Recreational NDL times ?

[Superlyte27]

In short, yes.

Rebreathers are very appealing to the technical savvy geeks : Engineers mainly.

Uh, no.

---------- Post added January 11th, 2013 at 04:01 PM ----------

Rob, while it is obviously true that keeping your O2 at a constant 1.3 (or whatever) during your dive is possible with a CCR and that will lesson your inert gas loading, the real question is, how much NDL time would a recreational diver actually gain in a dive to, say 100 feet wreck -- i.e., a fairly square profile as opposed to the time on say, 32%.

Not much, but take away "a fairly square profile" from your sentence and the answer is: Possibly, quite a bunch.

 

[Allan C Smith]

I don't think you can quantify the benefit in a broad since. We know that less N2 loading is good and as far as the no stop time goes we have determined it depends on the type of profiles you dive. So as with every thing there are Pro's and Con's. But at the end of the day at least on my part. I want to get in to Reabreathers because I think they are really cool. While there are a lot of benefits on the Tec side. Such as less money spent on breathing media and less tanks to drag around. The measure of the benefit to risk ratio in Rec diving is still a few years from being figured out. Also I would like to add I am not a rebreather diver. Just some one who is very interested in joining there ranks and has a lot of free time to spend on the internet between students at the moment. So if there is an expert to chime in I'm all for it!

 

[Superlyte27]

Allan, again, yes, of course having the constant PPO2 at the CCR set point will reduce inert gas loading. That isn't the real question. The real question (at least as I see it), is how much benefit would one actually get in reducing N2 loading/increasing no stop times, on a typical recreational dive?

While there may be many reasons to go to a CCR for recreational profiles, is no stop time/reducing signficant N2 loading, really one of those reasons?

I would contend that if the OP bought even a recreational rebreather, it wouldn't be long before he found he could do much longer dives (based on the availability of gas). He might decide he likes these longer dives and start pursuing a technical route.

Here's what you really need to ask...
Can you afford a rebreather? They cost a bunch initially, and a bunch to maintain/dive.
Are you good at following check lists? Are you diligent about maintenance? Do you find yourself becoming complacent? Are you ready to take diving to the next level?

Depending on the answer to these questions, you may or may not be ready for a rebreather. But if any of those above questions are answered improperly, don't bother with a rebreather.

 

[Packhorse]

They cost a bunch initially,

Yes they do!

and a bunch to maintain/dive.

No not really.
In fact I would argue that diving a CCR is as cheap as diving nitrox ( based on what nitrox cost in NZ) and if you consider the longer dive times, minute for minute the CCR is cheaper by far when you consider boat charter costs or fuel costs on a personal vessel.

If you then look at the cost of trimix dives CCR wins hands down.

 

[HIGHwing]

It's all about cost efficient run time.

Descend to 100'
100 for :28 (2-minute descent time not included)
090 for :20
080 for :20
070 for :30
050 for :30
020 for :03
Surface at 136 minutes

VPM-B +2 No decompression on the above dive at a constant 1.3 P02.

For the record, I would argue diving a CCR is equally as expensive as regularly diving Nitrox. However, once you load the Dil with Trimix, the rebreather can pay for itself over and over again.

 

[Superlyte27]

Yes they do!


No not really.
In fact I would argue that diving a CCR is as cheap as diving nitrox ( based on what nitrox cost in NZ) and if you consider the longer dive times, minute for minute the CCR is cheaper by far when you consider boat charter costs or fuel costs on a personal vessel.

If you then look at the cost of trimix dives CCR wins hands down.

Nah...
It costs me absolutely nothing to dive Air or Nitrox.
It costs me roughly $65 bucks to make an OC Trimix Dive
It costs me $10/hour to dive my CCR regardless of depth.

So, calculating the initial cost of the rebreather/training/sorb/sensors, etc., etc. How many dives is it going to cost even at $65/dive to pay for the 11k initially invested in a rebreather? 11,000/65 = 169'ish trimix dives to pay for the rebreather. I do about 10 OC trimix dives per year. That's a LONG friggin time to pay for that rebreather.

