Going to back gas while switching stages?

[rjack321]

There is bottom gas in his doubles, bottom mix in his stage (21/35), 50% and O2, There is no such thing as "travel gas" with an agency like ISE. On a hypoxic trimix dive (<15% o2) you usually have a 3rd deco gas, either 35/25 or 21/35 that is breathed until you can switch to 12/65, 10/70 or 8/80.

Normal DIR protocol (via ISE, GUE or UTD) on ascent is to switch from:
1) stage to backgas - this happens on the bottom, stages are breathed first
2) backgas to first deco gas (at 21m for this collection of gases)
3) at the last stop of the first deco gas go back to backgas (at 9m)
4) clean up the 1st deco bottle (at 9m)
5) switch to second deco bottle (at 6m)

Trying to learn more about tech techniques I came across the ISE deco stage switching procedure

Best to learn from an instructor

 

[jale]

its simply a video demonstrating a gas switching system by ISE - his 'bottom gas' is on his back and the 45m is his first switch - unlikely he'd be using a single cylinder to do a bottom part of a deep dive as a failure at depth would be problematic - computer switches are missed - would have been better to have them - anyone who has done similar diving will know that so i guess from an instructional video its a little tardy.

they don't use computer.
A lot of misinterpretations from.this video.
There is many ways to dive. ISE proposes one and the video demonstrates that and if you learn a bit about the why and how of this procedure, it all makes sense

 

[grantctobin]

When I dive with a back gas mix of 15/40 I usually descend on my 50% deco gas until I'm at 15-20m and change to back gas.

I was not taught to go to back gas between deco gas changes though.



His third tank is a bottom mix.

What year were you taught not to switch back to backgas during gas switches?

 

[Wibble]

Using two deco stages doesn't always mean a very deep dive. It could be a long 40m/130' dive where you need a second deco cylinder to give you the volume of gas and cope with a failure of the first deco cylinder.

Also, use of a bottom stage means longer bottom time, so more on-gassing, thus almost certainly requires two deco stages.

The problem with open circuit is the weight of the kit and the enormous cost of the helium. Hence nowadays few people do deep OC dives, they use a rebreather where the gas costs are a tiny fraction of that for OC. We're talking of $150++ for a deep OC dive, but the same on CCR is $10 or less!

 

[Searcaigh]

What year were you taught not to switch back to backgas during gas switches?

April 2014

 

[PfcAJ]

Hypoxic mixes, generally lower than 18% oxygen, have too little oxygen to breathe on the surface especially if you’re "working" hard, such as carrying yourself and your cylinders across a boat. Thus special hypoxic procedures are used where you breathe from the deepest normoxic (18 to 22%) mix you’re carrying.

For a dive with 13% oxygen, this would be a maximum of 1.4 PPO2 at depth,
Max operating depth (MOD) = dose over mix = 1.4 / 0.13 = 10.7 = 97 metres (318')

In reality few people would ever dive to this depth on open circuit nowadays, they'd use a rebreather.

You become open circuit when the rebreather breaks.

Still gotta know how to manage your gases.

 

[J-Vo]

As someone who still dives in the 250-300' range regularly on OC, let me run through the scenario.

On ascent at ~200' we switch to travel witch is now the first deco gas. The procedure is to loop it around the neck under the long hose leaving the long hose free to deploy in an emergency, validate the GAS, switch regs, then change PDCs.

When we clear the 80' stop, typically on ascent to 70' we switch to the short hose, switch PDCs, stow and cleanup the travel mix, deploy and switch to 50% as above. Same procedure for the O2 switch. For bigger dives there will be a switch to 32%, earlier, but then you have more bottles then the OPs example.

30 to 60 seconds on back gas at those depths is not going to cause ICD. If that was the case we would have to worry about it when testing our regs at the surface IMHO.

With the travel reg deployed under the long hose, it is a complete CF to attempt to deploy and switch to 50% before stowing the travel. You simply end up with a rats nest of hosed behind your head.

