Deco Question

[Line Squirrel]

I was wondering about how the rest of you guys deco out on a CCR? And I guess I'm talking more the 20ft, 15ft, 10ft stops (depending on your philosophy).

Do you stay on the loop or switch to OC?

I noticed, actually it was taught in class to expect it, that's it's pretty much impossible to maintain a 1.6po2 for a couple reasons. The primary one obviously is that you are actually off gassing into your loop.

[Randy Thornton]

I don't really know anyone who switches to OC from CCR for deco. Since you have a constant setpoint with eCCR, there really isn't any benefit to come off of the loop. Choose your set point, and you're there. More efficient, less hassle, and you stay warmer and more comfortable. Also, you don't have to worry about gas switching issues that come with OC. Most people I know, don't even change their set point during deco, but just manually add O2 if they want something higher than their set point.

[Luxrok]

Mike,

I set my last stop at 20ft on the ccr, little bit easier to maintain the higher PO2. If I bail out to OC, then I set the last stop at 10ft. Your right about maintaining a 1.6. I try for a 1.4. I flush the loop regularly with O2 to remove the inert gases in the loop. I make sure I keep the ADV shutoff to keep the ADV from allowing DIL in.

Because, I maintain a consistently higher PO2 than OC for the bulk of the dive, I don't try to get to that extra high PO2 at the end of the dive. I don't see the benefit of struggling with the unit and using all that O2.

We have discussed decoing on OC O2 to speed things along, but then you use a lot more volume for not that much benefit. On a typical 220ft dive the difference in deco time is like 4 minutes. I don't think it is worth the benefit of using all that O2 and exposing yourself to a 1.6PO2. And plus coming off the loop you have to breath cold dry gas. Yuck.

Hans

[Line Squirrel]

Hey guys, thank you for the replies.


Since you have a constant set point with eCCR, there really isn't any benefit to come off of the loop.

Well, on o/c O2 at 20ffw you can easily maintain a 1.6PO2, right now, on c/c I have a difficult time maintaining a 1.4 and I go through the O2 like crazy flushing the loop.

It's funny, I say "like crazy" but it's still so much less than breathing O2 on o/c :)



Because, I maintain a consistently higher PO2 than OC for the bulk of the dive, I don't try to get to that extra high PO2 at the end of the dive. I don't see the benefit of struggling with the unit and using all that O2.


O/C I plan my max PO2 at 1.2 during the working portion of the dive. On C/C I've got my set point at 1.1 and fly it manually at 1.2 but then theres that +/- .2 factor in the electronics to consider so I could be running as high as a 1.4 and not know it.

I've been looking at the numbers diving with a lower PO2, say around 1.0 - 1.1 and you don't ad a significant amount of time to your deco but you do however dramatically lower your PO2 exposure. I'm really considering running a 1.0 for the working portion of the dive and cranking it up a bit for the ride out.

[Randy Thornton]

Hey guys, thank you for the replies.



Well, on o/c O2 at 20ffw you can easily maintain a 1.6PO2, right now, on c/c I have a difficult time maintaining a 1.4 and I go through the O2 like crazy flushing the loop.

It's funny, I say "like crazy" but it's still so much less than breathing O2 on o/c :)



O/C I plan my max PO2 at 1.2 during the working portion of the dive. On C/C I've got my set point at 1.1 and fly it manually at 1.2 but then theres that +/- .2 factor in the electronics to consider so I could be running as high as a 1.4 and not know it.

I've been looking at the numbers diving with a lower PO2, say around 1.0 - 1.1 and you don't ad a significant amount of time to your deco but you do however dramatically lower your PO2 exposure. I'm really considering running a 1.0 for the working portion of the dive and cranking it up a bit for the ride out.

Yeah, I know what you mean about trying to maintain the high PO2 at deco, but I feel similarly to what the previous poster mentioned. On CCR, I do not feel the same need to maintain a 1.6 at deco. In fact, on CCR, usually the only time I will hit 1.6 on deco is when I am testing my cells to see if they are current limited. The difference in deco time between 1.4, 1.5 & 1.6 is fairly minimal. If I push the PO2 at deco to 1.6 for long periods on CCR I typically will not feel as good after the dive, which for me is the bottom line in the decision making process.

When cave diving, I have been running nothing greater than 1.2 for the bottom portion of the dive. If I am doing long, multiple dives over multiple days, I will kick it back to 1.1 or less, but I have found 1.2 works most of the time and doesn't get me too close to the CNS clock limits. That said, obviously each dive needs to be planned on its own merits. High work load = lower PO2 (somewhat)

Happy diving,
Randy

[Line Squirrel]

Good stuff Randy - Thank you.

