Rock bottom, 500 PSI, or something else?

[boat sju]

Kevrumbo, thanks for reposting Lamont's work. I was just using his rule of thumb. I said 400 psi because, if I recall correctly, the math works out for an 80 at 100ft that way.

 

[rsingler]

anyone know of a recreational course taught by one of the large agencies where calculating your sac rate is actually required?

SSI includes gas consumption calculations as part of one of their slides in the OW material.
Additionally, calculating SAC with formulas is in their Open Water student digital materials.
No exam questions on SAC, however.

They also include a 15min at 15' swim to actually derive an initial SAC as an optional part of Open Water Dive #4, and include a SAC formula in the Instructor's Manual.

 

[LordElrond]

I use normal Gue minimum gas rules. Maybe I'm coming up with a bit more gas, but I prefer having it and not needing it than the other way around.

 

[boat sju]

That’s fine, but results in kinda silly reserve volumes in big tanks and maybe small volumes in small tanks.

Yeah, but remember we're talking rec dives, so think of a multilevel reef dive in Bonaire. The rock bottom numbers I'm talking about are pressures when you need to go shallower - not necessarily to surface. It's just a way to tell if you're deeper than you should be for the amount of air you have left. And, the calculations were based on an emergency situation where two divers are stressed, hopefully ascending at 60 fpm, and a safety stop may not be in the plan any more.

 

[Rred]

Just how long is a piece of string anyway?

tbone, some of us still dive j-valves. In fact, when I had my pair overhauled a couple of years ago I was told that I had just consumed two of the last three spares in existence, and that leaves only those pricey new USN/NOAA spec J valves from XS SCUBA if I want to buy new replacements one day.

I always figured that I didn't want to trip the J, so surfacing with 500# didn't bother me. Note that I say SURFACE not "exit the water". If I can surface with 500# or even 300#, that's still a reserve and I'm no longer under water. If I'm 75 yards from the boat, or the beach, and I chose to drop two feet under the surface (I hate the splashly part) and happen to burn it down to 150# before I've exited, so what? I can drop the entire rig and surface swim or snorkel as I choose. It is no one's business or concern what is in my tank when I "exit".

And my long-term buddy felt the same way about that.

How much do I plan to have when surfacing? Depends more on what I'm doing and where I'm doing it, than any arbitrary numbers. Lobstering or spearfishing or flatfishing in 20-40' of open beach or jetty? Who cares, there's no real way we're getting "stuck" and in need of 500# at any point.

Doing a wreck dive with a planned deco stop...quite another thing. I'm going to be planning to have enough air to work around any gear failure, and to accommodate a longer deco stop, or a potential longer stop to clear a reverse squeeze. or other issues. But to pick an arbitrary number?

Right, a piece of string is this long. Strictly a personal matter between buddies.

 

[Lorenzoid]

Yeah, but remember we're talking rec dives, so think of a multilevel reef dive in Bonaire. The rock bottom numbers I'm talking about are pressures when you need to go shallower - not necessarily to surface. It's just a way to tell if you're deeper than you should be for the amount of air you have left. And, the calculations were based on an emergency situation where two divers are stressed, hopefully ascending at 60 fpm, and a safety stop may not be in the plan any more.

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.

 

[johndiver999]

Just how long is a piece of string anyway?

tbone, some of us still dive j-valves. In fact, when I had my pair overhauled a couple of years ago I was told that I had just consumed two of the last three spares in existence, and that leaves only those pricey new USN/NOAA spec J valves from XS SCUBA if I want to buy new replacements one day.

I always figured that I didn't want to trip the J, so surfacing with 500# didn't bother me. Note that I say SURFACE not "exit the water". If I can surface with 500# or even 300#, that's still a reserve and I'm no longer under water. If I'm 75 yards from the boat, or the beach, and I chose to drop two feet under the surface (I hate the splashly part) and happen to burn it down to 150# before I've exited, so what? I can drop the entire rig and surface swim or snorkel as I choose. It is no one's business or concern what is in my tank when I "exit".

And my long-term buddy felt the same way about that.

How much do I plan to have when surfacing? Depends more on what I'm doing and where I'm doing it, than any arbitrary numbers. Lobstering or spearfishing or flatfishing in 20-40' of open beach or jetty? Who cares, there's no real way we're getting "stuck" and in need of 500# at any point.

Doing a wreck dive with a planned deco stop...quite another thing. I'm going to be planning to have enough air to work around any gear failure, and to accommodate a longer deco stop, or a potential longer stop to clear a reverse squeeze. or other issues. But to pick an arbitrary number?

