Next step for longer bottom times on deep dives?

[jlcnuke]

No, I don't think they are necessarily ignorant. If they say they understand that they understand the increased risks with narcosis in an emergency situation when exceeding a density of 6.2 g/L and accept those risks, I'm good with that. I'm not good with people who dismiss the science. I have had a conversation with one individual who dismissed Dr. Mitchell as "just an occasional diver" who trusts the divers at his former dive center who "regularly" dive to 70 meters or more on air. A friend of mine in Europe is friends with a number of people who died diving that way. And they don't switch to trimix until 100 meters. Now obviously there is a big gap between 40 and 100 meters on air. However, the risks increase dramatically above a gas density of 6.2 g/L. That's ultimately my point. It does come down to risk assessment, and we all assume different levels of risk. But I'll never understand or respect those who dismiss science.

The problem I have with the gas density argument for 100' swapping is that "the data says the risk starts increasing significantly" around there, but there is no stated level of risk associated with it defined as far as I can see AND the data supporting that comes assuming that not only are you breathing that gas at the stated depths, but also doing fairly extreme exertion at the same time (all the test data that came up with the 8.5 kPa number, Warkander et al, for a high risk of incapacitation that I can find were using divers doing significant exercise for extended periods of time, a situation I've never been in while diving personally). Let's assume that you're at 130' on air, your buddy is at 130' on 21/35, and you both have a burst LP hose. What is the basic risk of a bad outcome using air assuming you're trained (10-45 seconds of activity to deal with the issue and then to start your ascent)? 0.000001%? 0.00001%? I mean if it's 100 more risky on air than trimix, but the $100 in trimix is going to change your risk of a bad outcome from 1 in 1,000,000 to 1 in 100,000 is it worth an extra $400 in gas every weekend for to do 4 dives, especially if you know that your dive isn't likely to be in a condition where you're doing the exercise levels that the testing was generally done in?

It would be safer for me to drive an armored vehicle, as the chance that a road rage incident etc. would be survivable would be significantly higher, but it's not cost effective to me based on the low probability of a negative outcome in the first place (despite it being significantly safer if that low probability event happens).

Now, if the science said "if you have a problem at 110' on air, there's a 5% chance you'll end up dying but if you were on 21/35 instead you'd have a 0.005% chance of dying" it would be a no-brainer to say it's worth it. That, as far as I can tell, has never been the message from anyone. Simply "it'll reduce the risk of X significantly", where X is a measurable item that doesn't necessarily equate to a specific negative overall outcome (i.e. death etc.) happening, but no statements or conclusions about what that risk of an actual bad outcome is mentioned in any of the things I've read or watched. If such an assessment exists I'd love to see it, but what I've seen is we have two general camps: those who say "the risk is really low on air so they're happy on air to a point", and those who say "the risk is lower on trimix so I'm going to use trimix regardless of how potentially safe it is without trimix". Often the first group omits the "to a point" and the second group omits the "regardless of how potentially safe it is without trimix" as each have decided that their person choice of "safe" is what defines "safe".

Usually, both camps ignore any other factors that impact risk. Personally, I'll dive air to 150-160' at my local quarry, with few if any entanglement hazards, no current to speak of, no overhead environments to be concerned with, navigation can always consist of "go up", etc. as the added expense of going to trimix for such a relatively safe dive doesn't seem worth it (especially with the vaguely defined benefit it would offer based on gas density and the conditional benefit from reduced narcosis). Now, if I were to be penetrating the Oriskany hanger at 176' in open water with currents and an overhead environment and all the other risk factors, the benefits of reduced narcosis start to make the dive seem significantly safer and the reduced gas density, when I could potentially encounter strong currents where strong exercise might be necessary provides another compelling reason to think that the benefits would be worth it.

So, for me, it comes down to relative risk. With actual risk being relative from person to person and dive to dive, and the science not giving a definitive answer as to what is "safe" to dive and what is "unsafe" to dive, I evaluate based on the dives I'm doing, the information I have, and the risks I'm willing to take. Which is what I suggest everyone do, which means we're likely to all come up with different answers as to what we consider "safe".

 

[Wibble]

OP would still need to get minimum Adv Nitrox cert to have access to 80%. As far as I can tell, no way around getting an advanced cert or two.

Should have been clearer...

For dives to 30m/100' diving on 32% will get you 30mins on the bottom before you're into deco.

Taking a stage of 80% (my preferred rich gas, other gasses such as 100% are available) means that you can easily do an hour on the bottom with circa 30mins of deco. This will require you to have Advanced Nitrox and Decompression Procedures (all agencies do this, gives you circa 45m).

If you really want to push your bottom time at 30m/100', then you need a backup deco gas. Typically you''d take two stage bottles, but extend your bottom time for, say 90mins with about an hour of deco. This will require you to have the next level of skills, for two stages, typically called Extended Range or Normoxic Trimix.

