Northwest Dive Club

One thing that still hasnt been clear to me, is gas choice. I understand the basic 50 vs 80+ wrt to deco time, but what other factors are involved in the gas choice. For a deeper stop of course, you can switch to 50 at 60-70ft, then switch to 80+ at 30'ish, but that assumes you want to carry 2 rich gases. If you dont want to carry 2 rich gases, do you just carry 50 and use for stops from 60 up(and longer deco), or carry 80+, and just switch to that gas above 30(depending on mix of course)?

thx.

Curious, what was the explanation given during your deco class? I thought no one taught use of 80% for your final deco gas anymore.

I'll start by saying I don't use 80%.

Total deco time from most dives above 200 feet is going to be very similar, whether you use a Bulmann profile and push shallow early, and switch to a rich gas at shallow depth and sit there, or whether you switch to a less rich mix deeper. The big difference is the rock bottom. If you have to calculate enough gas to get yourself and your buddy from your target depth to 20 or 30 feet (your first deco gas) you have a HUGE rock bottom. If you only have to get to 70, it reduces the rock bottom requirements.

fishb0y wrote:Curious, what was the explanation given during your deco class? I thought no one taught use of 80% for your final deco gas anymore.

80+ I said, so anywhere from 80-100% I guess. In my deco class we only ran 50% off of a tech dive, but I had to bring some bottles of 80% for the class but never used them.

Personally, I only see 50% and 100% as usable deco gasses but thought I'd ask about a lesser than 100% gas.

Why would you even want to use anything other than 50% or 100% for deco stops shallower than 71 fsw?

+1 on what Lynne said.

practically, it doesn't buy you much to deco on 80%, in fact you will spend more time and use more gas..

Its seriously more effort than what its worth in mixing, gas planning, deco profiles, etc. and only marginally more effective than 50% and less efficient than pure O2.. so keeping things as simple and efficient as possible, O2 is just a better choice when combined with 50% as a second deco gas.

For most dives of a bottom time of 20-30min at 170ft or shallower, I wont even bring O2, its more a hassle than what its worth.

If chose to take something like 80%, the other factor is that you because you can now start breathing this mix deeper, you will use more of it and will need to carry more of it and again practically it doesnt make sense..

If you are diving a bottom gas of for example 18% to 21% (oxygen) at 150-200 feet, your real deco doesnt really start racking up any significant stop time below 70ft and the benefit you will get to use 50% from 70-40ft and then 80% from 30-0ft will be very negligible, You might well as have just stayed on 50% all the way and not have to deal with gas switches, caarrying extra tanks, etc, etc.

And quite honestly, even adding a second deco gas of Oxygen in the above example, would probably get you a max 5-7 minutes less deco than if you carried just 50% and when adding 80% it might maybe like 4 minutes less deco.. and again, I would seriously just take 50% on these dives to make my life easier.

So - in my opinion. 50% on majority of 170ft and shallower dives and 50% + 100% on dives 250ft and shallower.

yea, I threw 100% and 50% into vplanner, and the difference in deco times on a 160ft dive were with a few minutes of just sticking with 50%.

I thought maybe there was something I was missing on 80%.

Now I guess I'll just burn this stuff off, and refill with 50% once I get the booster all setup.

thx Koos and everyone.

There seemed to be a fad a few years ago where people were using 80% in order to decrease 02 exposure. Most tech divers that I know never used 80% in the first place, but I saw people talking about it on the internet. It's not generally used by anyone who has any tech diving experience these days, though.

Jeff, I think this is a more complicated topic than it seems. The answer to "what is the best deco gas" is "it depends on the dive". As a general rule, I consider a dive with less than 30 minutes of deco a single bottle dive, and a dive with more than 30 minutes of deco as a two bottle dive (or more). Consider a few dives:

Dive 1: Fairly shallow (by deco standards) but long bottom time. Let's say 100' for 50 minutes. 100% is the correct deco gas. Easy to cover a lost bottle with back gas. Easy to reserve enough back gas to get you and buddy to 20'. Both of these scenarios need to be part of your dive planning.

