Next step for longer bottom times on deep dives?

[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.

 

[wetb4igetinthewater]

This reminds me of my first photography trip to the Colorado in late September of 1997. Now the previous month, 12 people died in Lower Antelope Canyon, one of the last slot canyons before Lake Powell that is fed by a huge area of smaller slot canyons, washes, etc.. That year, monsoon season dragged on a little longer than normal. I stopped at a store to ask about going into that slot canyon where people had died. A Navaho told me "I woulnd't recommend it." That was good enough for me. There wasn't a RULE against going into it (just pay an access fee), but there was a RECOMMENDATION against it. As the person I asked definitely had greater expertise on weather, drainage, risk, than me, I headed his RECOMMENDATION.

 

[Wibble]

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.

Put it this way; if planning a dive to 60m/200' and you turn up a the boat to be told that dive's off but we'll do a sheltered one at 30m/100'....

On OC you'd probably throw some toys around and take your kit home rather than face throwing away £100/€120/$140 of gas.

On CCR you'd say "lets go diving!"

 

[MichaelMc]

Put it this way; if planning a dive to 60m/200' and you turn up a the boat to be told that dive's off but we'll do a sheltered one at 30m/100'....

On OC you'd probably throw some toys around and take your kit home rather than face throwing away £100/€120/$140 of gas.

On CCR you'd say "lets go diving!"

But that is still an economic argument vs a missed dive day.

Say you have diluent bottles with 45, 30, 15, and 0% helium sitting ready. In increments of 15%, to avoid bickering about 5% helium. For a shallower dive, likely you pick a lower percentage of helium. At what depth do you dive the 0% as using helium diluent is just silly?
always, 180', 160', 140', 120', 100', 80', 60', 40'

 

[ginti]

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.

No, when I say community I mean the scientific community. I give you a parallel with medicine. Listen to a single doctor can be misleading: the doctor can make mistakes or, even worst, is a fraudster; if you have no competence in medicine, you cannot distinguish the good suggestions of this doctor from the bad ones. However, if you listen to the entire medical community (that is, the union of all the doctors, as, for instance, the WHO), you are less likely to make mistakes.

In research, it is slightly different. All good scientific papers are peer-reviewed, but these papers deal with really advanced stuff and can be controversial (and sometimes they should be!), so read with caution. But if a suggestion is not on a published paper, the same argument that I made for the doctors holds, and it is better to listen to what the scientific community has already accepted (which, in diving, it's a bit more complicated...). Although I have the feeling that nobody is a fraudster here, better be cautious, everyone is a human and can make mistakes (plus, there can be misunderstandings)

 

[macado]

Put it this way; if planning a dive to 60m/200' and you turn up a the boat to be told that dive's off but we'll do a sheltered one at 30m/100'....

On OC you'd probably throw some toys around and take your kit home rather than face throwing away £100/€120/$140 of gas.

On CCR you'd say "lets go diving!"

This is exactly what used to happen on New England dive charters. I mean it probably still does but it's not as common as it used to be with rebreathers being more popular.
For a ~60m dive (200ft) I would often bring 2 sets of doubles with me. If I was told we couldn't get out to the deep wreck due to weather in the parking lot well I guess I'm diving my other doubles. It some cases it meant bringing both sets out on the boat provided there was space.

Of course the economic argument can obviously be made but I simply don't care about wasting a 15/55 diluent fill on a 100' (30m) wreck. That's like a $5-10 worth of gas for me.
Most people on rebreathers don't care about wasting trimix on shallow dives.

Let's be honest, somebody that has already spent $7-10k on a rebreather plus associated training should really not be crying about wasting a $150 dollar trimix fill but it happens. Given the option if I had a lesser trimix fill or nitrox with me; I'd simply dive that.

I don't consider it cheaping out or being frugal. I've "wasted" trimix fills on 100-130' dives when the wreck got changed. I didn't cry about it. It was either that or I sat on the boat pouting and didn't go diving that day.

I've done the same thing with rebreather bottles but it typically bothers me less. I've brought multiple sets of diluent bottles with me. Example: We were supposed to dive a wreck in 190' but weather prevented it but we knew we could get to a wreck in 100' that was sheltered. It was either that or don't go diving. Luckily I had a bottle of 18/20 (it was an air top off from another trimix fillI). I swapped out to my other diluent bottle and was good to go.

 

[Wibble]

But that is still an economic argument vs a missed dive day.

Say you have diluent bottles with 45, 30, 15, and 0% helium sitting ready. In increments of 15%, to avoid bickering about 5% helium. For a shallower dive, likely you pick a lower percentage of helium. At what depth do you dive the 0% as using helium diluent is just silly?
always, 180', 160', 140', 120', 100', 80', 60', 40'

Have a couple of diluent cylinders; one's kept with a 45% helium mix, the other with about 15% (topped off, but also topped off with mix). Whilst it would be a pity to "waste" a rich helium mix in a 10m/30' puddle dive, it wouldn't really matter as I'd use very little anyway - mostly for the wing. So rocking up at a boat dive on a rebreather doesn't matter as the costs are negligible.

Spinning back to OC; I would choose not to go rather than waste £80/€100/$130 on backgas. If I had no choice, as in the relocated boat dive and on OC, I'd probably choose to do the dive if it's a "nice" but shallow dive.