Low pressure tank question

[Basking Ridge Diver]

Ah - so maybe I misunderstood the point or question - on guided dives - yes I have been asked for pressure during the dive so the team knows where everyone is - generally my son and I have low consumption rates and have been allowed to stay together and longer on the bottom than others.

But I have been on boats where the DM does not go in the water and you are responsible to get yourself back on board and there is no DM that is checking gauges back on the boat. Never on the boat have I been asked to show how much air I returned with... It is upon you to be careful with your time and your gas. Perhaps if I went OOG - someone would check but to date that has never happened to me on a boat.
Shore diving is different in that it is intentional to suck down that last bit of PSI...

 

[eelnoraa]

I agree with Bob too. The real message is basically "don't run out of air".

In practice, return to the boat with xxxpsi is an incorrect broad statement in general. What you should really consider is at what pressure you should start your ascent so that if you buddy has a catastrophic lost of air at the same moment, you have enough air to get both of you to the surface safely. This will depend on your tank size, current depth and SAC, safe ascent strategy. It sounds complicated at the first, but it is actually a very simple calculation.

 

[exposure]

Checking gauge on the boat is useles as you don't need air anymore But it can be useful to spot bad practices.
Asking you to ascent at some level of air is a safety practice that you can master for yourself easily.
Anyway, some french boat have to report air left on a form for all divers. Most of them think that it is just for administrative reaons.......

By the way, I am always amazed by your calculations in cuft

 

[geoff w]

My experience has been - not a single DM or Instructor has ever checked my SPG. They have cursed and looked at my PDC swearing I was some how in Deco when I wasn't - but they could not work my PDC and subsequently left me alone. But never has a single person ever checked my supply of air or gas after a dive....

So maybe they work differently on the West Coast or the islands? Have you ever had to show your SPG to a DM?

I have head a DM say to come back with 500#, but I've yet to see one check. I believe it is shorthand for "I know you are excited and want to have a good time but for Christ sake watch your F'n air"
Bob

^^^ What he said. I never said DMs check SPGs, and I don't think anyone else did. I've never had one check my computer either, in So Cal or anywhere else. They're just doing their job reminding divers to be responsible. On the flip side, I've heard of Course Directors running out of air, so it's not just for inexperienced divers

Back to the OP's original question, if a DM recommends to divers on the boat they return with 750 PSI, and most divers are diving Al80s (typically 3000 PSI fill pressure), they're suggesting coming back to the boat with 1/4 tank reserve. So if you're diving a low pressure 2400 PSI tank, 600 PSI will get you back to the boat with a quarter tank of air (or whatever gas you're breathing) in reserve. I've never seen them differentiate based on tank fill pressure. If you were diving with a high pressure tank filled to 3442 PSI, 750 PSI would be less than 1/5 tank full

If you're lucky and the DM is really thinking of you, they're also telling you not to come back with your tank completely empty to make sure you don't suck it dry and allow water to enter, which is generally bad for tanks

 

[boulderjohn]

Now, that said, you need to really look at rock bottom calculations to make sure 20cf is enough, but that's another discussion.

Rock Bottom is one way to calculate a reserve. It calculates how much gas you should have when you begin your ascent so that you and your buddy can ascend safely in a worst case scenario. That is not, however, what the OP is being asked to do. He is being asked to be back on the boat with that amount of gas, which is a different way to calculate a reserve. I would say that in all diving, having 20 cubic feet of gas after you are back on the boat is more than enough gas to be safe.

If you are using a system that says to be back on the boat with that much gas, then that system has in theory calculated enough for you to ascend safely with your buddy. The problem is that it does not give you any guidance as to when you need to begin your ascent in order to get on the boat with that much gas. To do that, you have to use a system similar to a rock bottom calculation, but if you use pure rock bottom along with the "be back on the boat with XXX PSI" system, you are really mixing two systems and will be beginning your ascent pretty darn early.