Further, this procedure translates wonderfully to stage diving in a cave. As your approaching the next stage, switch to back gas, stow and cleanup the stage hose, pick up the next stage as you fly by, clip it off, deploy the hose and switch regs. I even typically switch back to back gas before picking up my O2. It's just easier with all the stages stowed not having to stow a hose while holding a 20' stop in the flow.

 

[grantctobin]

As someone who still dives in the 250-300' range regularly on OC, let me run through the scenario.

On ascent at ~200' we switch to travel witch is now the first deco gas. The procedure is to loop it around the neck under the long hose leaving the long hose free to deploy in an emergency, validate the GAS, switch regs, then change PDCs.

When we clear the 80' stop, typically on ascent to 70' we switch to the short hose, switch PDCs, stow and cleanup the travel mix, deploy and switch to 50% as above. Same procedure for the O2 switch. For bigger dives there will be a switch to 32%, earlier, but then you have more bottles then the OPs example.

30 to 60 seconds on back gas at those depths is not going to cause ICD. If that was the case we would have to worry about it when testing our regs at the surface IMHO.

With the travel reg deployed under the long hose, it is a complete CF to attempt to deploy and switch to 50% before stowing the travel. You simply end up with a rats nest of hosed behind your head.

Further, this procedure translates wonderfully to stage diving in a cave. As your approaching the next stage, switch to back gas, stow and cleanup the stage hose, pick up the next stage as you fly by, clip it off, deploy the hose and switch regs. I even typically switch back to back gas before picking up my O2. It's just easier with all the stages stowed not having to stow a hose while holding a 20' stop in the flow.

This is the way. Not switching to backgas during gas switches is lame and ignorant training.

 

[Wibble]

This is the way. Not switching to backgas during gas switches is lame and ignorant training.

Switching to 80% from 50% (I never use 100% - too rich)...

At the switching stop, 9m/30', identify the rich gas, take out hose, follow back to first stage, read the gas label again, check depth, hold second stage with right hand, turn on cylinder, purge, hold out in front of you.

With left hand, grasp the second stage you’re breathing from and remove from mouth, unwind from behind your head, hold out to the left hand side with your arm fully stretched out (so the hose is completely away from your body).

With right hand, wrap new second stage around the back of your head, stuff reg in gob, verify depth then purge/exhale and breathe.

Tidy up the old stage hose, probably undoing the bottom clip and bringing it in front of you, cleaning up then reclip.


Obviously the side you hold the 'old' reg out depends what side the stage is clipped. Heresy or not, I mount my bailouts on both sides and if diving OC, will tend to balance my stages on both sides.

Simples.

 

[Gareth J]

This is the way. Not switching to backgas during gas switches is lame and ignorant training.

Very sweeping statement.

When I did OC. For years it was anticipate and switch as you hit the stop. If you where doing it right, you had confirmed the gas before you hit stop depth and it was ready to breath. It did mean, you always knew if you where reverting to gas fail runtimes before you hit the stop. Going to gas fail runtimes was always somewhat stressful - the more notice you had the better.
The only times I switched back to back gas was for planed air breaks.

The only other time you did it was to hand off deco' gas if your buddy had lost a gas and you had completed your deco and I could release the remaining deco gas.
In theory that should never have happened, spare deco gas was hanging on the trapeze. You where really having a bad day if you had missed the shot and one of you lost decompression gas.
It was wasn't a good day if you lost gas and where on your own.
All dives where planned so they could be run solo if the SHTF.
If you where on your own and lost both gases you really where having a bad day, and it was time for the KYAGB plan. You really hopped that the boat had spotted the DSMB's, and was chasing you down with the drop gas.

I only ever once had the boat drop divers back in on me (and my buddy). We where really short of gas by then, but had almost completed all the deco. We where approaching the point where a possible bend was preferable to drowning.