How much air to surface with?

[scagrotto]

You might want to take another look. All of my 1st stages are piston regulators. That heavy spring you refer to holds the valve (seat/cone) open until pressure reaches IP and the valve closes. And diaphragm regs work the same way.

Are you really a professional regulator technician?

Thanks for covering that, because I was getting very confused about how water will get into my empty tank that actually has 140 psi and a first stage that's firmly closed.

Of course I'm still puzzled about what's going to open the 2nd stage to let water in, since (as I understand it) that spring does hold the valve closed until inhalation reduces pressure.

As a practical matter, has anyone actually had a DM check their SPG?

I've never paid really close attention, but I'm pretty sure I've seen DM's take a quick glance at consoles a few times. I've never had any say anything to me, and I've probably come up with less than 500 now and then.

As far as air delivery at low tank pressures, on at least one occasion I breathed a tank down far enough that I couldn't really get another breath. I was laying in the sand at the time, in about 3 to 4' of water, and figured I didn't need to stop what I was doing any sooner than absolutely necessary. As I recall, the transition from easy to breathe to difficult to breathe was fairly abrupt. I knew I was very close to OOA, but I really don't remember getting a lot of warning from WOB.

 

[Iralub]

As a practical matter, has anyone actually had a DM check their SPG? Do such Boat Nazis really exist? I have done only a few hundred dives, all recreational, but in a good sampling of places in the world, and I have probably been asked to "return to the boat with 500 psi" on a fair percentage of those dives, and I have never once noticed a DM check anyone's gauge. They may have, but I sure have not noticed it. They work for tips, so I would imagine that even if someone were to return with less than 500 psi they would not scold but rather more likely would just make a gentle remark. My wife often is back on the boat with less than 500 psi in an Al 80, and we're working on solutions to that, but no DM has ever admonished her. I once heard a diver brag openly how he sucked his tank down to 50 psi, though I don't recall whether we had been given the "500 psi" instruction on that trip.

Is this really an issue? Divers know it's not a good practice, and even if they were to do it they know they would not get banished for life. And for a boat full of tech divers, it's totally a non-issue.

I guess it was intended as a theoretical, not practical question.

I have on several dive trips. The usual rule has been "come back with 50 bar", although one op was ok with 30 bar and one with anything above where your first stage can be removed from the tank without shutting the tank off

It may be because I tend to chose dive ops that are anal about safety, but frequently dms have either checked the gauge or asked the remaining pressure.

Never had a problem with that as any time I've gone below 50 bar it was with full awareness and knowledge of the DM.

I also found that rules are often relaxed once the dive op has a better idea of your air use and attention to air and also conditions you're driving in (eg. A dive op otherwise very strict about safety rules was fine with me coming back with 20 bar after a >1 hour dive where I was on an 8l tank, everyone else on 12l, with last 10 min spent at depth of less than 5m, so I could have easily popped up to the surface and where I communicated the pressure to the DM and indicated I was fine.)

 

[dbulmer]

I didn't see in your post anything about recalculating rock bottom......most dives aren't squares but multi level, you just recalculate your rock bottom.

The real benefit to rock bottom is that if you are on your ascent with your couple of hundred PSI for you and suddenly someone signals out of air you don't have to make a "his life/my life" decision....


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Disagree with this post specifically rock bottom.

The real benefit to rock bottom is that it is a protocol which binds together 2 or more divers into accepting that **** can happen at any time underwater and that it is better to have more breathing gas than you need than worry about not having enough.

 

[halocline]

Of course I'm still puzzled about what's going to open the 2nd stage to let water in, since (as I understand it) that spring does hold the valve closed until inhalation reduces pressure.


As far as air delivery at low tank pressures, on at least one occasion I breathed a tank down far enough that I couldn't really get another breath... As I recall, the transition from easy to breathe to difficult to breathe was fairly abrupt. I knew I was very close to OOA, but I really don't remember getting a lot of warning from WOB.

The 2nd stage valve is opened mechanically by a lever that sits inside the stage against the diaphragm. When you inhale, you effectively lower the pressure inside the case, which allows ambient pressure (water under you're submerged, air if you're on land) to collapse the diaphragm, which pushes the lever, which opens the valve. There is a spring that holds the valve closed, but on balanced seconds this is a very light spring, aided by air pressure that is diverted from the 1st stage.