Of course you can dive deeper too; 40m/130' for an hour with about 45mins deco. That's best done with backup deco gas as your backgas definitely won't last if your deco stage fails. Twinsets and deco stages give you easy access to far longer bottom times.

Beyond 40m/130' helium becomes your friend to reduce the narcosis. On open circuit it's very expensive.

FWIW, my ANDP & Normoxic skills were mostly used down to 40ish metres 130' ish. Mainly as one or two stages would always be used for 2 to 2.5 hour runtimes.

And a pee valve.

 

[ginti]

I'm surprised nobody has mentioned the fact that diving helium actually feels better. You see more of your surroundings, you have better awareness, and you remember more of the dive afterwards.

Isn't that a consequence of the lower narcosis? I believe everyone is considering it

 

[flymolo]

Isn't that a consequence of the lower narcosis? I believe everyone is considering it

I just hear it being framed as a safety aspect, but there's also an enjoyment aspect.

 

[wetb4igetinthewater]

Like it or not, the more "scientific" approach is not to follow one person's recommendations. Instead, follow the entire scientific community. A single individual, even if very knowledgeable, can make mistakes. The entire community too, but it's more unlikely.

I can’t speak for Dr. Mitchell as I don’t know the man personally but I have a number of friends who are scientists in different fields. Prior to publishing their work or results of studies, they consult other scientists to examine the data And conduct a mini peer review prior to publishing. Now as I said I don’t know if Dr. Mitchell does the same. But I’d be willing to bet money that he doesn’t operate in a vacuum.

When you say community do you mean us divers? Because if you do I would take the knowledge and recommendation of a single individual that studies decompression science over thousands of non-scientists.

Now people can accuse me of name dropping, but it isn’t that. It is citing a credible source. An expert source that overrides one’s hubris of knowing better than anyone else based upon whatever.

 

[Wibble]

I love my CCR, has really revitalized my interest in diving. I use it for pretty much every dive other than very shallow shore dives where I don't feel like setting it all up for macro work in 5 feet of water.

But you have to be doing a good bit of OC trimix before buying a rebreather saves you money. Just considering consumables alone (sorb, cells and maintenance), it's really not cheaper for the vast majority of divers.

Yes, there are big deco advantages, but for the kind of diving the OP is doing, that would be primarily in shallower dives where a fixed PPO2 of 1.3 will give you a lot longer NDL than any reasonable OC mix. For example, if you are doing a dive to 60 feet (2.8 ATA), you would have to be diving EAN 46 to get the same NDL as a CCR. With the OP diving a relatively square profile to 100 feet (4 ATA) on EAN 32, they would have similar NDLs on OC.

For what the OP posted, I don't see why anyone would be suggesting a CCR. I think that they are great, and while they add risks they also remove other risks, and that's a whole different argument. But given the limits of what they are asking, OC tech training seems to be a better use of time, money and resources for this particular situation.

CCR is brilliant for those 30m/100' to 45m/140' dives -- and all others for that matter. The reasons, for me at least, is that:

• I can use helium for every dive as the costs are minimal; £5/€6/$7 per dive. Literally nothing.
• For the deeper dives I can use a rich helium mix without breaking the bank £10/€12/$14
• I bank my own gas; just need some twinsets to stash air, oxygen and helium mix. Can get a dozen or more dives out of those fills.
• Planning's trivial. Down to 30m/100' use a single bailout of 32%. Down to 40m/130' use two bailouts (add a 80%). Beyond that use a standard mix (21/35) plus deco stage.
• Building the unit takes a little time, but it's the same every dive; just choose your diluent

On top of that comes all the other advantages of CCR; quiet, no gas anxiety, warm gas...

I'd even use CCR for a shore dive (not much chance of that around here).


The big tip for CCR diving: do your ANDP (advanced nitrox & deco procedures) on OC first and sort out your core skills (buoyancy, finning and trim). Doing your MOD1 (first CCR qualification) is trivially simple thereafter.

 

[Wibble]

Also, again, even the "safest ccr" still presents risks not present on oc.

Praise be!

As far as I'm concerned, I treat my box as if it's trying to kill me. We know where we stand thereafter.

Regarding helium for lower WOB (work of breathing) density; on open circuit it's hardly noticeable, certainly down to 45m/150'. The narcosis though... a whole other ballgame. Which is one of the major benefits of CCR; helium below 25m/90' without breaking the bank.

 

[wetb4igetinthewater]

Okay, let's summarize.

OP wants to extend bottom time and go into relatively short deco times. Typically will occasionally dive around 90 to 100 feet, depths where gas density isn't an issue. If the OP goes deeper, the OP understands that there is increased risk of gas narcosis inhibiting responding properly/quickly to emergencies. That is a risk the OP must make for themselves.