[Luxrok]

Just got back from an awesome dive down stream at Car Wash. Nando and I visited the end of the line and then the other line near the sign. Both dives were sidemount only for long portions. Very beautiful!

Like Randy said, I choose my PO2 based on the dive. Most of my diving can be done at a .7. If it needs to be higher, then I go to a 1.2. But that is mainly because I carry 1.2 tables and that is what we decided on. I will also use Nitrox as a diluent to squelch the swings in PO2. When I use nitrox, it is usually 28%. It really helps when the caves have a nasty saw tooth profile and I have to adjust loop volume often.

Most dives I fly my unit manually a little higher then the set point in my computer. I find that interacting with the unit keeps me engaged and saves batteries. ;) If I am really busy, I will let the machine take over.

I also have to agree with Randy about how I feel after a dive. High PO2 exposure for long periods make my lungs feel like crap and makes me congested. For some reason, I get a lot of snot and nasal congestion with long periods of high P02 exposure.

Hans

[Mike Edmonston]

Mike,

Something to think about is that depending on how many dives you're going to do on your breather that day, and how close you are to a filling station (or transfer whip), you might want to do your 20' stop on OC o2.

Draining your o2 on the breather during deco might be more of a PITA, unless you have another bottle for your next dive. Let's say you're diving the nest, and you deco on CC, after your first dive you're going to need to refill/replace your o2, whereas going to a AL40 of o2 hanging on the log would give you another dive on the meg without switching out anything.

Just food for thought. YMMV.

Safe diving bud,

[Line Squirrel]

Most dives I fly my unit manually a little higher then the set point in my computer. I find that interacting with the unit keeps me engaged and saves batteries.
Hans

Exactly what I do and for both the same reasons!



I also have to agree with Randy about how I feel after a dive. High PO2 exposure for long periods make my lungs feel like crap and makes me congested.

I ran my PO2 a little on the high side for the LR dive to the well casing the other day (135 min run time). I had a nasty dry cough for no apparent reason when I woke up the next morning. I wonder if that could have been the reason. I would have thought the cough would start up immediately after the dive if it was due to excessive O2 exposure???




Just food for thought.


Mike - That's some good stuff too! I don't have two sets of cylinders yet for my breather but I plan on getting a second set. I already have an extra set of the male side Shadow Mounts <G>

In the mean time, your point(s) are something to consider.

I need a baby booster :mrgreen:

[Luxrok]

I would have thought the cough would start up immediately after the dive if it was due to excessive O2 exposure???

That is one of the manifestations for me. It is an irritation in the airway. I find that I manifest those symptoms the day after. During the dive it is snot and congestion.

Hans

[FW]

I need a baby booster :mrgreen:
Or a gas switch block. You could plug in the offboard O2 on deco. Just don't forget, and go back down for something on pure O2 :roll:

[Line Squirrel]

Great thread guys! Thank you.

[runawaylobster]

The fact that you are offgassing makes maintaining 1.6 very difficult. Imagine all that Nitrogen or Helium you are breathing out.

I haven't done any dives switching to O2 for deco yet, but I did think about it for a dive on
the Lowrance recently but I ended up sitting out the dive (no trimix for sale in Miami :(...)


anyhow so far so good, I haven't needed to.

LL

[netmage]

Mike,

Lots of good stuff in the thread... the meg makes it very easy to jack in offboard gasses,... so for that eagles nest scenerio Mike E. mentioned, jack in a 40 of O2 for the long hang and manually maintain higher, leaving more residual....

Also, an LP120 boosted to 3000psi w/ O2, and an LP108 or two w/ 21/35 (for what I do, or whatever is appropriate for the nest and a transfill whip makes quick work of prepping tanks for the next dive on a liveaboard or remote location....

Also - forget about saving batteries... when I'm up in the area I'll bring my battery kit and give you some busy work... let you whip up some AA's or LithiumC's.... I'm sure you have soldering skills... the spot welder just makes life allot easier (http://www.flickr.com/photos/netmage/sets/72157604117539266/)

I experienced the lung irritation on a liveaboard trip recently. Multiple dives every day, and by day three I had a slight dry cough... Made more of a conscious effort to balance setpoint w/ deco benefit... Pushing 1.2 on a 60' dive didn't make much sense, whereas the benefit was there on the deeper reefs.

