Question Question for Experienced O2ptima CM Divers

[Superlyte27]

This has been an enlightening "thought exercise" (for me)!

Thank you, everyone who contributed.

rx7diver

Glad you thought so. I fire students like this. Ask for help, then don’t listen to any of the advice of the experts. They call that an “askhole”

Your dive planning is absolute garbage. Throw it all away and start again.

 

[rx7diver]

I just replied with the following to someone who DM-ed me about this thread. Posting it here for any other person wondering how I am thinking about this.

"Ha! Well, "obvious" now--after I ran the numbers.

"... Here is a simplistic way I am thinking of this now, in hindsight: Suppose, on open circuit, you require 80 cu ft from your double HP120's to get to your farthest point for this dive. Then you'll require ~80 cu ft to get back to the 70 fsw gas switch. And, you'll "require" ~80 cu ft as your reserve--either for your buddy (if you are buddy diving), or for yourself (if you are diving solo). So, you "require" a total of 160 cu ft for return + reserve, if you begin with 240 cu ft, open circuit. Okay so far?

"I am not a rebreather diver, but this is how I think these things work: Before this solo dive commences, your CM requires a "sip" of Tri-Mix from your (smaller) manifolded doubles, to fill it up. (Is this what is meant by "pre-breathing"?) As you descend, more "sips" are needed for equalizing both the CM's counter lungs and your wings. When you reach TOD = 165 fsw at RT = 2.5 min, the amount of Tri-Mix used from your manifolded doubles is not appreciable, maybe. Okay so far?

"As the dive progresses horizontally, your doubles remain virtually unused (since you're replenishing the spent oxygen in the CM's "loop", from the CM's onboard oxygen cylinder). Okay so far?

"You swim to the turnaround point (the point that would have required 80 cu ft of Tri-Mix open circuit, to get to), and then (1) your CM fails, and (2) you almost immediately have to isolate your manifolded cylinders (so, that now you're returning, open circuit, breathing from only one cylinder). You'd better have at least ~80 cu ft in that single backmounted cylinder, to get you back to 70 fsw for the gas switch.

"If you are wearing double 120's with your CM, you have slightly less than 120 cu ft in each backmount cylinder when you isolate. So, you have enough bail-out in a single cylinder, probably!

"If you are wearing double 50's (even 50's "cave-filled" to 3,500 [sic] psig) with your CM, you have slightly less than ~65 cu ft in each back-mount cylinder when you isolate. So, you do NOT have enough bail-out in a single cylinder! The 50's are too small for this dive!

"However, if you are wearing double 72's (cave-filled to 3,000 [sic] psig) with your CM, you have slightly less than than ~85 cu ft in each back-mounted cylinder when you isolate. So, you have enough bail-out in a single cylinder--if you can keep your RMV from elevating from the stress of losing both your CM and half your backgas!

"Upshot: If I am thinking about this correctly, the 50's are too small for this dive!

"Does this make sense? Comments?

"rx7diver"

 

[inquis]

Then you'll require ~80 cu ft to get back to the 70 fsw gas switch.

You said this was OW, so this is only true if it's mandatory that you ascend from the anchor line.

One thing I don't recall being mentioned on the CCR side is enough reserve to handle a CO2 hit. It's probably more than you think.

 

[inquis]

the stress of losing both your CM and half your backgas!

The point of using manifolded doubles is so you do NOT lose half your backgas. The failure mode that would require an isolation is extremely improbable. Even if I had to isolate, you can bet I would feather the valve on the problem tank until it was actually empty.

 

[lostsheep]

Why would you read past:

“I am not a rebreather diver but I think this is how these things work?”

 

[rx7diver]

Why would you read past:

“I am not a rebreather diver but I think this is how these things work?”

Good question. That quote was from post #22. Better question, why would you read past post #5:

Yes. (Sorry, not a Tri-Mix diver...

rx7diver

 

[lostsheep]

For the same reason I couldn’t turn away from a trainwreck?

You’ve been here since 06’ , Shirley by now you would have accidentally learned not to create threads like this.

 

[broncobowsher]

For the same reason I couldn’t turn away from a trainwreck?

You’ve been here since 06’ , Shirley by now you would have accidentally learned not to create threads like this.

 

[Superlyte27]

For the same reason I couldn’t turn away from a trainwreck?

You’ve been here since 06’ , Shirley by now you would have accidentally learned not to create threads like this.

I looked 5 minutes ago to see how long he’s been here. He’s a phd statician. He thinks he’s smarter than us dumb dive instructors, which is why he keeps trying to justify his own flawed logic.

Thank God I don’t have his brain.

 

[DiveTucson]

I think you need to call Ben/Tracy/Pete that have commented above and discuss this. This is a rather bizarre set of circumstances and a rather odd way of going about each of the details.

I’ll pass on this one. Don’t have enough time for someone that clearly is not concerned with learning the correct way to do things as illustrated in this thread.