Gas density has been discussed (and dismissed entirely by some) as a risk factor which has to be considered against one's own personal risk assessment, cost, and availability/convenience.

As the OP is not a prolific diver to these depths, a rebreather isn't financially viable/practical.

 

[macado]

This is probably going to come off as a rant but there is no hard rule at the depths being discussed here. The OP simply asked how he could extend his bottom time beyond NDLs at these depths and it took less than 1 page for the thread to devolve into bickering about OP needing to buy rebreather and to open his wallet for trimix. Seriously, did anyone read his original post?

You need to crawl before you can walk. Suggesting a rebreather to someone before they've taken a single entry level technical course, be it either Fundies or Intro to Tech is asinine.

For some people the line get blurred around 100-130ft (~40m) where they feel they need trimix but the vast majority of the world is not using trimix at these depths. You can argue normalization of deviance all you want. I'm also well aware of gas density guidelines, the thing is they are simply suggested guidelines. No more, no less.

I will directly quote Dr. Simon Mitchell where he responded to criticism of these guidelines on another forum. Please read it. These guidelines came from Gavin Anthony's rebreather testing data. See: "Respiratory Physiology of Rebreather Diving" by Gavin Anthony , Simon J. Mitchell

The paper is here: Dropbox - Rebreathers and Scientific Diving Proceedings 2016.pdf - Simplify your life

It is important to understand that Gavin's data identifies an inflection in risk, not an absolute prediction of a poor outcome. Put another way, no one is saying that if you exceed 6g/L means you will encounter problems. Indeed, in the data we published 42% of dives (not 100%) with a gas density greater than 6 ended with the diver retaining dangerous levels of CO2. Moreover (and I would have to confirm this with Gavin), none of those divers became incapacitated. You also have to remember that the test dives were deliberately provocative - involving moderate work, and it was during this work that the CO2 invariably peaked. If the divers had performed no work, probably few or none would have retained CO2 to dangerous levels, despite the high gas density.

Some examples..
At 100ft (30m) with 32%, gas density is 5.21g/l
At 110ft (33m) with 30%, gas density is 5.61g/l
At 125ft (38m) with 28%, gas density is 6.11g/l

While not ideal gas densities they are below the suggested maximum hard line of 6.2g/l

To paraphrase the great work that is The Pirates of the Caribbean, Gas density is "more what you'd call "guidelines" than actual rules." I'm not sure some people understand the difference between a rule and a guideline.

A guideline (definition) is simply "A plan or explanation to guide one in setting standards or determining a course of action." Guidelines are simply general recommendations. They are not mandatory.

Could most people benefit from some helium in their mix? The answer is yes.
Could most people do these dives on 28-32%. The answer is also yes.

How many thousands upon thousands of recreational divers do Florida Keys, North Carolina wrecks or Caribbean / SE Asia destinations every year on air or nitrox? The vast majority.

If I show up on a cattle boat in the Caribbean or SE Asia and demand a 21/25 trimix fill in a single tank for a 110ft (~33m) dive I'm probably going to get blank stares.

Good luck getting trimix at Olympus or Discover Diving in North Carolina. You'll be laughed at. You'll have to bring your own with you.

How many dive shops in Cozumel offer trimix? I can only think of a single shop out of about 100 dive shops.

How many cattle boats in the Florida Keys offer trimix? This is somewhat trick question, there are some shops that have it but vast majority don't.

How many people here dive on Ginnie Springs/Little River on 32%? There is high flow/current and you're at ~100' for majority of the dive. Do you honestly think everyone is diving 30/30? Nope. Most divers and cave instructors will be diving 32% if they're diving open circuit.

None of what I posted above means I'm against trimix. Quite the contrary, I think it's really useful at these depths but I don't always use it. It's entirely situational for me. It's simply not always available and for many not needed for a dive to 110ft (~33m)

My personal opinion is that I think getting the equivalent of "helitrox" training that these depths is very useful and should be encouraged. From here you can make an informed and educated decision if the dive you're doing requires it.

On a rebreather- I can't tell you the last time I didn't dive with some sort of trimix diliuent. Probably far more helium than is necessary for those dives.

 

[MichaelMc]

Have we had a poll of:
On a rebreather, how shallow does helium seem silly?
always, 180', 160', 140', 120', 100', 80', 60', 40'.
- For narcosis (as safety risk or enjoyment limiter)
- For work of breathing (which is worse on a rebreather)

Seems like that would put most of this side debate to bed. Then it is a matter of cost/benefit. Which has different tradeoffs for seeing pretty fish or hunting lots of them all week long to sell at a profit.

"Most people on cattle boats X Y Z", means very little. Given the many suboptimal things they do and their level of training and education.