Rebreather underwater time?

[Chris]

I've heard very little about rebreather other than no bubbles, and it scrubs what you breath out, adding in a little more oxygen. But I was very curious because the other thing I did hear was you can be underwater for a longer period of time (without surfacing) ? Just wanted to know the average UW time of a rebreather. Sounds cool.

[Tangfish]

(Take this with a mound of salt, b/c I'm just starting to learn about CCRs)

With CCRs, your bottom time is not as much limited by depth and gas consumption (as you are using SCUBA) as you are limited by the amount of scrubber you have with you. So, given a good amount of scrubber and mixtures of gasses - you can typically stay under for 3-5 hours on a dive, regardless of depth (though there are limits to the depths you can reach on given gasses).

John, Curt, Mel (and even Nailer) - did I butcher that?

[Dmitchell]

You've got it right for the most part Calvin.

Yes, on the rebreather, the limiting thing as to how long you can stay down isn't the air in you tank. The limiting factor becomes decompression (how long are you willing to hang), and your bladder!

With a set of 19's on my rebreather, I should have enough gas for about 5 hours depending on some variables like depth changes. My scrubber is rated for 3 hours so yes, I could be underwater for up to 3 hours.

Here's a place for some good reading http://www.jetsam.ca/News/magarticles/magarticles.html
Also of course rebreatherworld.com .

Dave

[Gill Envy]

When I was an OC diver I used to dive based on how much gas I had, then how much NDL I used up, then how cold I got. Diving CCR I have more gas than I want, more than double the NDL and i'm generally warmer since the air is moist and recycled. Now I go by how long I really want to dive, now how long I can. My dives are almost all over an hour and some end up being two hours. some folks do really extended dives for deep wreck or cave diving. My thing has and still is mostly extending NDL's in the recreational limits down to 140 fsw and seeing lots of critters. It's been well worth it. I can typically do two dives in a day without doing anything to my unit since full scrubber and tanks allow for 4+ hours of cold water diving.

[Curt McNamee]

Good job guy's !!!!!!!!

[YellowEye]

I was out at day island the other day after work doing another super-long dive (2hr+), and our limiting factor was that i was getting too hungry!

-Eric

[Chris]

Wow. Amazing to hear about this.. 2 hours underwater! Crazy. Someday I'll have to learn more about this when I'm home.

[Seth T.]

Okay, so if your dive time is mostly limited to your scrubber, what happens when your scrubber gets full? CO2 poisoning?

[Curt McNamee]

They call it a "carbon dioxide hit" and it can be a challenging situation.

When your scubber becomes full of carbon dioxide and cannot remove enough of it out of your breathing loop for normal breathing then you start to have problems, ie- feeling funny, anxious, hard to breath, your throat closing down. Everyone will have their own types of symptoms and will react differently.

When you feel the symptons coming on, you slow down your breathing and call the dive.

The plan is that every rebreather diver should be very conservative on their scubber time so they don't end up in that situation. It is just like planning your air supply on OC, no different.

Don't push your limits and all goes very well.

[Seth T.]

So, is there a real time way to monitor how full your scrubber is (like an SPG with OC) or is it simply an estimated run time?

[Curt McNamee]

So, is there a real time way to monitor how full your scrubber is (like an SPG with OC) or is it simply an estimated run time?

They have been working on a way to monitor this for years, but have not come up with anything that will work underwater.

For now, conservative self monitoring of the time the scrubber has been used is what we do.

[Seth T.]

I see. But as we all know, everyone's body is different. Some people breath like they're hypervenilating, vacuuming air, whereas other people breath like fish. The former would plow through the scrubber a lot faster than the latter.

So, if you wanted a very accurate time rating on the scrubber, would it be foolish to fill up your scrubber, recording the total time, and then switch to a stage bottle for the ascent (given that you're not too deep, obviously)? That way you could know exactly how long the scrubber lasts for you personally, not just a general recommendation?

[Dmitchell]

Co2 production is a result of metabolism not necessarily breathing rate. So that means that generally speaking we all pretty much produce CO2 at a similar rate. That's why you can estimate the duration of a scrubber.

Your CO2 production will increase if you start working hard so you avoid working hard on the RB just like your breathing rate on OC controls you air consumption.

There is of course conservatism built into the time that a scrubber is rated for.

The KISS is rated 3 hours. As the "kitty litter" gets used up it gets damp and clumpy. The KISS scrubber is about 9-10" deep. Since the air enters the scrubber at the bottom and exits the top it's the bottom that gets used first. When I dump the scrubber after 3 hours it's apparent that probably half the scrubber or more is still good since it's still dry and fresh the bottom will be clumpy. But, conservatism keeps you alive and scrubber is cheap so you don't toy with it.


Dave

[YellowEye]

The evo comes w/ a temp stick, which while it doesn't measure co2 directly, gives you a good indication if you're close to breakthru. (co2 scrubbing is exothermic... )

there are many factors to scrubber duration, including depth, temp, scrubber design, sorb age and storage condition...

there aren't necessarily warning signs to hypercapnia at depth, can't rely on that...

[Seth T.]

The KISS is rated 3 hours. As the "kitty litter" gets used up it gets damp and clumpy. The KISS scrubber is about 9-10" deep. Since the air enters the scrubber at the bottom and exits the top it's the bottom that gets used first. When I dump the scrubber after 3 hours it's apparent that probably half the scrubber or more is still good since it's still dry and fresh the bottom will be clumpy. But, conservatism keeps you alive and scrubber is cheap so you don't toy with it.

