diving thirds vs rock bottom

[j2s]

You plan a dive, calculate how much gas that you'll need, then take twice as much. Pony up. Hang bottles. Stage bottles. This isn't tennis, you know.

Thank you Moonglow......There's been a couple times when I've had to 'escort' a 'genetic defect' out when solo diving.....The double + xtra bottle(s) made it easy........

 

[lowviz]

Adobo, your concern points out the real threat of just heading straight out on a dive without thinking about the consequences. I tend to stay closer to the tie-in when conditions are bad, had some really nice dives just harvesting mussels and looking at everything that I usually just swim over.

I never saw the need to put distance between me and the tie-in unless I was looking for something in particular on a given wreck. The tie-in is a good hour offshore, another 500' away is just another place, it usually isn't any better...

 

[Adobo]

Two recreational divers go on this boat and are given those instructions. They descend the anchor line and proceed on their dive. Just when they are about to turn the dive (meaning, they are at their furthest point away from the anchor line), one of the divers experiences a catastrophic failure with his gas supply. They proceed to execute a gas share per their training.

​

Could happen, so you plan for it. Stay close enough to the tie-in to manage this potential threat.

Question: Is the captain expecting these two divers who are now sharing gas to return back to the anchor line before they begin their ascent? Or, is the expectation that they will begin their ascent immediately even though they are at their furthest point away from the anchor line?

Yes.

I might be on the minority here (perhaps a minority consisting of one) but this seems very much inconsistent with a primary premise of recreational diving - that a diver can begin his ascent at any point.

If excessive risk is incurred if the ascent is not performed on the anchor line, then the dive effectively has a virtual overhead and as such needs to incorporate an appropriate gas plan (straight thirds in this case would actually be insufficient). Some sort of navigation aid will also be required to ensure the divers can reliably get back to the anchor line. For example, a guideline.

Admittedly, my experience with dive boats is rather narrow. Outside a few boats in Hawaii (wherein no such mandate of ascending the anchor line was made), most of my boat diving experience comes from one boat in my area. If I am correct, the policy with that boat on recreational dives is that ascending on the anchor line is always strongly recommended but never mandatory.

I am a little bit out on a limb here but I think that if I asked the skipper of that boat, "in a situation where there is strong current or the site is in an area with heavy boat traffic or there is fog, would he mandate that recreational divers ascend the anchor line under all circumstances?" My guess is his answer would be, "I would not put recreational divers in the water in those conditions."

I never saw the need to put distance between me and the tie-in unless I was looking for something in particular on a given wreck. The tie-in is a good hour offshore, another 500' away is just another place, it usually isn't any better...

One need not be 500 ft away from the anchor line to lose sight of the anchor line. Its not unusual for us to have visibility of less than 20ft. In fact, the last boat I was on, the first dive, we had less than 10ft of vis. The second dive was better. Maybe 30ft of vis. In either case, swimming in any direction for 30 seconds would put the anchor line beyond sight.

 

[TSandM]

I vividly remember a dive from that boat, Adobo. I was with two EXTREMELY experienced local divers, and we were circumnavigating a pancake-like reef. Should have been a no-brainer to get back to the anchor line, but the viz was so bad I think we could have swum into it before we'd have noticed it. We ended up doing a free ascent on a bag -- it was embarrassing, a bit, but not dangerous. I am completely with you, that dives where it is truly dangerous to surface anywhere but the anchor are really pushing the limits of "recreational diving", and need more detailed gas plans, and contingency planning.

 

[boulderjohn]

dives where it is truly dangerous to surface anywhere but the anchor are really pushing the limits of "recreational diving", and need more detailed gas plans, and contingency planning.

..and the ability to shoot a bag from depth.

If you come up in heavy current off the line with no surface signal from the start of your ascent, you can be in trouble. That signal needs to be above the water before you ascend--if you wait until you have drifted 100 yards away to inflate it, it might not be seen at all. Even when you do deploy it in time, the captain cannot leave the ascent line to chase you until all the other divers are up.

 

[lowviz]

...//... Its not unusual for us to have visibility of less than 20ft. In fact, the last boat I was on, the first dive, we had less than 10ft of vis. The second dive was better. Maybe 30ft of vis. In either case, swimming in any direction for 30 seconds would put the anchor line beyond sight.

Sounds about right for where I dive too. A lot of common sense comes into play. If the tie-in is onto the propeller shaft, then I'll think nothing of following the shaft for quite a way, even in bad viz. Otherwise, I'll run a line.

