Scuba police

[Doc Harry]

How much of a pressure delta are you talking about? I assume you are hypothesizing that the diver not only emptied the tank but actually inhaled with enough effort to create this vacuum.

I dunno, I didn't do the test, I'm only informing you of the test results.

 

[AfterDark]

Nope. Consider a diving bell with the open bottom or a lift bag filled with air with an open bottom. But as someone alluded to, if the air in the tank is equalized for depth and you then move it lower in the water column, water will enter.

Originally Posted by Doc Harry View Post
Simple. It's not.

Ambient water pressure must exceed internal cylinder pressure for water to flow into the tank.

The only way to get ambient pressure to equal cylinder pressure is to either remove the valve or hold the purge button long enough for equalization to occur.

Seems to me that's what he's saying, that if ambient pressure is greater than internal tank pressure the air will compress as the water enters, wouldn't even cause bubbles. I was told by my LDS owner that my 72cu tank needed to be rolled because of internal rust only one year after I bought it!. He asked me if I'd had breath the tank down to nothing. I had done just that the previous summer because I got myself entangled on a diving job I had at a beach and sucked the tank and I mean SUCKED that tank dry before my dull dive knife's wet rope cutter freed me only 5' from the surface OA. So I somehow managed to get water in the tank because though I didn't see the water I saw the rust along the side of the tank starting about 3 inches from the bottom to the bottom. The tank was fine after that and I never did that again. The tank just failed it's last hydro this year, 40 years later.

 

[boat sju]

AfterDark,

I wasn't saying it's impossible to get water in your tank, my point is more that it's not easy though. Your example kind of demonstrates that. You probably actually did suck the air out of the tank to the point that the pressure inside was negative. Glad you survived that ordeal.

 

[captain]

Two things might be considered for discussion.

1/ diving after flying. There was limited literature about this, and the only reasoning I remember was the fact that after long flights body is extremely dehydrated. IMHO a valid point, but still only a reminder to hydrate after flying would do.

2/ . But tank valves were designed to keep air in, not water out. a standard valve has no where near the same holding power the other way .

Then explain why I can hook up a fill whip to an empty tank with the tank valve closed, pressure up the whip to 3000 psi and no air will go into the tank until I open the tank valve. If the valve can keep 3000 psi in it can keep 3000 psi out, air or water or what ever.

 

[redacted]

I dunno, I didn't do the test, I'm only informing you of the test results.

Playing around with a vacuum gauge, it seems to me that a person inhaling on an empty tank will produce about 1 psi of vacuum. Not very much. And that much vacuum is gone when the diver ascends just a little over 2 ft. So he would have to be pretty fast on that purge button.

If a person sets out to get water in a tank by breathing a vacuum into the tank and then purging, it can be done, easiest in very shallow water. The actions of an OOA diver are not a likely cause. The actions of a tester attempting to get water in the tank should be able to achieve that end with just a little effort if he understands the physics.

 

[captain]

The most common way water gets in a tank is being blown in when filling. It's not as common as in the past because 1, today's compressors are generally equipped with better water separators and driers than in the pass and 2, most shops fill dry rather than in a water tank. It can still happen if the tank valve is exposed to water and the valve not cracked open to blow it clear before filling.

 

[Papa_Bear]

The University of Rhode Island performs numerous scuba-related studies. One of their studies performed back in the 1970s was done to see if water could enter a scuba cylinder. The took scuba gear down to 10 feet in a swimming pool and breathed the tanks down to zero. Then the depressed the purge button on the 2nd stage regulator 10 times. Later, when they took off the valve, they found cylinder was flooded with pool water.

Cichy, Francis, Hilbert Schenk, and John J. McAniff. Corrosion of Steel and Aluminum Scuba Tanks, University of Rhode Island Technical Report 62, 1978.

I would have to see the study! If they breathed or emptied it on the surface and then went to 10ft it might happen if you held the purge open! Next in the Seventies most of the Regs were "Unbalanced" and would not draw below 150psi by any human I know. Next the Reg would have to be above the to of the tank as well or no displacement would a cure as in the "Hooka" principle! So I would say the tank was emptied on the surface and then take to pressure! Not something that would happen in real life! So I still say it is BS for a shop to use that excuse! I have never seen water in a tank from that in 32 years! I ask my buddy Tony Wiley as he has never seen it as well and he has seen thousands of tanks.....

