Future of OC trimix

[Tracy]

The hour decompression. I actually didn't realise the decompression times are a lot longer on a rebreather. Have you a large conservatism factored in. Is that the usual run time.

For a true square profile, which we often see in wreck diving, a properly planned and executed OC dive can yield faster decompression. It rarely happens as you don't often have the exact mix for the depth with OC.
OC will run a 1.4 po2 on the bottom and up to a 1.6 on decompression. Taking out the descent and ascent, you are running a higher po2 through the dive. The same dive on a rebreather, I will run a 1.2 from start to finish leaving a bit more deco time.
A multi level dive blows OC out of the water for deco as the po2 is always optimized.

 

[broncobowsher]

After putting transmitters on the rEvo and looking at the gas usage, square profiles are interesting to look at. The gas usage is highest on decent. Stable (and only slight) during the dive. O2 goes up a bit on ascent. The gas usage at the deepest part of the dive is actually the smallest. Completely the opposite of OC

 

[Wibble]

The hour decompression. I actually didn't realise the decompression times are a lot longer on a rebreather. Have you a large conservatism factored in. Is that the usual run time.

Checks log…. Left bottom at 1:05. Also padded the end by a couple of mins. So not 1:1, 1:0.81.

Actually OC deco can be more optimised as you’re switching gasses so helium is off the menu at the first (50%) deco stop. You’re also expelling all your off-gassed inert gas (nitrogen, helium) from your lungs to the bubbles.

A rebreather only scrubs CO2 with the off-gassed inert gas recirculating around the loop reducing the gas pressure gradient and slowing off-gassing. You actually notice the rebreather loop increasing in volume as the inert off-gassed gasses slowly fill the loop making you more buoyant — this requires occasional loop flushes (breathing out through one’s nose until the loop+lungs are collapsed then injecting oxygen to re-inflate and breathe).

 

[mr_v]

After putting transmitters on the rEvo and looking at the gas usage, square profiles are interesting to look at. The gas usage is highest on decent. Stable (and only slight) during the dive. O2 goes up a bit on ascent. The gas usage at the deepest part of the dive is actually the smallest. Completely the opposite of OC

This makes sense - add dil on descend, level O2 at the target depth, add O2 on ascend + flush. As someone who switched to CCR recently, I can't fathom trimix on OC. Even after running drills all day, I have gas plenty of gas left over.

 

[billgraham]

I think that the CCR, with the right training, is safer and easier for this type of diving. Compared to open circuit trimix dives, it's dead simple when everything is going right. That's the issue though, let's say you flood the unit after being on the bottom at depth, what then? You are back to your open circuit, trimix dive.

For myself, I feel like my open circuit skills have to be sharp before I even want to take the CCR in the water.

 

[billgraham]

Interest for trimix training for me is to be able to go deeper more safely, and to continue my training as I enjoy the process. I don't have dozens of deep trimix dives lined up per se, but I could see myself getting the occasional fill for certain dives below 100 or 120. There's a wreck in my area that goes to 160 and I'd rather to trimix certainly than air for that.

The 100-130 range should be ok with nitrox, depending on your tolerance. In the cold water, with (likely) limited visibility, some helium is a good idea for the 160' wreck.

 

[Tracy]

Its still not cheaper to drive the nail with a laptop, I know it can be done. If you enjoy your rebreather that's another thing.

You apparently didn't understand the analogy. If you have multiple tools, you don't have to use the same one for everything.
In this instance, you would use the hammer, not the laptop, for driving a nail.
But, you could use the laptop for email, instead of a hammer. Hammers are horrible for sending emails.

 

[Wibble]

I think that the CCR, with the right training, is safer and easier for this type of diving. Compared to open circuit trimix dives, it's dead simple when everything is going right. That's the issue though, let's say you flood the unit after being on the bottom at depth, what then? You are back to your open circuit, trimix dive.

There’s many more options available to the diver on CCR.

In the catastrophic event of a CO2 breakthrough, major flood or caustic cocktail it’s a full bailout and head to the surface ASAP.

However, those are rare failure modes. More likely are non catastrophic failures such as electronics, batteries, O2 cells, solenoid (open or closed), injectors, automatic diluent valves, gas supplies, regulators, wing, etc. All of which are inconvenient and easily worked around — some may not even end the dive early — providing you practice and think about them in advance.

Does depend upon the unit, some have sufficient redundancy, others far less.

 

[Celt]

You apparently didn't understand the analogy. If you have multiple tools, you don't have to use the same one for everything.
In this instance, you would use the hammer, not the laptop, for driving a nail.
But, you could use the laptop for email, instead of a hammer. Hammers are horrible for sending emails.

I understand where you're coming from, but I still wouldn't spend 6000 euro on a table saw where a 10 euro hand saw will do the job. There's dives that can't be done on OC, very long multiple hour shallow dives where its impossible to carry the gas needed. Or very deep with multiple gas switches. But for reasonable bottom times to 60m OC is absolutely an option. And for a diver making a small amount of dives per season there's no saving money switching to a rebreather. If a diver enjoys using a rebreather that's another thing entirely like the man or women who has a work shop in their garage to die for as a hobby. People are entitled to enjoy their hobby as they see fit but your not going to save money scuba diving with a rebreather.

 

[Wibble]

But but but some of us like collecting tools, there's hours of procrastination being able to select the right table saw, reading reviews, etc.

Just like an underused table saw, an underused rebreather is an expensive ornament and a dangerous diving tool; competence is earned through lots of practice and it's rather volatile, slowly ebbing with disuse.

The point is I like my rebreather. I'd happily dive it in a shallow puddle, a pool, anywhere.

To that end I must get around to disposing of all my open circuit diving kit that cannot be re-purposed for CCR. It needs a loving home.