So water will only get in your tank if you press the purge, or were to take the completely empty tank deep enough so that the ambient pressure exceeded the pressure in the tank.

As far as the abrupt transition from relatively easy breathing to nothing, that's a function of balanced 1st and 2nd stages. Once the tank pressure gets below IP, the first stage stays open, but with a balanced 1st stage IP will stay pretty much steady right up until that point. This is relevant because if the IP drops, WOB goes up. The balanced 2nd will be more tolerant of an IP drop (less of a WOB increase) because the spring pressure holding it shut is mostly air pressure from the 1st stage. (aka IP) When the tank pressure drops below IP, so does the air pressure holding the 2nd stage shut.

With an unbalanced piston 1st stage and an unbalanced 2nd stage, there's a more gradual increase of WOB. Most divers would start to notice it at around 250-300 PSI, and it would get increasingly difficult until the tank is empty. But it could still feel fairly abrupt when there's no longer enough air to actually fill your lungs.

 

[Jax]

Such a lot of debate over what is really a simple answer - regardless of tank size.

Recreational divers are to be back on the boat with 500 psi because that is the only thing the boat can easily measure!

 

[Lorenzoid]

I have on several dive trips. The usual rule has been "come back with 50 bar", although one op was ok with 30 bar and one with anything above where your first stage can be removed from the tank without shutting the tank off

. . .

If those trips were in Australia, I wouldn't be surprised if they gave you a full physical exam after returning to the boat.

 

[Rusty Roo]

50 bar ideally, 40 not an issue.

can exceed to 20 bar if cool shark sitting right underneath the boat

can go to 5 bar if doing your dm skill set and just need to get that last person done.

 

[geoff w]

Forget about the nuances of different tank sizes and pressures, and balanced vs. unbalanced regulators. If I was a dive operator and providing the tanks, as typical for vacation divers everywhere, I would be unhappy if any diver ran out of air and got water into a tank. At best, that would mean the diver makes an emergency ascent and the dive operator has to visually inspect the tank. Worst case scenario is there's water intrusion in the tank because it's empty (water pressure higher than tank pressure) and the tank requires service (possibly elsewhere in some remote parts of the world) and/or a vacation diver needs rescuing. There doesn't seem to be anything magical about 500 psi minimum except it's something bozo vacation divers can remember if DMs the world over drum it in to their heads. When I lead a dive and give instructions, I usually try to re-enforce what divers should have heard over and over so they don't have to think, then still periodically have them check their SPGs

Personally, I don't have much confidence in my SPG when it's down to the last few hundred PSI. Not that I ever breathe the tank that low on a dive, but when I'm purging the tank the needle doesn't move smoothly those last few PSI until hoses are purged so I can take off my first stage

 

[redacted]

Forget about the nuances of different tank sizes and pressures, and balanced vs. unbalanced regulators. If I was a dive operator and providing the tanks, as typical for vacation divers everywhere, I would be unhappy if any diver ran out of air and got water into a tank. At best, that would mean the diver makes an emergency ascent and the dive operator has to visually inspect the tank. Worst case scenario is there's water intrusion in the tank because it's empty (water pressure higher than tank pressure) and the tank requires service (possibly elsewhere in some remote parts of the world) and/or a vacation diver needs rescuing. There doesn't seem to be anything magical about 500 psi minimum except it's something bozo vacation divers can remember if DMs the world over drum it in to their heads. When I lead a dive and give instructions, I usually try to re-enforce what divers should have heard over and over so they don't have to think, then still periodically have them check their SPGs

Personally, I don't have much confidence in my SPG when it's down to the last few hundred PSI. Not that I ever breathe the tank that low on a dive, but when I'm purging the tank the needle doesn't move smoothly those last few PSI until hoses are purged so I can take off my first stage

Wonderful idea. That way nobody really has to think. It is just a lot of wasted effort.

When I have reason to question the accuracy of one of my SPGs, I avoid problems by testing it.

 

[scagrotto]

(water pressure higher than tank pressure)

I can think of others, but that's one reason not to descend again if your tank is empty. If you don't descend after the tank stops delivering air the only way tank pressure will be lower than the water pressure is if you give the tank a really good squeeze and it pops back into shape.