How does moisture enter tanks?

[Naked diver 2]

Apparently a tank without much air pressure in it can get moisture entering. How is this possible if the tank is air tight? Surly it's therefore also water tight?

 

[rjack321]

Apparently a tank without much air pressure in it can get moisture entering. How is this possible if the tank is air tight? Surly it's therefore also water tight?

If there is any pressure in the tank at all, moisture won't enter just sitting there

Water gets in when the valve is wet during the fill process and the whip blow those water drops in. They usually splatter on the bottom of the cylinder and cause patchy flecks of corrosion or pits.
or
Water gets in when compressor filters aren't maintained and excessively wet gas is coming straight out of the fill whip and then condenses in the tank. This tends to be more widespread all over corrosion not just a half dozen flecks.

 

[broncobowsher]

Apparently a tank without much air pressure in it can get moisture entering. How is this possible if the tank is air tight? Surly it's therefore also water tight?

Corrected for you.

I have seen a tank from someone taking a rescue class that was empty but took many dives where the water pressure exceeded the air pressure and that backfilled through the regulator getting a fair amount of water inside.

Empty tank with the valve left open will 'breath' as it warms up and cools off. In the correct environment, cooling off during high humidity, it can suck moisture in and condense as it cools off.

 

[BlueTrin]

How does water is prevented from entering during a fill if the valve is wet?

 

[rhwestfall]

How does water is prevented from entering during a fill if the valve is wet?

A quick blast of what is left in the tank to purge/dry the orifice...

 

[boulderjohn]

Explaining all this was a real challenge for me as an OW scuba instructor, because the language in the course seemed to indicate that just getting low pressure in the cylinder would allow water to get into the tank. As a consequence, I believe a lot of people, including dive shop employees and instructors, believe that is true.

 

[Subcooled]

Apparently a tank without much air pressure in it can get moisture entering. How is this possible if the tank is air tight? Surly it's therefore also water tight?

If there is an overpressure in the tank then NO moist (or dry!) air can enter.
Water vapour might still creep in if the valve would be open - but then the pressure would equalize!

 

[Gone for diving]

How is this possible if the tank is air tight? Surly it's therefore also water tight?

You are correct,


A quote form some I know,

Air is 1500 times more searching than water,,,,

Not sure if the number is correct, but it makes a point...

Air is what you want to leak test with, (helium is even better)

Just sitting there on the surface, tank at 20 psi. No water can get in, (valve closed)

Now,
if it was filled with humid air you could have water condensation in the tank when exposed to freezing temperatures.

Also
Underwater you could have water coming in, If the tank pressure was lower than the surrounding water pressure,
( .5 psi per ft aprox),

Example:
On a 800 ft dive the last (aprox 350-400 psi) is unusable at that depth.

Mainly water gets in from compressor, or a wet valve, as stated above,

 

[BoltSnap]

Also
Underwater you could have water coming in, If the tank pressure was lower than the surrounding water pressure,
( .5 psi per ft aprox),

Example:
On a 800 ft dive the last (aprox 350-400 psi) is unusable at that depth.

If the gauge is showing 350 - 400 psi while at depth, then it is a none issue since the gauge is showing 350 - 400 psi above ambient pressure or PSIG. If you start from the Surface with 350 - 400 PSIG then go down to 800ft, then that maybe an issue.

 

[Joebar]

If the gauge is showing 350 - 400 psi while at depth, then it is a none issue since the gauge is showing 350 - 400 psi above ambient pressure or PSIG. If you start from the Surface with 350 - 400 PSIG then go down to 800ft, then that maybe an issue.

I guess that is not true.
The High pressure gauge is directly connectet to the tank (of course via the first stage) and not manipulatet by the first stage. On the gauge you don't have a correction of the ambient pressure.