Now, lets talk about "maintaining the rebreather".
Plan on losing at least one sensor per year. $100 bucks each. I'm in my first year, and I've lost two sensors already.
Plan on losing a set of mushroom valves every year $40 bucks.
Miscellaneous o-rings, opv springs, etc... 50'ish bucks


Now on the flip side...
I've had the same exact rig for my OC setup for 10+ years.
VIP Tanks .... Free, I have a PSI Cert
Hydro Tanks.... $12 dollars x 2 tanks
Rebuilding 3 sets of regs.... 36 dollars in parts.

That's it. Same bcd, same regs, same tanks for a decade.

-edit
I'm reminded of the first five minutes of my class. There I sat with Paul Raemakers, Pete Sotis, and Jim Wyatt (my three instructors for rEvo). The very first bullet point for class that we spent more than 5 minutes on was that "rebreathers aren't cheap". They went on very adamantly that if I'm cheap, like to cut corners, can't afford to buy sorb, sensors, mushroom valves, or like to squeeze every last second of value out of a part, rebreather diving isn't for me. They were explicit in saying, you can't be cheap and be a rebreather diver. It was even stated literally in the powerpoint slide. The point is, rebreathers are expensive, short term and long term.

 

[Packhorse]

Nah...
It costs me absolutely nothing to dive Air or Nitrox.
It costs me roughly $65 bucks to make an OC Trimix Dive
It costs me $10/hour to dive my CCR regardless of depth............

While I can appreciated that what its costs for you where you live it is quite different here.
Air is about $8 Nitrox $20 Tx is close to $100 ( single tank)
Diving my rEvo costs me less than $10 per hour in sorb and O2 but add to that the occasional air fill for DIL and inflation, O2cells, O2 storage tank hire and it gets closer to $20. Servicing is on par with OC diving.

Yes there is a high initial outlay but that is the price we pay for longer better quality dives. I off set this by driving a cheap car .

 

[Dive-aholic]

Rob, while it is obviously true that keeping your O2 at a constant 1.3 (or whatever) during your dive is possible with a CCR and that will lesson your inert gas loading, the real question is, how much NDL time would a recreational diver actually gain in a dive to, say 100 feet wreck -- i.e., a fairly square profile as opposed to the time on say, 32%.

Sorry for the delayed response. I left for Jamaica right after you posted that and have been busy since getting back.

Let's look at an example:

For a square profile dive to 100 ft breathing 32%, the NDL per the NOAA EAN32 table is 30 minutes. This is assuming a 60 fpm descent and a 30 fpm ascent. So it takes 1 minute 40 seconds to get to depth. On a CCR during a descent at that rate you have to be really good to maintain a 1.2 ppO2 and it will essentially do nothing for your NDL. The advantage of a recreational CCR is in multilevel dives, not square profiles.

Now let's look at a square profile dive to 150 ft for 30 minutes breathing 21/35. Here's the profile per V-Planner

Dec to 150ft (3) Triox 21/35 50ft/min descent.
Level 150ft 27:00 (30) Triox 21/35 1.14 ppO2, 69ft ead, 86ft end
Asc to 80ft (32) Triox 21/35 -30ft/min ascent.
Stop at 80ft 0:40 (33) Triox 21/35 0.71 ppO2, 30ft ead, 40ft end
Stop at 70ft 1:00 (34) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 1:00 (35) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 2:00 (37) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 2:00 (39) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 4:00 (43) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 5:00 (48) Oxygen 1.59 ppO2, 0ft ead
Stop at 10ft 11:00 (59) Oxygen 1.29 ppO2, 0ft ead
Surface (59) Oxygen -30ft/min ascent.