I would not expect a single ~2:15 exposure to be enough to cause pulmonary symptoms unless you were pushing 1.4-1.6 the entire time. If you were diving allot the days prior, this should also be considered. I haven't seen formal guidance in textbooks on the subject other than 'if your doing multiple days-lots of diving - drop the setpoint', was flipping the pages on my Oxygen Hacker's, and bam, right on page 142... table 11, REPEX Exposure Limits in OTU's credited to Dr. Bill Hamilton, 1989...

[MengTze]

Mike,
My last stop is 20ft and usually run the main part of the dive at 1.1. With longer dives (4hrs) it does make a difference. I found it easier to do a good reverse O2 flush by compressing (bear hug) the lungs with your left arm while injecting O2. Then subsequently adding O2 to make up for volume. I usually end up decoing out between 1.4 and 1.5. Just watch your PO2 as you offgass into the lungs.

[Line Squirrel]

Tim,

Gregg was going to show us how to make batteries we just ran out of time...even though the shortest day of eight was 11-1/2hrs long!!!!

Can you get me a materials list with specifics (I'm not much of an electronics guy) but you need to spell out exactly what type of solder/flux, pigtails, etc.

I'll try and pick up what I can and I would be grateful if you showed me how.

Gregg is more than willing, just he's 1-1/2hrs away.

[netmage]

Tim,

Gregg was going to show us how to make batteries we just ran out of time...even though the shortest day of eight was 11-1/2hrs long!!!!

Can you get me a materials list with specifics (I'm not much of an electronics guy) but you need to spell out exactly what type of solder/flux, pigtails, etc.

I'll try and pick up what I can and I would be grateful if you showed me how.

Gregg is more than willing, just he's 1-1/2hrs away.

I have it all - just have a soldering iron - or hell, I can bring mine along... it's a radio shack cheapo...

[PacketSniffer]

Just some more data points from another CCR diver.

-Yes, maintaining a relatively higher setpoint on the shallow deco stops will be hard to obtain in the *beginning* of deco. If you watch carefully, you'll notice closer to the end of each stop, the PO2 will be easier to maintain. As everyone else said, it helps to flush often. I flush at least when reaching a new stop after a few minutes (near the shallow stops). At the end of your 10 or 20 foot stop, you should have a very stable PO2. That's one sign to look for that your deco is "thorough".

Another thing that helps tremendously is minimum loop volume. By keeping no more than what is needed for one full breath, you're not diluting the O2 as much. This will become second nature after so many dives.

- I never deco out on OC.

- I have typically run 1.2, the entire dive, for almost all of my dives. Lately, I've been using 1.0 going in and switching to 1.3 on the turnaround for long dives (2~3 hours).

- I've never experienced dry cough or lung irritation from the RB even during multi-dive liveaboard trips that were four and five days long (diving "non-stop"). To add, honestly on a 4 or 5 day trip, I'll start with clean lungs (using Virkon S) and wash them mid-way through and that's it. Same when cave diving; wash them mid-week on a 5 day trip unless something unusual happened that warrants a wash.

- Get some lithium cells (either 2 C primary cells or rechargeable AA Ni-Mh cells). I had two primary C cells that lasted at least 6 months (lots of diving) until ISC yanked them out during a firmware upgrade and then charged me for new batteries. lol ....@#%@$#%

[tflaris]

I was wondering about how the rest of you guys deco out on a CCR? And I guess I'm talking more the 20ft, 15ft, 10ft stops (depending on your philosophy).

Do you stay on the loop or switch to OC?

I noticed, actually it was taught in class to expect it, that's it's pretty much impossible to maintain a 1.6po2 for a couple reasons. The primary one obviously is that you are actually off gassing into your loop.


If you love your Meg (what at a cruel mistress she is) you will stay on the lop for a couple of reasons:

1: You now hate open circuit and dread going to it unless you have to
2: Your mistress will become jealous

Seriously

By using the rebrether you are maintining a higher setpoint therefore will have reduced deco obilagationa at the end of the dive.

Using the 1.6 setpoint on OC allowed the diver to start the offgassing process faster by switching to richer mixtures sooner.

Just my 2 cents. Stay on the loop. It is much warmer than OC. No bubbles.

[Line Squirrel]

Who, BTW, was the one that dumped all that sand & gravel on my head during our deco??? Was that you or the other goof <G>.

[tflaris]

Who, BTW, was the one that dumped all that sand & gravel on my head during our deco??? Was that you or the other goof <G>.


That was your buddy from Wakula Dive Center. I had no part of that but I did enjoy it.

