question : water in the tanks ?

[cast]

I read an article in latest SCUBA Diving magzine, which talks about an instructor called TOM had an incident becasue at his previous dive he depleted his tanks and water was siphoned into them.
So the article recommends that alway surface with at least 200-300PSI air in the tanks. I was taught the same. But when I started to think about it, I wonder where this number come from.

200PSI is about 13-14 standard atmosphere. that means even you are at depth of 300+ feet. the tank still has higher pressure than the water. so the water can't get in. If the air is really low, say, close to the pressure of water, I think the pressure can't keep the airway open at the first stage of the regulator. That means you can't suck the air out of it, that means you are out of air, I don't think in this situation, the diver would still go deeper. so my question ist what is the chance that water can actually get in the tanks? when this "siphon" can actually happen?

 

[MyDiveLog]

1. You'd be surprised at how inaccurate pressure gauges can be...certainly 200 to 300 PSI in error. Put a few different ones on the same tanks and compare, or put yours in a pressure pot and you'll see what I mean. I dive with two air integrated computers and they are usually a couple hundres PSI different.

2. Remember that the pressure of the tank seals the o-ring against the yoke, not the hand-cranking of the knob. Less pressure means a less strong seal, especially if your guage is reading a bit high, which makes it more likely that a 'bump' to the valve while submerged will allow the o-ring to get blown out of shape, letting the remaining air escape, letting water in ....

I've seen what a tiny bit of water in a stell tank can do over night, and it's not pretty; the tank had to be tumbled, the dive computer and guages were shot ... certainly not worth the hassle of ending a dive with a couple hundred extra' PSI.

 

[teknitroxdiver]

Yeah, I read that article too. I definitley agree that you should surface with a few hundred pounds, even if you aren't worried about water getting in. It's not going to cost you any dive time, and someday you may be really glad you've got a little air left over when you find out that (1) your pressure gauge was wrong and (2) you've got a nice boquet of mono line around part of you.

 

[pescador775]

Pressure forces the regulator away from the valve face and puts some lateral pressure on the O ring which makes it want to extrude but pressure does not aid in the seal in any important way. An empty tank will not leak through the valve's fascia O ring. However, take care when handling an empty tank as most dive shops do not torque the valve into the tank and the valve is subject to rotation when handled. Check to make sure that the valve on an empty tank is tight. A regulator will breath hard when pressure drops to <100psi. The residual pressure is still enough to prevent water entry although the regulator knob should be screwed down tight at this point. Why no water entry? This has to do with the fact that the actual pressure inside the tank is gauge plus ambient. However, when checking on shore and finding that NO air emits from the tank it is a good idea to unscrew the valve and look inside. Fill the tank or put in enough pressure to ensure that the valve will not accidentally unscrew. A 100 psi will do. If the valve face is wet and there is not enough pressure to blow it clear then the valve must be removed and blown clear, then reinstalled with a new O ring. Otherwise, the compressor will blow droplets into the tank. If a new ring is not available, do not remove and do not open the valve. Close it if opened and use another tank to blow the valve fascia clean. Watch for a flying O ring.

 

[Rick Murchison]

There are essentially three ways water gets into tanks.
(1) By far the most common is when water gets into the tank valve orifice (just a drop or two - salt spray over the deck, etc.) and the person filling the tank just hooks up the whip and starts the fill. Bingo, especially if it's salt water, instant rust freckles in your nice new steel tank, or aluminum oxide pimples in your Luxfer. I have seen this far too often, all over the world.
The solution is simple - always give your tank valve a last momentary blast (by momentarily opening the valve) and a quick wipedown if there's any detectable moisture just before taking it into the fill station. Do it yourself. Don't depend on the fill station.
(2) Water separator failure (or more likely a failure to bleed it often enough) at the fill station. I have never seen this happen but I've heard of it happening. As a user there's not a thing you can do to prevent this except to pick the right fill station.
(3) Last and least likely, it is possible to get water into a tank by breathing it dry and then descending with the valve open. The other things that have to be "lined up" (barring a failure of the integrity of the tank o-rings) for this to get water in your tank are that the purge valve on at least one reg must be opened and the overpressure from descent must be sufficient to force water all the way back through the reg into the tank. This isn't likely but it can happen, so any time a tank is breathed completely dry it's a good idea to inspect the tank and the regulator for water - especially if you see water when you detach the reg from the tank. (Notice that if this happens the reg should be broken down completely and thoroughly cleaned immediately - especially if salt water is involved) I've never seen this one either, and the few cases I've heard about are, shall we say, on a long thread. But it is theoretically possible.
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Last but not least, I reitterate that, as Pescador775 says, "If the valve face is wet and there is not enough pressure to blow it clear then the valve must be removed and blown clear, then reinstalled with a new O ring. Otherwise, the compressor will blow droplets into the tank. If a new ring is not available, do not remove and do not open the valve. Close it if opened and use another tank to blow the valve fascia clean. Watch for a flying O ring."
Rick

 

[mustfajohnson]

2) Water separator failure (or more likely a failure to bleed it often enough) at the fill station. I have never seen this happen but I've heard of it happening. As a user there's not a thing you can do to prevent this except to pick the right fill station.

Rick is correct. A few years ago I worked at a LDS that we found had a couple of inches of water in the cascades. I guess no one inspected them in quite some time. Had to send all the cascade bottles out to be tumbled. That shop is long gone but the regulator rebuilds and tanks that got tumbled are still a bad memory!