Rock bottom, 500 PSI, or something else?

[2airishuman]

We are talking about recreational divers in this thread, and I am going to make some assumptions for this when I am diving under those circumstances. First, I will assume it is not going to be me going OOA. Next, it is not going to be any of my usual buddies. The only way we are going to have a problem is a gear malfunction, such as a burst regulator hose, which gives you time to act before all is gone.

I used to think like that.

There have been, in the relatively short time I have been participating here, three reports of abrupt, complete losses of gas delivery from a 1st stage that could not be resolved at depth.

One was due to blockage from corrosion. One was due to delamination of the HP seat. Another was thought to be related to incorrect assembly of the ACD components of a regulator during service, but later analysis and assessment did not support this conclusion; in any case, the failure was abrupt and complete at depth.

In addition there was one report of a diaphragm coming dislodged from a 1st stage at depth that resulted in loss of all gas before an ascent could be completed, requiring using of a pony cylinder. (The depth and personalities involved in the incident do not change the reality that all gas was lost very quickly, nor are they in any way related to the root cause)

The vast majority of OOA incidents are caused by sloppy diving. That should not blind us to the small but significant number of cases that could affect any of us, no matter where we may be on the skill continuum.

 

[johndiver999]

If we start to discuss the importance or validity of various assumptions....

Why assume a safety stop will be performed? If the victim (or the donor) is really freaking out, it is unlikely that they will both manage to control buoyancy at a shallow depth and actually complete the stop.

Why assume an ascent rate of 30 fpm? If the divers are very stressed, chances are the ascent is going to be pretty fast.

Why assume 200 psi is not usable? If you think your Gage is accurate, then it probably is and 200 psi is actually a good bit of usable air - even if it starts to be delivered a little slow.

Why assume that it will take a bunch of time to start the ascent or swim back to an anchor line? In the very unlikely event that you will have to share air MAYBE it is reasonable to assume that the divers will just go up. I mean, if I had a panicky diver all over me, I would try to get a regulator in their mouth and the next instant, I would be pressing the inflator button on a BC. A panicky diver is going to feel a lot better going up rather than trying to swim around on the bottom or exchange a bunch of hand signals. It is not going to take me two minutes to start going up.

If the situation is totally controlled, and both divers are extremely calm and skilled, well then they are not going to be sucking air like crazy and they will not consume so much gas, so a very slow and controlled ascent will be possible. If they show up at 20 feet with 4 or 500 psi, then I would expect them to perform a safety stop. So I'm not saying that the safety stop should be skipped or the ascent rate should be made fast, but these are less important than actually reaching the surface before the air runs out. Getting to the surface as fast as is reasonably possible is going to be the overwhelming objective for most victims in a scary situation.

When you string together a series of independent (to some extent) conservative assumptions, the end result is a calculated reserve pressure that few people will follow. Possibly a more reasonable result is attained by stringing together some more aggressive assumptions, but as I mentioned before, I am going to assume the victim will be sucking air like crazy.

I know I would not be happy if a prospective dive buddy said that I had to leave the bottom (on a 60 ft dive) with 1,100 psi or whatever is calculated by some techniques.

 

[seeker242]

However, these numbers do not agree with the calculations of others. For instance, NWGrateful Diver's has a very nice page with a similar table, but his RockBottom for an AL80 at 60 feet is 1902PSI,

Because he uses a sac rate of 2.0 in that chart, that's A LOT! Although, he does say to find out your own working consumption rate and using that, whatever it might be, would be appropriate.

 

[tbone1004]

As I see it, the problem we have is that some authorities are using assumptions which, taken as a whole, are so conservative that they result in rock bottom PSIs that are at odds with actual, reasonable, prudent practice.

A "OOA diver" SAC of 1.0 may be reasonable.
A "stressed assisting diver" SAC of 1.0 could perhaps be reasonable.
A 30 FPM ascent and 3 minute safety stop may be reasonable.
A 30 second problem determination time at depth may be reasonable.
A one minute surface reserve may be reasonable.
A 200 PSI unusable gas allowance, may also be reasonable.

Assuming all these things together, however, is not reasonable, and it leads to calculations that can't be taken seriously, and so we end up back to rules of thumb. The same logic spills over into the sizing discussions for pony cylinders.

I think that what is needed is some leadership on what values are actually reasonable based on behavior in actual emergencies.

From 70ft for easy math.
2cfm combined
30sec at depth is 3 cubic ft.
Average of ascent is 2 min*2ata*2cfm=8cf
safety stop is 2*1.5*3=9cf
That's 800psi. Add 200psi of regulator buffer, and 1000psi. 70ft by Lamonts "rule of thumb" is 1000psi on an AL80. To me the 200psi is the surface minute *200psi in an AL80 is 5cf, so that's a 2 ish min reserve up there*

so yeah, I think assuming all of them together is reasonable.

Big wreck? increase the bottom time so you can travel around to make a safer ascent. If you only get 2 mins from the safety stop? oh well, etc

 

[Bob DBF]

Good questions, and the assumptions can affect your reserve a lot.

Knowing ones SAC when stressed is probably the most handy bit of information, and having a realistic measure of ones SAC is even better. I do my calculations for an entire tank rather than picking a specific segment of time diving, it isn't flattering but it is realistic.

Why assume a safety stop will be performed? If the victim (or the donor) is really freaking out, it is unlikely that they will both manage to control buoyancy at a shallow depth and actually complete the stop.

I would assume a safety stop would be made. If everything was back under control, it can be used to moderate a fast ascent, and it gives one fudge factor rather than many. In addition, I can check for boat traffic before breaking the surface.

Why assume 200 psi is not usable? If you think your Gage is accurate, then it probably is and 200 psi is actually a good bit of usable air - even if it starts to be delivered a little slow.

I wouldn't bother, I'll cut short the safety stop if I empty the tank.

Why assume that it will take a bunch of time to start the ascent or swim back to an anchor line? In the very unlikely event that you will have to share air MAYBE it is reasonable to assume that the divers will just go up.

This would depend on your dive plan, normally I would give half a minute to sort things out and go. However, if you are diving around hazards, fishing line ect., or have to get back to an anchor line, it's better to give a reasonable amount of time to account for loitering at depth.

Going straight up, in some cases, is a last resort because of conditions, ripping current on the way up putting you out of visual range by the time a marker is deployed, as an example. Don't start another problem when you already have one.



Bob

 

[DanielRJones]

40bar reserve
25L SAC for each diver
10m/min + 5min stop

Gives me 110bar on a single 12 at 30m

As i am going in with 240 bar gives me 1560L to breath

 

[boat sju]

I agree it's a great way to do a multi-level dive, but on wrecks, unless you are happy to do a touch-and-go on the wreck with that Al 80 you may need to carry more gas than many of your fellow divers who use the "back on the boat with at least 500 psi" strategy.

I understand what you're saying but that discussion might lead to another 10 pages.