Prism2 vs rEvo?

[stuartv]

With TDI at least if you have done OC deco procedures the first level of CCR training allows air dil deco to 45m, I think you even have the option of helitrox now as dil for that entry level CCR course, an option I would have taken up if it was available when I did my first CCR course.

You are correct regarding what the TDI standards say. It's kind of weird, though, since the training (as I understand it) is to not exceed an END of 100'. But, the air dil deco cert is for 45m. And, as I'm sure you know, there's no way to dive past 100' with air dil and not exceed an END of 100'.

I don't mind diving deep-ish on air on OC. It has not been an issue for me yet. But, I would not choose to do it on a CCR. The extra complexity of a CCR really seems to merit being extra acute, mentally.

 

[kensuf]

TDI Air Dil Deco cert is for 150', but the TDI Air Dil cert (no deco) is 100'. The TDI Helitrox CCR cert is for 150' with helitrox (no limit on the END, but 25/20 is the DIL so you're really limited on END to about 110').

Things to think about...

The Prism is an eCCR with a solenoid. The Revo comes in two basic flavors, an mCCR with a leaky valve or an hCCR with both a solenoid and a leaky valve. There are differences between a solenoid and a leaky valve, and personally, I think learning to fly an mCCR with a leaky valve first (or an hCCR but not using the solenoid) will make you a better CCR diver in the long run.

Front mounted CL's will have some clutter, but whatever. Backmounted CL's won't but you'll have some hydrostatic work of breathing issues in some positions. However, the hydrostatic breathing issues won't be as bad as everyone makes it sound like it will be.

The Prism has a simple and straightforward method of de-watering in the event of a loop flood. I'm not sure if you can get all of the water out of the scrubber can, but you should be able to get most of the water out. The Revo has no method of draining a flooded loop.

I believe you should be able to block off / shut down the ADV on a Prism. This is a HUGE safety benefit:

1. If your ADV sticks open, your PO2 will drop. With a shut off you can stop that and maintain PO2 (and should be able to manually add DIL).

2. If you're running a hypoxic DIL, you will likely want to disable the ADV when shallower than 15' to minimize the risk of your PO2 dropping when you're near the surface (think doing a surface swim).

My understanding is you cannot disable the ADV on the Revo without completely disabling it. I may be wrong here, I'll openly admit I don't know.

The Revo has 5 cells and an interesting philosophy on why they want 5 cells. I'm not sure I can argue with their logic, but I find 3 works for me and I can always do a DIL flush to verify cell integrity if I have any doubts.

The dual scrubber bed idea in the Revo is interesting and in theory should reduce the risk of channeling. IMHO, I think CO2 is one of the greatest risks on diving a CCR -- excessive CO2 causes heart attacks, massive narcosis, and blackout, and so this may be a good thing.

Enjoy the courses, I'd be curious to find out how they went.

BTW -- grab yourself a copy of the Bozanic book. It's worth reading.

 

[stuartv]

TDI Air Dil Deco cert is for 150', but the TDI Air Dil cert (no deco) is 100'. The TDI Helitrox CCR cert is for 150' with helitrox (no limit on the END, but 25/20 is the DIL so you're really limited on END to about 110').

Things to think about...

The Prism is an eCCR with a solenoid. The Revo comes in two basic flavors, an mCCR with a leaky valve or an hCCR with both a solenoid and a leaky valve. There are differences between a solenoid and a leaky valve, and personally, I think learning to fly an mCCR with a leaky valve first (or an hCCR but not using the solenoid) will make you a better CCR diver in the long run.

[snip]

BTW -- grab yourself a copy of the Bozanic book. It's worth reading.

Thank you, Ken!

On the END limit, my calculation for using 20% Helium is 132' would give me an END of 100' (treating O2 as narcotic also). So, with Helitrox Dil CCR, I could dive to the limits of normal recreational diving. Treating only N2 as narcotic, the END limit would allow me to go even deeper. Am I missing something? What I would LIKE to be able to do is dive to normal rec limits (130') and, eventually, do some deco on dives to that limit, before I have to step up to the full mixed gas training. I was thinking Helitrox Dil CCR would let me do that.

I have learned enough about CCRs over the last year that I actually understand your paragraph about the solenoid versus leaky valve.

But, for some reason I had the impression that you can get a rEvo configured as an actual eCCR. Is that not true?

I am only waiting on an email back from Best Publishing to clarify the offerings on their website and I will be ordering Bozanic's book.

 

[tbone1004]

I think the revo only comes as an hCCR.
any eCCR can be configured as an hCCR by the addition of some sort of leaky valve whether CMF or needle. It just swaps in for the O2 MAV. With a CMF you need to block off the O2 first stage and make sure it is a diaphragm to do it *Prism IIRC comes with pistons, so you'd need to get a new first stage*, but with a needle valve you just buy one from Chris Kennedy and plumb it in as normal in lieu of the stock O2 MAV.
conversely any hCCR can become an eCCR by removal of whatever sort of leaky valve it has and put a normal O2 MAV in place.