How much time do you think a CCR will save you on this dive by keeping the setpoint at 1.2 during the bottom time and 1.6 during decompression? 4 whole minutes:

Dec to 150ft (3) Diluent 21/35 1.20 SetPoint, 50ft/min descent.
Level 150ft 27:00 (30) Diluent 21/35 1.20 SetPoint, 67ft ead, 87ft end
Asc to 70ft (32) Diluent 21/35 1.20 SetPoint, -30ft/min ascent.
Stop at 70ft 0:20 (33) Diluent 21/35 1.60 SetPoint, 1ft ead, 48ft end
Stop at 60ft 1:00 (34) Diluent 21/35 1.60 SetPoint, 0ft ead, 42ft end
Stop at 50ft 1:00 (35) Diluent 21/35 1.60 SetPoint, 0ft ead, 37ft end
Stop at 40ft 2:00 (37) Diluent 21/35 1.60 SetPoint, 0ft ead, 31ft end
Stop at 30ft 3:00 (40) Diluent 21/35 1.60 SetPoint, 0ft ead, 26ft end
Stop at 20ft 4:00 (44) Diluent 21/35 1.59 (1.60), 0ft ead
Stop at 10ft 11:00 (55) Diluent 21/35 1.29 (1.60), 0ft ead
Surface (55) Diluent 21/35 -30ft/min ascent.

The only difference between these dives is during the 60, 50, 40, and 30 foot stops the CCR diver is breathing a 1.6 O2 mix while the OC diver is not.

Increase the bottom time to 60 minutes and the difference is still only 11 minutes.

A lot of people believe that CCR means you save tons of time on deco but it doesn't. It means you save a few minutes here and there. Even a 30 minute dive to 250' only gets 20 minutes shaved off by a CCR versus OC. There are benefits to rebreathers, but saving a lot of time on deco is not a major one.

 

[HIGHwing]

A lot of people believe that CCR means you save tons of time on deco but it doesn't. It means you save a few minutes here and there. Even a 30 minute dive to 250' only gets 20 minutes shaved off by a CCR versus OC. There are benefits to rebreathers, but saving a lot of time on deco is not a major one.

To be quite fair your example somewhat ignores the sweetspot of a rebreather. In conducting a RT vs RT comparison using a precisely square dive and coming to a generalized conclusion as it applies to diving requires the assumption most dives are square to be true.

So let's say we're wall diving instead of wreck diving dragging the sand for the entire dive. If we make a very similar dive and accept a multi-level strategy the total run time could be drastically different OC v CCR. As you no doubt know, there are all sorts of ways to tweak the numbers to make one tool look more or less efficient. I once saw an example where someone compared a 200ft dive OC using Best Mix for Bottom, Travel Gas, 50% and 100% deco gas versus a rebreather at a constant 1.2 PO2 (yes even for deco). The room of unknowledgable divers were shocked and amazed the CCR diver required more deco.

When I look at dive profiles, what I generally see is most divers go deep first and then work their way up the water column in some fashion thoughtout the dive until the eventual ascent. While this isn't always absolute, I think most people naturally multi-level to some degree because that's where the cave goes, the wreck has more than props to see, the air supply is running low, NDLs are approaching or whatever. Looking at the online repository of dive logs appears to support my theory, though I'm not going to count through thousands of dives to come up with a number.

I think what can be said is we are at an exciting time in dive technology as their is a right tool for the job available.

 

[Dive-aholic]

I agree. If you go back and read my previous post you'll see that I stated the advantage of a CCR is in multilevel dives not square profiles. Now, I did include the word recreational there because it was specifically in reference to a question that was asked about recreational dives. There definitely are advantages to CCRs. But the advantages are not what most people think they are. I own 2 SCRs, one of which is convertible to a CCR. I may be purchasing a dedicated CCR in the next year as well. I still do a majority of my diving on OC. I use the rebreathers for specific dives in which they are required or in which the advantage makes it desirable. But I dive OC enough that I still have a low enough RMV to do a 2.5 hour long dive in the 90 foot depth range with only 1 stage. And I really don't want that to change!