Question Question for Experienced O2ptima CM Divers

[rx7diver]

Quick question. (This is a thought exercise. I am attempting to understand how one dives a Cho2ptima.) Suppose you are solo diving your Cho2ptima using full, back-mounted, isolation-manifolded doubles containing an "appropriate" Tri-Mix (say, 1/4 He + 3/4 EAN32) for a 150 fsw deco dive in open water.

I am thinking that you should plan your dive so that you can bail out ("dil out") to open circuit, and return to the surface from depth, using at most 1/2 of the contents in your doubles + EAN50 (which you are slinging) + 02 (which you are slinging).

In addition, your bail out plan should allow for the loss of either your EAN50 or your 02.

Is this a reasonable plan?

EDIT: Corrected "2/3" to "1/2", and added "solo."

TIA,

rx7diver

 

[Tracy]

It is a very conservative plan, but certainly one that would work. You are planning for a stack of failures. You are going to flood your rebreather, have your buddy run out of gas at max penetration, and somehow misplace your deco bottle all on one dive. OC isn't your primary, you don't need a third left after the bailout. You need enough. Same goes for the deco gas. What you do need to know, is how much gas you actually need for a given TTS.

 

[rx7diver]

It is a very conservative plan, but certainly one that would work. You are planning for a stack of failures. You are going to flood your rebreather, have your buddy run out of gas at max penetration, and somehow misplace your deco bottle all on one dive. OC isn't your primary, you don't need a third left after the bailout. You need enough. Same goes for the deco gas. What you do need to know, is how much gas you actually need for a given TTS.

@Tracy, I should have mentioned that I am thinking of this as a solo dive. I will add this now to the OP.

Thanks,

rx7diver

 

[lostsheep]

What is 1/4 He and 3/4 Nx32?

Would that be like Tx 24/25? Never seen someone describe it like that.

 

[rx7diver]

What is 1/4 He and 3/4 Nx32?

Would that be like Tx 24/25? Never seen someone describe it like that.

Yes. (Sorry, not a Tri-Mix diver. Maybe better to describe it as Helitrox 25/24 or 24/25?)

rx7diver

 

[rx7diver]

I thought a bit more about this. The real risks here (other than a CCR, itself) are:

1. A doubles free-flow
2. The gas switch

So, maybe the plan should be to use at most 2/3 of one doubles cylinder during the bail-out ascent--assuming the free-flow could be solved only by isolating the doubles (while one of the doubles cylinders drains empty), ...

... and to sling two EAN50 cylinders (and no 02), for redundancy, planning to deco out only on EAN50. (Maybe the boat could provide overboard 02, though not necessary here.)

What do you all think?

rx7diver

 

[broncobowsher]

This is where a good instructor comes into play. You pay an instructor to learn this stuff in a class. You may get a single point of information figured out, but that doesn't fill in for the whole class.

Sounds like you are trying to plan for multiple major failures all at once and trying to use OC planning to cover for it.

Why are you planning on making the bailout plan harder with loosing your deco O2? I can understand bailout for the loss of O2. But the bail out should not be tied to onboard O2.

 

[Tracy]

I thought a bit more about this. The real risks here (other than a CCR, itself) are:

1. A doubles free-flow
2. The gas switch

So, maybe the plan should be to use at most 2/3 of one doubles cylinder during the bail-out ascent--assuming the free-flow could be solved only by isolating the doubles (while one of the doubles cylinders drains empty), ...

... and to sling two EAN50 cylinders (and no 02), for redundancy, planning to deco out only on EAN50. (Maybe the boat could provide overboard 02, though not necessary here.)

What do you all think?

rx7diver

Why are these risks? You are trying to plan a rebreather dive as an OC diver. It doesn't work.
1. A doubles free flow
a. won't happen as you aren't breathing off of them.
b. If it did, and it took you 10 minutes to shut it down, it still wouldn't matter. You are breathing off your rebreather.
2. There is no gas switch, you are diving a rebreather.
I am leaving Thursday for a week of 200ish feet diving on a CM. I will be carrying backmount mini doubles with 18/45 and a 40 of 50%. Dive times won't be long enough to warrant an 80 of deco gas. There is no thermocline yet, temperature is 37 top to bottom.

 

[Superlyte27]

This is a mess. Too much to fix here. Find an instructor.

 

[rx7diver]

Why are these risks? You are trying to plan a rebreather dive as an OC diver. It doesn't work.
1. A doubles free flow
a. won't happen as you aren't breathing off of them.
b. If it did, and it took you 10 minutes to shut it down, it still wouldn't matter. You are breathing off your rebreather.
2. There is no gas switch, you are diving a rebreather.
I am leaving Thursday for a week of 200ish feet diving on a CM. I will be carrying backmount mini doubles with 18/45 and a 40 of 50%. Dive times won't be long enough to warrant an 80 of deco gas. There is no thermocline yet, temperature is 37 top to bottom.

Hmmm. If you bail out and go to open circuit, and then get a free-flow, then you are not breathing off your rebreather, and you are experiencing a free-flow, correct?

If you bail out to open circuit with the intention of ascending to 70 fsw to switch to a deco gas, then you will make a gas switch, correct? Then, having two deco gases means the possibility of switching to the wrong deco gas, correct?

@Tracy, re your "I am leaving Thursday for a week of ... diving on a CM. I will be carrying backmount mini doubles with 18/45 and a 40 of 50%." Ha! You have stumbled upon what is motivating this "thought exercise" of mine: You see, I recently assembled a set of mini doubles (LP50's), and I am thinking of the possibilities ...

Thanks,

rx7diver