How much air to surface with?

[Lorenzoid]

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.

 

[halocline]

You don't have to breathe the tank down. Just shut off the valve and breathe. You will know right away how low you can breathe the 1st stage. I'm betting you can breathe it till you can remove the 1st stage from the tank.

In fact that is exactly how you remove a double hose reg from a tank. DH regs have no purge button.

 

[HalcyonDaze]

Well, I don't re-calculate it because I don't use rock bottom in the first place, for NDL diving.

Sure, I get the concept of multi-level allowances, but, on many of my local dives, 80' (which was just used because it was the example offered for rock bottom) is actually often the highest point of the dive and when the ascent begins. Also, I don't want to do math during the dive. I have my PSI to begin my ascent planned (and programmed into my computer), and follow that. I want to enjoy the sights, not try to do mental math fumbling with my slate or wetnotes when moving from 120 to 80', while imperceptibly narc'd, probably dealing with a current, and (hopefully) distracted by the joy of 80 goliath grouper or a dozen sharks.

How do you presume from my example that I begin my ascent from 80' with a "couple hundred" left in the tank, only for me? The example clearly shows that ascending at 700-800 gives 250-300 for me and an additional equal amount for my buddy, with still a 200 potential cushion. Guess what--we are going to make it.

The "bottom" line is that rock bottom cuts way too much into dive time for NDL diving, unnecessarily, in my view, as Tbone's example above shows. I do not plan for dallying on the bottom, for a leisurely ascent, or for any stops with my OOG buddy. But, it will never be "who dies" because I will have enough gas to get us up, as my alternate example above shows. I called rock bottom for rec diving a "crusade" precisely because of the attitude that if you don't do it that way and only that way, you are endangering lives. It is a "you're gonna die" cliche that is just not true.

Yeah, the "one minute to collect everything before leaving the bottom" idea is a bit nuts. If my buddy is OOG - which, unless we're both horribly irresponsible about checking gas throughout the dive, implies a severe equipment failure - I am not spending 60 frigging seconds at 80 feet to get my bearings under those circumstances. My primary is going to be in his mouth in 5-10 seconds and we're going to be on the way up - not "bolting," but ascending calmly. I'd rather spend that minute getting at least 30 feet closer to the surface.

I'm not that familiar with the nuts and bolts of rock bottom gas management, but from this discussion it reminds me a lot of the one time I tagged along with a local GUE group in Seattle for a shore dive, max depth 70 feet. I turn up on a February night with a single AL80 and, aside from a backup second stage, one of everything I need. Everyone else is in loaded down with double tanks and triple-redundant everything else, and they spend a good 5-10 minutes after entering the water checking it all over. Me, I'm freezing my skinny butt off in a wetsuit. The dive lasted about 35 minutes, after which there was more checking of the multiple redundant everythings while I stayed in the water, because at 45 degrees it was warmer than the snow flurries. After that I ran to my car in full gear like a Marine charging ashore and was half out of my suit before they came plodding along and asked why I did that because it was "really bad for bubble formation," never mind the ice crystals forming in my circulation by then.

Long story short, there's different types of divers and different types of diving. I'm a person who dives to see things or do work at depth, not a meatbag following a painstakingly graphed optimum dive profile. I'll keep my open-water drift dives; they can keep the caverns. I'd rather float in the middle of 20 bull sharks with a chum box than stuff myself down a hole underwater. We can each keep the dive techniques that work for what we're doing.

As far as "dive boat nazis," I've had some of those - usually when I go someplace that caters to tourists and I get an instabuddy who hasn't dove in over a year. Then I fully understand being a pain in the ass about getting people back on with 500 psi. As stated I try to stick to that one myself, but for other factors I try to stick to boats that cater to experienced divers and don't get the third degree about how much air I came up with.

 

[RJP]

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.

My experience - also in a good sampling of the world - is that when you comport yourself as a knowledgeable, confident, and competent diver, the vast majority of dive ops, DMs, boat crew etc give you pretty wide berth to do your thing.

DM's etc tend to focus their attention on two groups of divers:

- Those who step on a boat and make it obvious that they know very little
- Those who step on a boat and make it obvious that they know EVERYTHING

I've been on plenty of boats that request "back with 500" and many of them will proactively ask for remaining PSI along with max depth and dive time when you're back on board. I've been under 500 on occasion, and when asked I've always told them what I had left - 400, 350, 250, whatever. No one has ever given me grief, no one has ever scolded me, I've never been told to modify anything I was doing on subsequent dives.

I have to admit it never occurred to me to flail about like a new OW student unable to tame a free-flowing reg on their first pool dive.

 

[TMHeimer]

Some boats I've been on mentioned the 500, others not. Can't recall a DM ever checking gauges.

 

[Zeagle Eagle]

Plenty times the DM has said back with 500. I am mostly back in the boat with 200-300. I like to hit the hang bar with 500.

 

[AdivingBel]

Dived with an OP in Keys as a relatively newly minted diver (20 dives or so) where the captain was the shop owner. The boat had more rules than a catholic girl's school. He would join the dive brief and ask OW exam questions of the divers, most of which tried to ignore him and be about their business. When he didn't get an answer he would berate the divers in general as well as their non-present instuctors. I actually went back for a second trip as it was the closest shop to where I was staying at the time and I thought maybe the first time out was an aberration (hey, I was new and you don't know what you don't know). Turns out it wasn't so I took my c-card (and my cc) and moved on down the road. I didn't see it happen, but if someone had boarded that boat with less than 500 psi I'm sure he would have gone off like a roman candle. Might have been worth it to hang for the show. Other than that I've never had an issue. Some of the DMs/Captains would ask for starting and returning pressure as they checked roll, but no one ever looked at a gauge.

 

[SeaHorse81]

I have to admit it never occurred to me to flail about like a new OW student unable to tame a free-flowing reg on their first pool dive.

Well, now that it has occurred to you, I guess you just have to do the experiment now, don't you?

On a more serious note, my experience is another in the category of them saying 500 psi, but not fussing at all if you come in a little under if you've done nothing to seem like a train wreck. I actually try to hit pretty close to 500 psi except for the last dive of the week (live-aboard trip) at which time I will just do a zen thing on the hang line until about 200 psi and then come up.

 

[redacted]

While I have dove with some ops that say (recommend) 500 psi, I have never dove with any that checked.

 

[HalcyonDaze]

Well, now that it has occurred to you, I guess you just have to do the experiment now, don't you?

Sort of the opposite of this routine ...

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