[Line Squirrel]

That was your buddy from Wakula Dive Center. I had no part of that but I did enjoy it.

lol - next time he's losing all his reels at deco. I've stripped more than one dive buddy of half their gear on deco :mrgreen:

[MORGAN]

lol - next time he's losing all his reels at deco. I've stripped more than one dive buddy of half their gear on deco :mrgreen:

Probably even easier to be stealthy and pick off your buddy's reels when you're not making any bubbles!

Mike

[BillGraham]

I also just deco at my bottom setpoint, I don't like giving myself extra stuff to do if I can avoid it.

[Bob Cree]

I was wondering about how the rest of you guys deco out on a CCR? And I guess I'm talking more the 20ft, 15ft, 10ft stops (depending on your philosophy).

Regarding deeper dives like at the Nest and the Deco that accompanies. As has been pointed, CCR deco "time" is definitely improved over OC because you can push the PO2 up all the way through, but what about O2 toxicity? How do you deal with exceeding 100% of your CNS limits which you will do if you are taking advantage of the RB over OC.

Example, on OC, for dives I do at the Nest upstream, I get 25 minutes of bottom time before I hit the 100% CNS limit during deco using 15/40 bottom gas + 50% and 100% O2 for deco and I am turning before 1/3rds. For this dive I actually hit 104% on the CNS clock.

Now - same dive, same bottom mix for comparison sake but now deco using a constant PO2 of 1.2 all the way up and I still just hit my 100% CNS limit at the end of deco. Virtually no difference on the O2 toxicity side, although I do gain 11 minutes on deco (91 minutes of total deco versus 102 min for the same OC dive - use decoplanner 20/80 GF's).

So, are we all just blowing off the CNS clock and exceeding 100%? If you reduce the PO2 setpoint to drop the total CNS% for the dive, you lose the deco time advantage...so,

Back to mmcauliffe...I would focus the question - what do you do during deco to reduce the risk of O2 tox for longer than OC dives like at the Nest? Air Breaks? How do you use them CCR?

[netmage]

Regarding deeper dives like at the Nest and the Deco that accompanies. As has been pointed, CCR deco "time" is definitely improved over OC because you can push the PO2 up all the way through, but what about O2 toxicity? How do you deal with exceeding 100% of your CNS limits which you will do if you are taking advantage of the RB over OC.

Example, on OC, for dives I do at the Nest upstream, I get 25 minutes of bottom time before I hit the 100% CNS limit during deco using 15/40 bottom gas + 50% and 100% O2 for deco and I am turning before 1/3rds. For this dive I actually hit 104% on the CNS clock.

Now - same dive, same bottom mix for comparison sake but now deco using a constant PO2 of 1.2 all the way up and I still just hit my 100% CNS limit at the end of deco. Virtually no difference on the O2 toxicity side, although I do gain 11 minutes on deco (91 minutes of total deco versus 102 min for the same OC dive - use decoplanner 20/80 GF's).

So, are we all just blowing off the CNS clock and exceeding 100%? If you reduce the PO2 setpoint to drop the total CNS% for the dive, you lose the deco time advantage...so,

Back to mmcauliffe...I would focus the question - what do you do during deco to reduce the risk of O2 tox for longer than OC dives like at the Nest? Air Breaks? How do you use them CCR?

Select an appropriate setpoint for the duration of the dive... 1.2ppO2 gives you 3.5 hours single exposure..., 1.0ppO2 gives 5 hours.

Or flip between setpoints... lower on the bottom, higher on ascent...

[Bob Cree]

Select an appropriate setpoint for the duration of the dive... 1.2ppO2 gives you 3.5 hours single exposure..., 1.0ppO2 gives 5 hours.

Or flip between setpoints... lower on the bottom, higher on ascent...

OK - say I select 1.0ppO2 then for this Eagles Nest Upstream 25 minute BT, I start my deco with 9% on the CNS O2 clock - BTW, selecting 15/40 gives only 17% on the CNS clock going into deco, so not much difference since BT is short...most of the CNS clock ticks go by while in deco, not on the dive.

And deco adds over 100% to the CNS clock somewhere around 25-30 minutes of bottom time even for the most optimum setting of constant ppO2 (happens to be around 1.4ppO2 is optimum for deco and minimizing resultant deco addition to the O2 tox clock).

So you are going to go over the tox clock if you are really using the RB to its capabilities...back to my expanded CCR deco question...how do you handle O2 tox issues during deco?