Dave

So, what's the deal with these crazy dudes doing 400 FSW dives on rebreathers with 4.5 hours of deco on the way up? (Source: Advanced Diver Magazine featured some dudes that recovered the bell from a ship in 120 meters of salt water and had 4.5 hours of deco headed up.)

Do they have some kind of extra large scrubber or a double unit to get these kind of times? So, what happens if you're waaay stinking down there and you get that "CO2 Hit" that Curt mentioned? Are you hooped?

[Joshua Smith]

Radial scrubbers can extend dive time- how much, I'm not sure- 6-8 hours, maybe? And anyone doing these kinds of dives has open-circuit bailout and safety divers, as a rule- a team of 3 would carry enough tanks to bail out 1 and a half divers from whatever depth they planned, for example- if more gas was needed, they would shoot a SMB with a note attached- surface support would then bring down whatever was needed.

[Seth T.]

...if more gas was needed, they would shoot a SMB with a note attached- surface support would then bring down whatever was needed.

Yeah, surface support has got to be crucial for this crazy stuff.

So, am I correct to assume that there would be zero nitrogen narcosis at these depths because the gas they're breathing is simply helium and O2?

[Joshua Smith]

Well, maybe not zero narcosis- I'm not trimix trained, yet, so I'll let someone who knows more answer this one. I'm pretty sure nobody uses a mix with zero nitrogen in it, but now that you mention it, I don't have a clue....

[Seth T.]

Well, maybe not zero narcosis- I'm not trimix trained, yet, so I'll let someone who knows more answer this one. I'm pretty sure nobody uses a mix with zero nitrogen in it, but now that you mention it, I don't have a clue....

That's what heilox is, helium and O2, and is what saturation divers use. I'm pretty sure that hardcore (read: fricking insane) techies use that stuff.

[John Rawlings]

I'm pretty sure nobody uses a mix with zero nitrogen in it, but now that you mention it, I don't have a clue....

Yes, they do. It's called Heliox. There are several interesting effects:

1) You are COMPLETELY clear-headed at depth.
2) You freeze your ass off in cold water due to the heat sapping properties of Helium.
3) Tissue loading is extraordinarily fast as Helium molecules are tiny compared to Nitrogen molecules. Hence, your tissues get saturated faster than they do with Nitrogen.
4) For the same reason, tissue off-loading is also fast and your tissues will DUMP off Helium at a high rate as you ascend, so a slow ascent rate is even MORE important that with Trimix.
5) Your wallet will physically shrink....a phenomenon related to the high cost of Helium!

- John

[Seth T.]

Yes, they do. It's called Heliox.
- John

Sweet. I thought so!

Have you ever dove the stuff John?

[Seth T.]

3) Tissue loading is extraordinarily fast as Helium molecules are tiny compared to Nitrogen molecules. Hence, your tissues get saturated faster than they do with Nitrogen.
4) For the same reason, tissue off-loading is also fast and your tissues will DUMP off Helium at a high rate as you ascend, so a slow ascent rate is even MORE important that with Trimix.

- John

So can you still get "bent" using heliox? Nitrogen free DCS has got to be bizarre.

[John Rawlings]

So can you still get "bent" using heliox? Nitrogen free DCS has got to be bizarre.

DCS isn't a by-product of only Nitrogen. You can get bent on any inert gas that enters your tissues and then has to get out of those tissues as a result of decreasing pressure.

- John

[dsteding]

Yes, they do. It's called Heliox. There are several interesting effects:

1) You are COMPLETELY clear-headed at depth.
2) You freeze your ass off in cold water due to the heat sapping properties of Helium.
3) Tissue loading is extraordinarily fast as Helium molecules are tiny compared to Nitrogen molecules. Hence, your tissues get saturated faster than they do with Nitrogen.
4) For the same reason, tissue off-loading is also fast and your tissues will DUMP off Helium at a high rate as you ascend, so a slow ascent rate is even MORE important that with Trimix.
5) Your wallet will physically shrink....a phenomenon related to the high cost of Helium!

- John

Interesting point re being completely clear-headed, because I've been taught to treat 02 as narcotic. Now, maybe on heliox your END ends up so low because of PPO2 limitations, but it would be something to consider . . .

[John Rawlings]

Have you ever dove the stuff John?

I have used it twice. Both times were training dives while I was at the Florida Institute of Technology in the early 1980's. The dives were done in the Indian River and were basically to familiarize us with Heliox, both as divers and as members of a surface support team, (this was surface-supplied training, rather than scuba). I have never used it outside of a training environment.

During the recovery of the skeletal remains of Russell Warren in Lake Crescent, the National Park Service dive team used 20/80 Heliox. I was an on-site observer throughout their dive planning and the actual dives. They were following then current NPS deep dive protocol.

I believe that Trimix would have been far superior to the 20/80 Heliox blend that they used in the frigid waters of the lake. Down to 200 FFW a simple Normoxic Trimix would have allowed for extremely clear thinking, would not have had NEARLY the heat-sapping properties of Heliox, and as a side-line would have cost the tax-payers FAR less while being safer, in my opinion.

- John