Scuba police

[boat sju]

A tank and regulator emptied at depth would function like your bc and it's exhaust hose. You'd have to hold the regulator (while purging) higher than the tank to allow air to escape and water to enter the tank. So while not really "gravity" I see awap's point on this one.

 

[MauiScubaSteve]

No diving AFTER flying? Not seeing the logic in that one.

That's the point - zero logic applied. But in the melange of mish mash quasi regulations who knows where logic starts and stops.

I believe if you look at the statistics the vast majority of accidents/incidents happen on the first dives of a destination trip, and when you break it down to dives the day of landing the risk to john q public diver is the highest. Resort Insurers and lawyers have probably caused this policy, but for john q public divers it is in their best interest too.

 

[Doc Harry]

A tank and regulator emptied at depth would function like your bc and it's exhaust hose. You'd have to hold the regulator (while purging) higher than the tank to allow air to escape and water to enter the tank. So while not really "gravity" I see awap's point on this one.

Nope. Positioning of the hose and the cylinder is irrelevant. Air doesn't need to escape for water to flow in (remember, air is compressible). It's not gravity as you describe, it's hydrostatic force that allows water to enter the cylinder.

 

[MauiScubaSteve]

At 5 ATA the psi is 73.5 so if you could suck the tank down under 70 psi you could have the tank upside down and the 2nd stage stretched to its lowest possible depth and still take on water by pushing the purge, right?

 

[knotical]

After you've drained the cylinder and pressed the purge, even a small movement of your system deeper in the water column could move water into your cylinder.

 

[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.....

 

[MXGratefulDiver]

If you think the scuba police are bad, check out what the golf police are up to ...

Canadian judge rules Happy Gilmore golf shot illegal | csmonitor.com

Those zany Canadians ... I have heard that they won't let you dive in Quebec unless you're certified there, but now you can get arrested for using the wrong golf swing!

... Bob (Grateful Diver)

 

[boat sju]

Nope. Positioning of the hose and the cylinder is irrelevant. Air doesn't need to escape for water to flow in (remember, air is compressible). It's not gravity as you describe, it's hydrostatic force that allows water to enter the cylinder.

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.

 

[PRL]

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/ Empty tanks. I once talked to a small dive boat operator about emptys, and his reasoning in his case was valid imho. After pulling back to shore he would throw all tanks into the water for pickup, and than tie off the boat some distance away. his reasoning was that if the tank was empty in the hot sun, and than cooled in cool water, there was a possibility that water would get sucked in. Yeah, we can argue that its almost impossible. 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 . And his reasoning was that 1 out of a hundred tanks was to much, and he would have to inspect every empty tank, since it was impossible to keep the tank dry when unloading the tanks.
Another thing to consider that if a valve is properly tightened, than the valve oring has very little force on it when tank is empty, and creates another entry point for water when the tank is cooled

 

[redacted]

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.

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.