I do my lollygagging on the way out, into the current if possible. This stacks the deck in my favor for the return trip if turning on thirds. After turning, I begin to switch to rockbottom rules and find myself killing time again, but now it is with the gas I got by dropping thirds and going to RB. I don't push rockbottom on the outbound leg, it just makes for a stressful dive...

Diving true thirds plus a rockbottom reserve is what I do when things are really crappy. Short dive, but often memorable due to the conditions.

 

[DaleC]

I believe Rock Bottom has its origin in the GUE system.

I do not know the origin of those words as they pertain to diving but the whole concept of rock bottom (or minimum needed gas for safe return) probably began in aviation and was quite commonly used by WW2 bomber pilots under the term "bingo". Bingo was the fuel level needed to return to a friendly landing field and planes would turn at that point or risk ditching. Many divers before GUE were applying that concept to their gas management.

 

[boulderjohn]

I do not know the origin of those words as they pertain to diving but the whole concept of rock bottom (or minimum needed gas for safe return) probably began in aviation and was quite commonly used by WW2 bomber pilots under the term "bingo". Bingo was the fuel level needed to return to a friendly landing field and planes would turn at that point or risk ditching. Many divers before GUE were applying that concept to their gas management.

Sure. I'm talking about the official term and the specific protocols.

 

[Kevrumbo]

How about both divers have significant different SAC?
You will need a conversion table.
It does get complicated in Basic Scuba discussion.

I think you're referring to that IANTD SAC Ratio Factor Table, but I'd rather do the full analysis exercise and just use straight thirds of the volume amount of usable gas left after first subtracting out the Rock Bottom Reserve. If dissimilar tanks are used, take one-third of the smaller tank volume after first subtracting out Rock Bottom, and then match that amount to the remaining tanks of the team by their respective metric cylinder ratings. For conservatism, figure out Rock Bottom using the teammate with the higher Surface Consumption Rate (SCR).

Example:
28 litres/min SCR, with 18 meters (2.8 ATA) depth NDL with one minute stops every 3 meters to surface:

2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28

Sum Total: 372.4 litres gas needed to ascend to surface for an emergency contingency.

Suppose you have two divers with different cylinder ratings:

In a 12 litre/bar tank, 372.4 litres is 31 bar (divide 372.4 by 12); in a 15 litre/bar tank, 372.4 is 24 bar pressure.
For these two divers sharing gas in an out-of-gas contingency, multiply the respective pressure readings by 2 for actual Rock Bottom:

12 litre/bar tank: 62 bar; 15 litre/bar tank: 48 bar.

The 12 litre/bar tank has a starting pressure of 180 bar; subtract out Rock Bottom of 62 bar yields 118 bar usable.
The 15 litre/bar tank has a starting pressure of 150 bar; subtract out Rock Bottom of 48 bar yields 102 bar usable.

Which of the two tanks above has the smaller remaining volume of usable gas left?
12 litre/bar x 118 bar = 1416 litres
15 litre/bar x 102 bar = 1530 litres

So the 12 litre/bar tank is the smaller, hence we apply the Rule-of-Thirds to the usable pressure of 118 bar:
118/3 = 39 bar; so 39 bar delta down from 118 equals an actual turn pressure SPG reading of 79 bar.

39 bar delta in the 12 litre/bar tank equals 468 litres (39 x12 = 468);

Therefore 468 litres divided-by the other cylinder's 15 litre/bar tank rating equals 31 bar;
So 31 bar delta from 102 bar equals an actual turn pressure SPG reading of 71 bar with regards to the 15 litre/bar tank.

See also:
Post #15
http://www.scubaboard.com/forums/ba...ving-thirds-vs-rock-bottom-2.html#post6469083
Post #20
http://www.scubaboard.com/forums/ba...ving-thirds-vs-rock-bottom-2.html#post6469264

 

[dumpsterDiver]

..and the ability to shoot a bag from depth.

If you come up in heavy current off the line with no surface signal from the start of your ascent, you can be in trouble. That signal needs to be above the water before you ascend--if you wait until you have drifted 100 yards away to inflate it, it might not be seen at all. Even when you do deploy it in time, the captain cannot leave the ascent line to chase you until all the other divers are up.

Why would you say that? The captain most certainly SHOULD be prepared to leave the line to recover a diver if necessary. Why must he stay?