 

[Rhone Man]

Reverting back to the OP just for a minute, I was thinking about this last night. For all the talk about the dreaded scuba police, to my mind they come in two garden varieties.

- Dive operators who insist on highly stringent and over cautious safety rules. Realistically, I can understand this. They must see a lot of really cr@p divers, so to their minds it makes sense to treat everyone as cr@p. You might know you are a really good diver, but they don't, and they don't want an unpleasant surprise that involves either a lot of paperwork or scrubbing blood off their rental gear. I can't say I would be much different if I was taking someone diving on my boat and I didn't have any idea how good they were.

- Internet experts on what is safe and what is not. You get internet experts on everything, and diving is no exception. Spend too long on SB and you would assume you would drop dead instantly if you descended below 100 feet on air, or allowed your ppO2 to climb above 1.4 on the bottom, or took a certification class from PADI, or did about 1,000 other things that divers do safely every day. Fortunately, almost nobody cares what internet experts think.

In terms of real scuba police, ie. local authorities who create rules and regulations about diving safety standards (as opposed to hunting game for example), I suspect that is actually quite a rare beast.

Of course I want you to think there are no scuba police because I actually work for the scuba secret service. And we don't want you to know that we are watching you...

 

[Gilldiver]

The Rhode Island study is more then a bit bogus. What they did was to put 500 ml of salt and fresh water water into tanks and then store them at an elevated temp for 90 days or so. Significant corrosion was noted for salt water and much less for fresh.

Now 500 ml is half a liter or just about the same as a pint of beer. I can see getting a few drops in each time if wet filling with poor practices or from a poorly maintained compressor. But just how do you get a pint into a tank? I think you would hear that much sloshing around if nothing else.

I have seen dry suit fill bottles go bad quickly, say over 2 years, but that is from running out of fill at depth and keep pushing the fill letting in water and then just refilling the bottle without checking to see if any water did get in. In a small fill bottle, 20-50 ml of salt water will cause significant damage over time. But this is the result of poor fill/use practices and poor maintenance.

If you drain a tank all the way or if you think water could have got it, drain the tank, take the valve off, and have a look.

 

[drdaddy]

I don't blame the resort guys for testing out their clients.

I was thinking of taking my boat out on the ocean with some diver friends, but I rethought it after a bay dive.

I had a group of 7 divers, some from our local college, who I took out on a trial dive. We loaded our gear and I motored up the bay and anchored in 30 feet of water in a slack/low exchange tide in a little cove. We were looking for crab. Oh my goodness!!! (put your own !@%$#%@ expletive in there).

You wouldn't have believed the bozo stuff people were doing, with one of the worse being a so called "divemaster" girl. After getting in the water and a equipment malfunction, she had to call her dive with a, "oh yeah, I've been having problems with the regulator". Another guy got in without his weights and another forgot to turn on his air. Anyway, being that it was in shallow water, it was hilarious - and eye opening.

Oh, and I decided to let the pros take divers out on their boat. Since this time, on a very limited - by invitation only, I have taken my boat out in the bay crab diving. Most of the time we jetty dive - what a blast!

Speaking of "Scuba Police". We do get looks from the Coast Guard when we get in with a brisk chop or are practicing smb releases. But only once did they ask, "Where's your flag?"*, when I popped up in the middle of the bay by a tourist/whale watching boat (on purpose ).

* Dive flags are not required in Oregon as dive flags in Oregon seem to attract boats to it, but in the case mentioned, I did have one in the distance on the jetty. We put our dive flags out on the jetty so that fishermen know we are down there and hopefully will go elsewhere and not snag us with a cast. For the most part, boaters stay away from the rocks, with a notable exception a couple of months ago where I stood on the jetty screaming at an idiot boater motoring towards us, to get the heck out of there as there were divers below. I did recover a freshly severed line and anchor that day - served him right!