How does moisture get into a tank?

[Soggy Paul]

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?

[Chrisch]

If there is water in a tank it came out of the compressor.

[nigel hewitt]

Air contains gaseous H2O. Not water vapour but as a gas.
Compress it and it can't stay a gas so it becomes first water vapour then just plain water.
Compressors filter it out and need it draining off.
Some however avoids the filter/traps and ends up in the tank.
Better quality compressor, less water.
Dodgy fill, more water.

The problem comes that at high pressure water is a catalyst to the oxidation of iron.
Hence corrosion in a tank by the oxygen in the fill.
Worst. A catalyst is not consumed in the reaction so it just keeps happening.

[notdeadyet]

If it's pressurised then the moisture came from the compressor as Chris said. If it's not pressurised and the valve is open or removed then in theory it's exposed to atmospheric air which has higher humidity than breathing air. But even if it is just 1 bar higher than ambient then nothing is getting in. If it was run very low under water, enough that ambient pressure was higher than cylinder pressure, then I suppose there is also the possibility that water can makes it way in. How realistic that is I don't know.

This sounds like it's heading towards a story about rusty cylinders and some dive shop bollocks about how it's your fault and not their compressor.

[WFO]

"Low pressure" is fine
"Totally empty" can let water in if someone for example left the valve open

[Nickpicks]

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 the tank has higher pressure than ambient atmospheric pressure, air from outside cannot get in.

Ways water can get inside a cylinder:
The compressor filter has failed
A self inflating dSMB cylinder was emptied underwater (so got water inside it), then connected to the main cylinder (so water can flow to the main cylinder once pressures have equalised)
There was water in the mouth of the valve before the whip was connected (could have been left on deck and splashed)
Water in the end of the whip before connecting (if the whip was dropped in the water bath that some people like to use)
The cylinder was opened underwater when empty/near empty (with 5 bar cylinder pressure, water can flow in at depths >50m)
Leaving the valve open for a long time can theoretically let atmospheric air (including water vapour) diffuse through the tiny opening of the valve.
Cooling the cylinder could form condensation on the inside (but assuming the air inside conforms to BS EN12021, would have to be less than -11 degrees (most air tests I've seen have a dewpoint of around -50 degrees)

[Energy58]

All the above are correct but if you do the maths its obvious that the only practical way to get any appreciable moisture into a cylinder is to pump it in (either from a filling whip full of water, which is unlikely, or most probably from the compressor) - for every other mechanism the pressure gradient is too low or non-existent to drive water into a cylinder (unless you leave it with the valve open at the bottom of a water bath for hours).

[graham_hk]

If it's pressurised then the moisture came from the compressor as Chris said. If it's not pressurised and the valve is open or removed then in theory it's exposed to atmospheric air which has higher humidity than breathing air. But even if it is just 1 bar higher than ambient then nothing is getting in. If it was run very low under water, enough that ambient pressure was higher than cylinder pressure, then I suppose there is also the possibility that water can makes it way in. How realistic that is I don't know.

This sounds like it's heading towards a story about rusty cylinders and some dive shop bollocks about how it's your fault and not their compressor.

Seen water come pissing out of drysuit tanks on multiple occasions

[notdeadyet]

Seen water come pissing out of drysuit tanks on multiple occasions

I had it happen when I played around with a 0.5l cylinder for my suit now that you mention it. I'd completely forgotten about that. In fact, I think I might've bought it off you years ago? I vaguely remember whoever I got it from warning me not to be extravagant with gas in case that happened.

[graham_hk]

I had it happen when I played around with a 0.5l cylinder for my suit now that you mention it. I'd completely forgotten about that. In fact, I think I might've bought it off you years ago? I vaguely remember whoever I got it from warning me not to be extravagant with gas in case that happened.

I can't remember what I had for breakfast ... 10 years ago no chance