[netmage]

OK - say I select 1.0ppO2 then for this Eagles Nest Upstream 25 minute BT, I start my deco with 9% on the CNS O2 clock - BTW, selecting 15/40 gives only 17% on the CNS clock going into deco, so not much difference since BT is short...most of the CNS clock ticks go by while in deco, not on the dive.

And deco adds over 100% to the CNS clock somewhere around 25-30 minutes of bottom time even for the most optimum setting of constant ppO2 (happens to be around 1.4ppO2 is optimum for deco and minimizing resultant deco addition to the O2 tox clock).

So you are going to go over the tox clock if you are really using the RB to its capabilities...back to my expanded CCR deco question...how do you handle O2 tox issues during deco?

CNS is always a concerns, the same rules\rates apply, even more so because of the fixed pO2, and resulting CNS uptake rate thruout the dive.

re: "really using the RB to its capabilities"

In deeper/longer dives like this, deco optimization tends to not top the list... Logistics, helium $$$, and time pressure sit closer to the top.

[jpdiver]

I always do my CCR deco on the loop at 1.6 (or around 1.55 in reality). On most cave dives most of the deco is at 30 and 20 feet so I will get my loop at 1.55 at 30 feet. I pull the 10' stop down to 20'. I think that gives me the widest O2 window and cleanest deco.

On deeper mix dives, I put my loop at 1.55 at the 70' deco stop and try to maintain that for the duration of the deco. That is assuming an easy ascent.

[ed pavey]

Mike,

Something to think about is that depending on how many dives you're going to do on your breather that day, and how close you are to a filling station (or transfer whip), you might want to do your 20' stop on OC o2.

Draining your o2 on the breather during deco might be more of a PITA, unless you have another bottle for your next dive. Let's say you're diving the nest, and you deco on CC, after your first dive you're going to need to refill/replace your o2, whereas going to a AL40 of o2 hanging on the log would give you another dive on the meg without switching out anything.

Just food for thought. YMMV.

Safe diving bud,


I am wondering why you can not plug your 40 into the loop and manually add?
Just a question.. I'm a novice..

[Bob Cree]

Let me restate my point a different way. On deeper dives in particular, deco induces "time pressure" because of CNS toxiticity limits that will occur during deco if you increase your BT beyond normal OC bottom times.

I am going to go out on a limb here and suggest that many of us want to use a RB to get more BT as the primary reason for purchasing. One can make a heck of alot of OC dives with all the $$$ for HE and never spend near what would be spent on a decent RB...

So - do the Eagles Nest dive and extend the BT. But now you have >100% on the CNS clock pop up during deco. Perhaps you wouldn't do this, but if you do, how do you handle the situation?

...or does everybody essentially do an easily OC capable dive with a RB?

[Jeff H]

So - do the Eagles Nest dive and extend the BT. But now you have >100% on the CNS clock pop up during deco. Perhaps you wouldn't do this, but if you do, how do you handle the situation?

...or does everybody essentially do an easily OC capable dive with a RB?

Just an FYI, but a lot of people will pull a 40+ min bt on OC in those depth ranges vs. the proposed 25 min. On the rebreather I generally run between 1.0 and 1.2 constant PPO2. Most deco is done at 1.2 and shallow stops at 1.4. I always do a 15ft and 10 ft stop independent of what I'm diving because there is always a pressure difference between 20 ft and the surface. IMHO the O2 window is only part of the equation. PP of inert gas and overall pressures gradients should also be considered.

I've always consider the true benefits of CCR deco to be the warm moist gas and not having to inspire 100% O2 since it's easier on my respiratory system. If proper deco is done on OC then the time difference is usually negligible.

If additional O2 is needed for longer dives then plugging in a 40 cuft bottle is easy on the meg. I also travel w/ a full 80 of O2 to transfill post dive. I find 2000 psi in my faber 19 is enough for a 210 min dive. The 40 is for backup on deco.

There's lots of benefits to CCR diving beyond deco and bottom times but they will differ based on the type of dives you're doing. One of the biggest CCR benefits for me is the logistics of shipwreck diving and available gas. I also find gas usage a huge benefit in cave country as well as the warm moist air which makes me feel less fatigued post dive.

There aren't really any dives that can't be done on OC, there's just some that are better suited to CCR.

-Jeff

[Nick Mislak]

With dives like the Nest. I usually run a 1.0-1.2 PO2 at depth with 10/50 or similar and carry 32% which I drop at the mound at 120ft and O2 at 30ft or so. I usually run a 1.4 for deco with the mix still in the loop until I flush with 02 at 20.

Nick