O2ptimaCM and Trim

[mr_v]

What things do you guys do that helps you find that perfect neutral buoyancy sweet spot on the rebreather for sawtooth dive profiles? I can never seem to find it. Always just slightly negative or slightly positive or worse. Even with the MAV and closed o2 I struggle to find that perfect buoyancy when still.

Practice... I wish there was a different answer, but you gotta know your unit well. I am with @lostsheep - no ADV (or turn it off), min loop volume. I don't turn off tanks unless I am out, so on deco I monitor loop volume and adjust as I offgas into the loop.

The "new to me" skillset is CCR Cave DVP buoyancy. Still getting that tuned in, but the same approach applies. Make sure that there are no unsuspected gas additions, stop often, and develop a feeling for when you're negative or positive.

P.S. I am not a Choptima diver so please pardon my ignorance on the subject... Your comment regarding turning off O2 does not sit well with me. Is that a standard practice?

 

[mr_v]

Here a clip: (ready to be trashed).

I think you're hard on yourself when you say "buoyancy and trim all over the place." From the angles in the video, it does not seem like you're trying to arch - superman - your back. Perhaps that could help.

 

[PEDiver]

Your comment regarding turning off O2 does not sit well with me. Is that a standard practice?

It's not standard practice.

 

[Tracy]

P.S. I am not a Choptima diver so please pardon my ignorance on the subject... Your comment regarding turning off O2 does not sit well with me. Is that a standard practice?

Definitely not standard practice.
I doubt it would be recommended by anyone.

 

[lostsheep]

I am a Choptima diver but pardon MY ignorance…

Isn’t shutting O2 off an MCCR thing?

 

[addohm]

It's not standard practice.

I misspoke. I meant to convey that I tried turning it off during deco and fully manually controlling all gas additions.

 

[addohm]

Practice... I wish there was a different answer, but you gotta know your unit well. I am with @lostsheep - no ADV (or turn it off), min loop volume. I don't turn off tanks unless I am out, so on deco I monitor loop volume and adjust as I offgas into the loop.

The "new to me" skillset is CCR Cave DVP buoyancy. Still getting that tuned in, but the same approach applies. Make sure that there are no unsuspected gas additions, stop often, and develop a feeling for when you're negative or positive.

P.S. I am not a Choptima diver so please pardon my ignorance on the subject... Your comment regarding turning off O2 does not sit well with me. Is that a standard practice?

Yeah man, I'm waiting for that damn "click" moment but I'm also trying to facilitate that moment by asking questions no matter how ridiculous they might be . I've got maybe 100 hrs on this particular unit and maybe I'm being too critical. Thanks. All I know is that on OC I am rock solid on deco stops and that's my self-inflicted "standard". I need to lock this down so I can push MOD certifications smoothly. I know and understand the differences but putting them into practice makes me feel like a damn bum in the water.

 

[mr_v]

I misspoke. I meant to convey that I tried turning it off during deco and fully manually controlling all gas additions.

Ok, I hope you tried and decided not to do it again. Keep O2 open. There are legit scenarios where you must close O2 , but normal deco stops aren't that scenario. And it is not closing O2 that will kill you - it is forgetting that you've closed O2.

@lostsheep - I dive hybrid and electronic units. For both units, the standard practice is to select a low setpoint on deco. If the unit is on you, gases should be on and the setpoint should be non-surface. I've heard of MCCR/HCCR divers trying to save O2 by shutting it off unless they're in water. That's a bad practice and a poor application of math skills.

A 3L tank at 200 bar contains 600L of O2. CMF or needle valves are typically configured to leak O2 at less than diver metabolic rate, so less than 1L/min. But suppose a diver is an O2 hog and metabolizes 1.25L/min and must have 1L/min flow. If the O2 tank is on in that scenario, then the diver will lose 40L while waiting 40 minutes to get in the water. With 1.25L/min O2 consumption rate, the diver would have had 480 min on the O2 tank, but now has 440 min or 7+ hours. Oh, the horror. Given O2 prices, bring extra O2 tanks if you're that concerned. (And wtf are you doing waiting for 40 min with the unit on your back?)

 

[Capt Jim Wyatt]

The only strategy I use in sawtooth profiles is to anticipate depth/buoyancy changes and get ahead of the curve adjusting for the change.

Ever see handprints in the mud in a cave at the bottom of a hill? The diver did not anticipate the buoyancy change.

 

[addohm]

I do this already. At depth it's not really a problem. It's at shallower deco stops that I sometimes just want to forego the rebreather all together. Lol