Right, a piece of string is this long. Strictly a personal matter between buddies.

I don’t think some of those numbers are arbitrary at all. They are based up well defined assumptions and the results are
Entirely defensible.

I personally think some of the numbers thrown around are a
Little over conservative, but that is due to the assumptions used.

If you want to justify lower reserves, then do it by modifying the underlying assumptions . The math is correct.

Also when making assumptions, it seems risky to assume a consumption rate that is anything but “high” for the uncontrolled element- which is the potentially panicked buddy.

 

[boulderjohn]

Also when making assumptions, it seems risky to assume a consumption rate that is anything but “high” for the uncontrolled element- which is the potentially panicked buddy.

It would be interesting to somehow do a study that would give a reasonable expectation of what would happen in an OOA sharing situation.

I have only been in the vicinity of one OOA episode in my life. The OOA divre apparently put her gear on a used tank and did not check the gas supply. She went OOA very early in a 100-foot dive, took her buddy's alternate without signalling, and was then very calm and able to participate in an orderly ascent. Both she and her buddy were fairly new divers, with no advanced training. After that incident, I surveyed the other instructors where I worked, and among that lot, only a couple had been near OOA experiences, and they all reported something along those lines.

On the other hand, we used to have a prolific poster on ScubaBoard, an instructor/DM working with a company somewhere in the Micronesia area. (He or she was very sparse with personal details.) According to his or her posts, someone went OOA on nearly every dive he or she led, and it was always a panicked, terrifying, death-defying horror show. Now, if I were that person's employer, I would have fired him or her long ago, so I assume there was some degree of exaggeration in those descriptions. The joint PADI/DAN study on dive incident from a few years suggests that the truly panicked OOA diver is more likely to bolt to the surface rather than attempt to share air.

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. No, if I have to share air with someone during a recreational dive, it will be with someone I don't know, either an instabuddy or someone who happens to be randomly near me during a dive. I have no way to predict that composure of that individual, so I am going to assume a worse case situation, which I think will be something between the two examples I gave above.

In that case, I think the 0.75 SAC rate for an air share is giving to much credit to the composure of OOA diver. If that diver went OOA at depth during a typical recreational dive, then that diver was probably breathing at that rate or more before the OOA incident. My composure will hopefully help compensate for that, but a heavy breather before OOA will be a heavier breather after OOA. I think the 1.0 SAC rate used by most people is better, both for that reason and because it makes for easier calculations.

As for ascent rates, the 10 FPM rate baffles me. As I said earlier, the only study on ascent rates that I know of that includes 10 FPM as an option (the Maronni study) concluded it was the worst of the 3 that were compared--worse even than the 60 FPM ascent rate. I have asked for the scientific basis for this decision, but the only explanation I have been given so far is that it is a training exercise for people planning to go into technical diving some day. Since I don't think bringing an OOA diver to the surface is the best time to be teaching that diver technical diving skills, I will not use that number. I use 30 FPM for my diving, and I will plan for that. In an actual event, I will judge the situation and act accordingly. I will not be opposed to a 60 FPM ascent if it seems prudent.

 

[topher10]

I wanted to say that as a novice this discussion has been helpful. I have read about RockBottom several times previously, but a table like in the posts from Lamont is what I need because I am unlikely to do the math on the boat each time, and my specific depth is not always known beforehand. For the foreseeable future I will be diving AL80s between 40 and 80 feet of water, and likely doing out and backs with the occasional 'circle the wreck'. Most dives I pass by the upline more than once.

I find the table generated from his formula interesting for few reasons. First, it reflects what is practically done on most boats; i.e, be at the upline at 1000psi and on the boat with 500psi. For depths between 50 and 80 feet that is pretty close. It is also nice because the ascend (rock bottom) pressure changes by 100psi for every 10 feet, with 60 ft = 1000 psi. So if you just remember that you can calculate up and down easily (20 feet deeper means save 200 more PSI. So I would like to print this table off and put it in my log book for reference:

Depth Turn Ascend (RB)
30 1850 700
60 2000 1000
80 2100 1200
100 2200 1400

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, while the above for Lamont its 1000 PSI. That is a very big difference. I have also never seen someone start their ascent from 60 feet with 1920PSI left.

Therefore, I would like to ask others if they feel this table is excessively risky? For what its worth my calculated sac rate is around 0.5 to 0.6, but there is some inaccuracy and variability there. I assume 1.0 for calculations. I also apologize for not actually doing the math myself, but the point as noted above is that all these people are doing math correctly, but arriving at different numbers.

Thanks

 

[2airishuman]

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.