual molecules.

Go forth and fluidize.

 

[Liquid]

I'd risk here a guess that a part of the answer might be that if compressed in air, the risk of rust is increased becouse of the bigger partial pressure of the oxigen. When in liquid, the partial pressure of the oxigen stays quite the same, no matter how deep.