Gas planning question

[Subcooled]

You're postulating the OOA diver loses BOTH tanks simultaneously, either of which individually could get him to the exit?

This usually happens as the ppCO reaches a limit during descent :|
This sort of OOG-situation may hit the whole team at the same time.

 

[rongoodman]

This usually happens as the ppCO reaches a limit during descent :|
This sort of OOG-situation may hit the whole team at the same time.

Some days you're better off just staying in bed. The scenario implied three major failures, though, two lost tanks and a buddy who completely mismanaged his gas, so it's hard to see how relevant it is to real world planning.

 

[flots am]

Basically a Cave example, which is where SM should only be used.

Really?

Is SM dangerous somehow in OW, wreck or deco?

 

[Subcooled]

If you shut down a tank in sidemount you have lost that tank; effectively you have just lost half your gas.

Not quite correct. The tank can be easily opened and closed with a minor twist of the wrist. Which needs to be practiced, of course. Hardly any gas goes wasted. I know as I did it. This of course only applies to 1st stage or 2nd stage free flows and some flooding problems of the regulator. A ruptured hose will make the tank unusable or cause great gas loss. In that situation a back mount system has benefits. Unfortunately, not all back mount divers are able to operate the valves, or at least they are not able to operate them fast enough. Two minutes to close the valve -> not cool.

But it is not the gear that kills but our minds.

Just plan for the foreseeable hose rupture and CO and you'll be fine. It would suck to die because one was too lazy to carry decent reserves.

Unless needed for the task don't dive SM. Why would you want to?

Because, you should learn and practice sidemount in open water, not in a cave. Another reason is that entanglements are easier to deal with. And anyone diving solo (I know that happens) would be safer with independent tanks. And it feels better. I can bend my back and pull and carry things under water. Great for certain project aware dives... Some people have physiological reasons, too.

But yeah, there are reasons for side mount as there are reasons for back mount, no mount and rebreathers (that kill).

The BM system can compensate for one divers' SAC being higher than the other, a SM system can't.

True. But this was accounted for in the planning, wasn't it? Using 1.5xSAC and 1.5x Exit time in contingency calculations is a good thing. In addition to the thirds, quarters, fifths, sixths and other rules of thumb.

 

[Ste Wart]

Really?

Is SM dangerous somehow in OW, wreck or deco?

It is task specific tool.

Shallow reef bimbles on SM, overkill

OW deco with sidemount. Too cluttered

SM wreck penetration. Maybe, but I'd find a bigger wreck if the restrictions were so tight :cool2:

Evaluate the dive and make the correct choice of equipment. SM has its place, but standard OW diving is really not it. And they take up WAAAYYYY to much room on boat charters

Unfortunately, not all back mount divers are able to operate the valves, or at least they are not able to operate them fast enough. Two minutes to close the valve -> not cool.

I have yet to see a diver take two minutes to close one valve on BM YMMV.

However, a team member should have responded to the situation by that time and donated gas. After that point it doesn't matter when the valve is closed, dive is over.

I remain sceptical on anyone taking two minutes to close a valve though

Great for certain project aware dives... Some people have physiological reasons, too.

project aware!

I mentioned the physiological reasons

But yeah, there are reasons for side mount as there are reasons for back mount, no mount and rebreathers (that kill).

Agreed

True. But this was accounted for in the planning, wasn't it? Using 1.5xSAC and 1.5x Exit time in contingency calculations is a good thing. In addition to the thirds, quarters, fifths, sixths and other rules of thumb.

As already mentioned, simple multiplying by 1.5 does not necessarily mean you will have enough gas for an air-sharing emergency

 

[flots am]

Shallow reef bimbles on SM, overkill.

Or maybe just easier to get back up the ladder and on the boat.

OW deco with sidemount. Too cluttered

SM wreck penetration. Maybe, but I'd find a bigger wreck if the restrictions were so tight :cool2:

You could say the same things about SM in a cave.
Evaluate the dive and make the correct choice of equipment. SM has its place, but standard OW diving is really not it. And they take up WAAAYYYY to much room on boat charters

Only if your boat is too small or the gear is spread out all over. Mine fits nicely under the seat.

 

[Ste Wart]

Shallow reef bimbles on SM, overkill.

Or maybe just easier to get back up the ladder and on the boat.

?

OW deco with sidemount. Too cluttered

SM wreck penetration. Maybe, but I'd find a bigger wreck if the restrictions were so tight :cool2:

You could say the same things about SM in a cave.

No I'm referring to practices like this:

[video=youtube;t9wbJUm9i1Q]https://www.youtube.com/watch?v=t9wbJUm9i1Q&noredirect=1[/video]

Evaluate the dive and make the correct choice of equipment. SM has its place, but standard OW diving is really not it. And they take up WAAAYYYY to much room on boat charters

Only if your boat is too small or the gear is spread out all over. Mine fits nicely under the seat.

That's great, except when you're wearing it and are wider than everyone else on the boat

UK dives, you don't get tanks handed to you. You jump fully kitted and return fully kitted.

 

[FoxHound]

well glad to see this thread got way off topic into a SM vs BM argument again. I like how most people that give their opinions have not used both systems. But that is neither here nor there.

Long and short of it is, the gas planning in both systems is the same. No matter how you look at it, there are failures that can happen to both systems that allow you to lose half of your gas no matter how unlikely.

 

[flots am]

That's great, except when you're wearing it and are wider than everyone else on the boat

UK dives, you don't get tanks handed to you. You jump fully kitted and return fully kitted.

That not an equipment problem.

Pick a boat that you and your equipment can fit in, and dive how you want.

I can tell you that I'm not going to switch back to BM just to make the dive op happy.

 

[Kevrumbo]

well glad to see this thread got way off topic into a SM vs BM argument again. I like how most people that give their opinions have not used both systems. But that is neither here nor there.

Long and short of it is, the gas planning in both systems is the same. No matter how you look at it, there are failures that can happen to both systems that allow you to lose half of your gas no matter how unlikely.

Correct . . .the only difference is that your breathing Depth Consumption Rate (SAC rate at depth) will be twice as much breathing independent doubles alternately versus breathing gas equally off of two manifolded combined tanks --all of the same volume & tank cylinder rating.

In other words for example, if you nominally consume 30 bar in 10 minutes at 30m depth diving on conventionally doubles backmounted manifolded Alu 11L cylinders, then be aware that you will obviously consume 30 bar in 5 minutes at 30m breathing off of only one cylinder --if alternately breathing one cylinder at-a-time those same Alu 11L cylinders now configured as independent double sidemount tanks. . .