I know mine is caused by fills on dive boats. Never a lot of rust but some every viz- On multidive days and multiday trips it is almost impossible to get all the water out of the valve before they do the fill, and I am sure the filters are not catching everything.
That is not the case. You need to crack the valve for a couple seconds to blow all the water out of the valve (and the threads with a DIN valve) and it costs you a couple cubic feet of gas, but you will have a dry valve. They also need to do the same with the fill whip, but again you will ensure no moisture enters the tank that way.
If the fill monkey on the boat is not doing that, you need to educate them.
Otherwise, a more likely culprit is poor moisture separation on the compressor itself.
---------- Post Merged at 08:51 AM ---------- Previous Post was at 08:40 AM ----------
Bill, think about that for a second.....the IP is created by tank pressure, so it can never be higher than tank pressure, at most it can be equal to tank pressure. Once the tank pressure reaches the IP of the reg, the first stage will go full and stay that way, keeping tank pressure and IP equal. And even it it did, were is the water going to get in from?? The ambient wate pressure would have to be higher than either the tank pressure or the IP and even then, one of the second stages would have to be open.....if that were the case, you have much bigger problems than a little rust in the tank.
I agree with you Herman.
It could happen, but you have to work really hard to make it happen. If the IP is, for example, 145 psi the first stage seat is closed anytime the IP is at 145 psi. Once the tank pressure falls below 145 psi, the seat could stay open. But you still have 145 psi above ambient in the first and second stage. So you'd have to run the tank pressure to near zero (relative to ambient pressure) to allow the second stage seat to be open with no gas flow pushing water out. Consequently, if a diver were at say 50' and ran the tank dry and then descended to 100' and depressed the purge and held it open long enough to flood the first stage, you might get some water in the tank, if you orient it so gravity moves it there. It's not a very likely scenario and I'd have problems making it happen even if I tried.
Another slightly more likely cause of first stage and valve flooding is to shut a valve off and purge a reg (for example, a stage bottle left on the bottom and accidentally bumped/purged) and then accidentally unscrew the DIN fitting enough to flood the first stage and valve. In this case, you could have a water upstream of the first stage seat, that could migrate into the tank if the reg were not flowing gas, after the reg is re-tightened and the valve is turned on. However, this can happen at any tank pressure, so it has no bearing on a 100 psi tank argument.
---------- Post Merged at 08:57 AM ---------- Previous Post was at 08:40 AM ----------
I was running late today, so I'll try tomorrow to get a look inside. The kid (the one with the scientific explanation, above) said the whole inside was coated.
If it always happens and it's legitimate, then it's bad (overly wet) gas.
The cynic in my however suspects it's either an overly picky inspector who can't tell the difference between flash rust and a problem, or it's just a revenue source for the shop.
Flash rust is pretty normal, and it is not a problem unless it reaches a point where it obscures the tank underneath, and or starts to for spots and clumps that both hold moisture and obscure the tank underneath. That's where rust pits get their start and if that's happening it needs to be removed. But if it is a light more or less even coat of flash rust, it's a non issue.
That said I am pretty anal about my tanks and use steel LP 45s for O2 tanks, so I like flash rust free tanks. I also tumble, clean and inspect my own tanks, so any flash rust gets removed if any is found - but generally with quality gas fills, I'll go 3-4 years before I find any flash rust.