Dive 2: Deeper dive, less than 30 minutes deco. Let's say 150' for 25 minutes. 50% is the right choice. Again, easy to cover a lost bottle with back gas and easy to get to you and your buddy to 70'.

Dive 3: Deeper yet. 200' for 30 minutes. This dive needs more than 30 minutes, so 2 bottles are necessary (and two bottles make deco more efficient). Since we only plan for a single major failure we only need to cover a single lost bottle on back gas. If you planned to deco on 50% only, you would need to reserve 160 cubic feet or so of back gas to cover an 80 of 50%. That's not reasonable, as you'd need to turn the dive with most of your back gas remaining.

Dive 4: 150' for 45 minutes. Longer bottom time here requires more than 30 minutes of deco, so we choose to use 2 bottles (50% and 100%) to manage back gas reserves. Usually I don't adjust deco when adding a 100% bottle (just use it to make deco more conservative).

Dive 5: 220' for 40 minutes. Deco for this dive on 2 bottles will bust your CNS clock. This is a controversial subject, but, if you believe in the CNS clock, 80% starts to make some sense over 100% when the deco times get really long. Most people are not doing dives like these, at least not on OC.

As you can see, there are a lot of variables, and the important pieces change as depths and bottom times become more aggressive. It's best to discuss this with your instructor.

100% agree with Casey!

The only thing we do differently, would be for the 220' profile, we still only use 50% and 100%, CNS clock overages is not an issue with back gas breaks. For me personally, this makes deco planning easier with standardized profiles and gasses.

For example a recent dive on the AJ Fuller was 230ft for 25min, we had about 60 min deco from 70ft up in a 75min total ascent and we used only 50% and Oxygen for deco and burnt maybe 140cft in back gas. 30cft in 50% and 20cft in Oxygen. Thats totally doable on OC..

With a stage tanks 250+' dives is completely within OC range for around 30min or less bottom time and any longer than that and for the 300+ dives, then it for sure starts to be a few more bottles than i might care to lug around on OC, time to call in the rebreather-erers.

thx Casey and Koos, so far I'll only be certified to 160ft, so we didnt cover the deeper dive profiles and gas planning for those yet. 160ft is far simpler

This question came up as I was looking forward to deeper certification, and planning what tanks I need for OC bailout, plus of course why bottle up 80% and not use it.

I know most have probably seen this article, but it's just so damn great, written by the legend that I gotta post a link.

http://www.baue.org/library/bakers8020.html

we had about 60 min deco from 70ft up in a 75min total ascent

Not to hijack, but this statement makes me shiver . . .

pensacoladiver wrote:I know most have probably seen this article, but it's just so damn great, written by the legend that I gotta post a link.

http://www.baue.org/library/bakers8020.html

Interesting read.

I've been debating plugging my Deco gas into my Gem, which with FIO drop at 20ft, my 100% O2 becomes more like 88%'ish I think, and/or 60% which by the time I get to 20ft is like 48%.

a> for gas savings
b> just plain ease of use, I've got the switchblock and everything, just havent plumbed it in yet.

Lots of good info given so far. And like Casey said, "it depends on the dive"...

For me personally, on single deco bottle dives (to 170fsw), from a pure decompression/how do I feel after the dive standpoint, I prefer 100% with a very nice, slow, shaped ascent to the bottle.

However, looking at RB for dives at 140-170', 100% becomes unrealistic due to the travel time to 20'. So, I use 50% on dives deeper than 140' or so for RB purposes. It may not be the "best gas" from a purely decompression standpoint, but logistically it makes sense.

80% doesn't gain you anything logistically, nor a decompression advantage.

BDub wrote:Lots of good info given so far. And like Casey said, "it depends on the dive"...

Isn't the answer to most diving questions.. "it depends on the dive" ?

A quick question about Casey's examples, are they all assuming 80 cu ft deco bottles?

How often do you guys use 40 cu ft deco bottles if ever?

fnerg wrote:How often do you guys use 40 cu ft deco bottles if ever?

I use a 40cuft for 50%. For O2 I typically use a 30, but I have a couple of 40s for O2 as well.