None of these are permanent or particularly expensive mods to the rebreather *except going to a CMF if you need a new first stage*.

i.e., if you buy a needle valve MAV from Chris Kennedy, you can turn any eCCR you use into a needle valve hCCR in all of 3 minutes.

 

[kensuf]

Thank you, Ken!

On the END limit, my calculation for using 20% Helium is 132' would give me an END of 100' (treating O2 as narcotic also). So, with Helitrox Dil CCR, I could dive to the limits of normal recreational diving. Treating only N2 as narcotic, the END limit would allow me to go even deeper. Am I missing something? What I would LIKE to be able to do is dive to normal rec limits (130') and, eventually, do some deco on dives to that limit, before I have to step up to the full mixed gas training. I was thinking Helitrox Dil CCR would let me do that.

I have learned enough about CCRs over the last year that I actually understand your paragraph about the solenoid versus leaky valve.

But, for some reason I had the impression that you can get a rEvo configured as an actual eCCR. Is that not true?

I am only waiting on an email back from Best Publishing to clarify the offerings on their website and I will be ordering Bozanic's book.

I consider O2 as narcotic as N2 and the way I do END calcs is based on the fraction of O2+N2 in the mixture. 25/20 at 150' = 113'.

OK, let's talk Revo's and solenoids for a second.

Here's the Revo manual. http://www.revo-rebreathers.com/wp-content/uploads/2016/02/Manual_rEvo_III_CE_2014_v05_ENG.pdf

My understanding is when you buy a Revo with the setpoint controller (solenoid) you still get the leaky valve with the unit, hence it has both and we call it a Hybrid. Personally, I'm not a fan of hybrids and if I were to buy a Revo, I would probably buy the mCCR model.

An mCCR makes sense -- it's a safe and simple system that forces you to develop good/safe habits. Failure to properly monitor and maintain your PO2 leads to hypoxia and death -- that's a pretty strong motivation right there . But there are some limitations to a CMF driven unit (depth, potential to have the orifice get jammed up, too high an INP on the blocked reg, etc).

An eCCR also makes sense, but it has some limitations too (do you trust HAL 9000? buoyancy shifts when the solenoid fires, solenoid firing when you don't want it to causing a oxygen spike -- such as going over a hill, etc). Diving an eCCR as if it were an mCCR and controlling your PO2 by manually adding Oxygen is a good way to do it. And the benefit of being on an eCCR is that if your PO2 drops below your programmed setpoint, it'll act like a parachute and fire. In practice, the way this works is I will manually maintain a PO2 0.1 to 0.2 higher than my setpoint. So if I want to do a dive and maintain a 1.2 SP, I will program the solenoid to fire at a 1.0 and then just keep the PO2 up by manually injecting O2 when it's time.

The solenoid as a parachute is a good thing, and I can understand the desire to add a solenoid to an mCCR in order to gain that parachute, but in my opinion, adding a CMF to an eCCR is a waste of time and does nothing beneficial for you but add the negatives of diving a CMF based unit.

Please understand, my views are based on my limited CCR experience (about 400 hours, 250 of which were on a strict mCCR and 150 on a pure eCCR), so others with more experience, especially on hybrid units, may have a different viewpoint.

 

[tbone1004]

@kensuf

so genuine question. I'm not a fan of CMF's on principal, so this is restricted to needles for my line, but could be orifice driven for others. Needles remove the three limitations that you listed by being able to adjust the orifice as IP changes, and if it gets jammed, just open it all the way for a second to clear it as well as screw it all the way in to stop it completely. Downside is having to adjust it with depth if you want to minimize use of the MAV.

What is to stop you from telling the computer to keep you at 0.8-1.0 or whatever you want as a parachute, and instead of having to hit the button, you have the leaky valve set to keep you at whatever setpoint you want and hit the button as necessary? or are you saying there is no point in having the pair of parachutes and if you're going to pay for a solenoid you may as well use it?

The orifices are doing the same thing as the solenoid in terms of maintaining a "parachute" setpoint, with CMF being passive, needle being semi-passive/interactive, and the solenoid being active.

 

[kensuf]

I only have 12 minutes before my next meeting, so I'll try to get this all in there quick.

TL;DR: Needle valves are great on fixed IP first stages (blocked), but just say no on unblocked first stages.

Long version:

Let's talk about an orifice for a sec.

It allows a fixed amount of gas (molecules) to come through the hole.

Let's talk about needle valves for a minute.