 

[BRT]

One other reason the divemasters want you back on the boat with a reserve is so nobody breaths them out of air. We dove with a divemaster that finally signaled he was OOA and going up. I swam over to offer him air, which he declined, but his gauge read zero. I hadn't ever seen an SPG reading zero under water before. We still had 1000 lb. I'm sure he thought if he just hung on we would get low and go up and no one would know he was low. He couldn't fake it anymore when the tank was dry.

 

[WetAndHappy]

This gets worse when you dive LP tanks with gauges and computer reporting metric (bar) instead of PSI. I had a DM continually send me back to the boat at 70 bar (on LP tanks) because 70 is a smaller number than 500. I took it as an opportunity to get him familiar with when to close out a dive in bar. My information was dismissed. On subsequent dives, I switched my computer to PSI and, and per BRT's comment, had him call the dive because he was out of air. Heck, the needle on my SPG at 70bar is on/near the interface between the yellow and green arcs.

DMs should not be embarrassed to have their air deplete earlier than others, especially if I see them tending to an inexperienced diver.

If it's more safe than less safe I almost always defer to / respect the call of a DM, as I am not a DM. Working in safety roles myself, I never tolerate people attempting to override the standards I use, even if they are not in line with accepted standards. (I always operate on the safe side of safe.) If a DM is creating a less safe condition by pushing a standard, they should be challenged. For example. a different DM was turning my valve back a quarter turn from full open just prior to entry. I buddied up with someone (a fellow working as a safety diver in Antarctica) who knew better and we ensured each other's valves were wide open, then kept a close eye on the other divers.

Sorry for rambling, lots of thoughts on this subject...

 

[Nemrod]

Have you ever had a captain or assistant actually check your pressure when you got back? I haven't.

Actually yes, and every time in the Keys and a few times, though not as stringently in Cozumel. They did a gauge check and wrote them down and then checked them coming out. Nanny diving to the max. One was on a Vandie dive and the entire group, guided (ughhh) were on 100 cf tanks but for me on an aluminum 80. The dive turned and I was back on the boat still with 1500 psi :/ . They were all, including the guide, at around 500, on the deck. N

 

[NAM001]

When I go on flower gardens trips. the boat issues a diver # tag and you attach it to you rig. when you get on the boat they record bottom time, max depth and tank pressure. If you are coming up with 1500 psi at end of dive for several dives and you later do not come up with the majority of the group they have an idea if you have likely gone ooa or perhaps just decided to max out the time on the last dive. Most of the time I have been told 500#. As I dive lp95's also I translate to 400# and nothing is ever said. Of coFace iturse this takes no consideration for a spg error. Face it if you are diving an lp120 and you berath it down to 20 cu ft that is 450 psi and if you have a 10% gage error then you may only have 100 psi. Most of the time I hear 700 or 1000 psi it is 500-700 psi pluss gage error margin of safety. that same 20 cuft is around 700-800 on an al80 (77).

Ah - so maybe I misunderstood the point or question - on guided dives - yes I have been asked for pressure during the dive so the team knows where everyone is - generally my son and I have low consumption rates and have been allowed to stay together and longer on the bottom than others.

But I have been on boats where the DM does not go in the water and you are responsible to get yourself back on board and there is no DM that is checking gauges back on the boat. Never on the boat have I been asked to show how much air I returned with... It is upon you to be careful with your time and your gas. Perhaps if I went OOG - someone would check but to date that has never happened to me on a boat.
Shore diving is different in that it is intentional to suck down that last bit of PSI...

 

[Dive Right In Scuba 2]

Back in my DM'ing days we would tell divers to be back on board with "XXX" psi(actual number dependent on the operation). While I wasn't one to "nipple twist" and read gauges of divers coming up the ladder I would often glance at some of the divers gauges who piqued my interest(either through their lack of experience or cavalier attitude towards safety). If I noted one well into the red, I would pull them aside and nicely readvise them of the rules.

As far as differing cylinder sizes, I understand and made the allowance in the difference between "XXX" psi in an AL80 and "XXX" psi in say...an LP 121.