BDub wrote:

fnerg wrote:How often do you guys use 40 cu ft deco bottles if ever?

I use a 40cuft for 50%. For O2 I typically use a 30, but I have a couple of 40s for O2 as well.

Ditto. I don't use an 80 until the 190' bottle.

All mine are 30's and 40's. I havent gotten any 80's yet, but for my cert level, dont need them.

I dont care much for the 30's though, the 40's give a better reserve depending on the dive. I blew a 30 of 50% on a training dive. That was fun.

But I can see a 30 of O2 would have its place. But if I'm carrying a 40 slung for B/O, I'd prefer to have a 40 on the other side.

kdupreez wrote:CNS clock overages is not an issue with back gas breaks.

Sorry Koos, I can't let this one go. Back gas breaks address pulmonary toxicity, not CNS. The half-time stated by NOAA for CNS tox is 90 minutes (admittedly this number is highly suspect), so a 6 minute back gas break isn't going to change things very much. The math just doesn't work out.

The main issue being debated with the CNS clock, as I understand it, is that CNS toxicity may or may not be related to exposure time. An alternative way to look at it is in terms of odds. In a fixed period of time, your odds of having a CNS hit go up with increasing PPO2. The odds of taking a hit at 1.6 during deco are probably very low, so you see people doing dives with some unbelievable CNS clock numbers and not taking hits. Still, I view going over 100% CNS as an increase in risk, small as it may be.

The WKPP guys do those huge O2 exposures in dry habitats. I would guess this is both for comfort, as well as the increased risk of a CNS hit.

cofford wrote:Back gas breaks address pulmonary toxicity, not CNS.

From my understanding, lots of research seems to prove back gas breaks is very beneficial in managing CNS tolerances.. math might not add up, but the empirical data seems to, at least in my understanding.

If you look at many of the In Water Recompression models, you will note air/gas breaks are a fairly big part of managing CNS tolerances while being submerged and exposed to very high oxygen partial pressures.

The NAVY Diving manual has also a been a good source for me to understand for how air breaks are applied in DCS treatments to reduce CNS events in the chamber.

There is also great piece of CNS write up in the GUE Tec2 materials and discusses application of primarily back gas breaks along with other strategies for postponing hyperoxic-induced seizures. (Be happy to send you the research material if you'd like to peruse over it.)

Here is excerpt from said research material:

"Intermittent exposure to hyperbaric oxygen breathing with air breaks at the same pressure is a technical approach for increasing the total time of exposure to hyperoxia. This procedure is routinely used in the clinical setting. Various other pharmacological strategies have also been tested for postponing hyperoxic-induced seizures"

One of the big influencing factors as I understood from research materials and training and talking with the people who do those 25 hour deco, it is that high PO2 exposure with small duration (for example of PP02 of 3.0+ for very short time) is statistically less prone to produce a seizure than a very long exposure of a lower (but still high) PO2

We (read GUE) actually apply back gas breaks for both CNS and pulmonary tolerance management. On typical relatively short deco times of lets say 1-3 hours, the pulmonary concern is insignificant and the application of a back gas breaks benefit here is leaning more towards being useful in managing CNS tolerance than pulmonary issues (AFAIK, USN diving manual dont even regard treatments 5,6 or 6a as pulmonary significant)

All of this stuff quite honestly is very inaccurate science and much guess work based on highly variant empirical studies.. So I'm not trying to argue with you here. its just that what the books and formulas sometimes say is not always how its practically applied and I think we agree that in all of this there is some medium of truth that says gas breaks help with CNS.

and if I can apply some strategy thats empirically proven to potentially reduce my risk of doing the funky chicken down there.. I'm probably going to apply it..

fnerg wrote:How often do you guys use 40 cu ft deco bottles if ever?

If I need to spend 25min or more on 50%, its an AL80.

With oxygen its very very rarely needed to take more than AL40. AL30's will do the trick most of the time.

but carrying a 40 one side, a 30 on the other. The lack of symmetry would drive my OCD crazy...

thats why we carry it all on the same side