They allow you to adjust the rate of flow by adjusting the diameter, which means you can adjust the amount of molecules coming into the loop. If you need more oxygen, you can open the valve more. Need less, you can close it off. This is a neat feature to get on a mCCR, and my friends that are diving the Fathom mCCR love their needle valve because they can adjust the flow based on workload.

Now let's talk about blocking off the IP versus not blocking off the IP with an orifice.

On a standard regulator, as you descend your IP goes up on your regulator which allows you to continue to put the same VOLUME of gas through the hose, but you get more molecules of gas in the process. On a CCR delivering Oxygen through an orifice, if the IP goes up, you will get more MOLECULES of gas through the orifice. At a depth of 33' you're getting twice as many molecules of Oxygen as you were on the surface. Let's go to 100', and now you're getting 4 times as many molecules.

Sound like a bad idea? Yeah, that's why people block the first stage so it has a fixed IP and the amount of molecules stays constant, regardless of depth, until you reach a depth that's about equivalent of 1.5ATA less than your IP (in BAR). So if you have your IP set for 10 bar, you can probably continue to get a flow of Oxygen down to about 8.5 ATA, or so.

Some may say "but now I'm depth limited!" and they'd be right. So the solution may seem to get a needle valve, unblock the first stage, and allow the IP to go up, and just adjust the needle valve accordingly.

The thing is, with this strategy, any change in your atmospheric pressure will require you to adjust the needle valve.

How many times do you do a dive where you're staying at just one depth the entire time?

Even a depth change of 10' is a 0.3 ATA change -- that may be enough to have a significant impact on your needle valve when your IP swings.

I don't know about you, but I view constant fiddling with a needle valve for any depth change to open up a variety of potential problems --

1. Could open too much, accidentally. This would be significant at great depths (300', yikes!)

2. Could fiddle with the needle valve enough that the locking mechanism (I hope your needle valve has a locking mechanism!) doesn't engage properly and your needle valve changes setting on you.

 

[tbone1004]

@kensuf you forgot to factor in the IP for your numbers. The adjustment is actually fairly small because of the initial IP. At say 135psi of a standard first stage, it doesn't go up 100% at 33ft, 200% at 99ft, and 400% at 264ft. It goes up 10%, 50%, 100% at those depths.

With that, in say a "shallow" dive in HITW, JB, or Ginnie, you can set it when you get to 100ft and leave it alone until you get back on deco. You just have to hit the button slightly more frequently at 70ft than you did at 100ft. Still considerably less than an eCCR in parachute mode where all of the O2 add is done manually though obviously a bit more than if you had a CMF. Adjusting for workload is essentially the same, you can just leave it where it is and hit the button a bit more often, or you can adjust to the situation. Pros and cons and based on if you want to adjust the needle or hit the button I guess.

Points on opening too much and the failure modes are obviously valid, just have to be weighed against a dedicated first stage that is depth limited and the risk of the cmf getting blocked. Especially when turning an eCCR into a hybrid where the depth limitation would require a separate o2 source for the solenoid since we aren't all as lucky as Brett and can use custom beryllium spring to get an IP of like 250psi out of his first stage.

 

[kensuf]

Dude, you need to stop.

Anyone with $30 can dive a byrillium spring.

Tec Equipment für Nitrox, Rebreather Höhlen Tauchen

Part #T0200A.

I had one in my KISS.

 

[stuartv]

I consider O2 as narcotic as N2 and the way I do END calcs is based on the fraction of O2+N2 in the mixture. 25/20 at 150' = 113'.

Gotcha. I misunderstood your previous post.

I thought this:

25/20 is the DIL so you're really limited on END to about 110').

meant that with 25/20 I would only be able to dive to a max of 110' before I exceeded my END limit of 100'.

I see we are on the same page. I.e. With 25/20, I could dive to 132' and have an END of 100'. And that would be fine with me. I would anticipate wanting to do a lot of my "experience" dives on CCR, once I'm trained and ready to leave the quarry, to be NC wreck dives that are normally NDL dives in the 80 - 120 range. With Helitrox CCR, I could spend a lot of time doing that before I would start feeling chapped by any limitations.

Also, thank you to you and @tbone1004 for the back-and-forth discussion. I think I'm generally following it and "pickin' up what you're puttin' down."

I definitely appreciate what you said about flying an eCCR manually and using the controller setpoint as a parachute. That definitely makes sense. It seems like a skill that you would actually HAVE to use if your controller died and you only had a backup Shearwater with O2 sensor readouts but that was not a controller. So, developing that skill by flying the unit that way as a normal practice doesn't sound like it has a downside.

I don't know yet whether my shop delivers and trains the rEvo as an eCCR or hCCR. I'm pretty sure it's not an mCCR. I guess I'll find out pretty soon!