Deep Bailout Gas Choice for ~70M / 230FT

[packeteer]

How long was a typical dive on a single 12ltr to say 25m?

been a while, but I seem to remember it being about 20 mins (total dive time)

I do remember the day I made a 12L last an hour at 10m. I was rather chuffed

 

[Mark Chase]

been a while, but I seem to remember it being about 20 mins (total dive time)

I do remember the day I made a 12L last an hour at 10m. I was rather chuffed

a 20 min dive??? Wow i bet you were a popular buddy

burt even then you must have spend a fair chunk of the dive well above 25m

45RMV allowing 50 bar reserve at 25m would be a max 14min dive on a 235 bar fill

ATB

Mark

 

[matt]

Why not 100? its just as impractical in the real world

Yes, why not. I do not know where the upper limit is. Reports I read say 50-60lpm (as an average) with higher peak are realistic. And there's a fair few of them to read, and even a couple of videos.

It doesn't take a genius to figure out you cant carry enough gas to plan for a 50RMV at 70m

This we agree on.

Matt.

 

[emmbee]

Hi,

I'm not competent to discuss bailout, I just lurk here to see what the future may hold, so of course I read all the links and stuff

Still, I don't think the KISS scrubber test result is relevant. They're talking about someone in tiptop physical condition exercising as hard as he can, so consuming as much oxygen and generating as much CO2 as he can, until he can't do more.

He didn't stop because the CO2 concentration was too low and he passed out or something. He stopped because of muscle fatigue. In particular activity at that level is not like a marathon. It is largely anaerobic. He ran out of the supplies for this reaction (actually, for one of them) and the aerobic reaction cannot provide the amount of power required. And yes, duration of anaerobic exercise is a couple of minutes (google "anaerobic exercise")...

Breathing is still in accordance with the metabolic needs of the body. This is not hyperventilation (I think the term is hyperapnea). All it means it that an athlete can produce enough CO2 to require 75L/min of breathing to remove it for 2 minutes.

Hyperventilation is when increased breathing does not serve those needs. When O2 consumption and CO2 concentration are normal or low, yet breathing is increased. It is not governed by the blood pCO2, so can be higher. Presumably the time limit then is when the pCO2 drops too much? That may take much longer (I would imagine that as the driving pressure drops and/or the rate of breathing increases the amount of CO2 removed by a single breath diminishes). And much longer again if the victim is actually exercising, so producing CO2. Or if he had hypercapnia to begin with (I'm not clear if increased breathing due to hypercapnia is hyperventilation in and by itself, or if hyperventilation just follows, after pCO2 is back to normal, because the body overshoots, or because of stress).

Somebody with more medical knowledge can correct me?

Cheers,

Matthieu

 

[Mark Chase]

Yes, why not. I do not know where the upper limit is. Reports I read say 50-60lpm (as an average) with higher peak are realistic. And there's a fair few of them to read, and even a couple of videos.



This we agree on.

Matt.

Seriously, its just not sustainable. Such high SACs can be possible for very short periods without serious adverse effects.

Watch the Dave Shaw video with the sound on. Work out how long he was suffering high SAC before he passed out and died. Tragically it wasn't long.

What annoys me here is the concept that we should be planning for 50-60LPM SACs on bailout. This is ludicrous.

I have had forced bailouts a couple of times and my SAC didn't break 15lpm? Why should it. Its zero drama bailing out from a flooded or broken CCR. My first thought when I had to bailout was shit this is going to cost me to refill my tanks. Even when i bailed at 90 odd m in the arse of a cave I only hit 18lpm.

Its a simple scenario. Something goes wrong, you go OC via a BOV or reg and once on OC its all good again and you go home. I know i have planned a 25SAC for bailout so i know i have shed loads of gas. There is no drama

If bailing out from a flood or handset failure on a CCR is going to result in elevated SACs for a prolonged period Id strongly suggest the diver isn't dived up enough to be doing this sort of diving.

If the diver is competent but for reasons unknown hes got a natural SAC over 25, you have to ask if deep diving is the right hobby for said diver. Hes got to cut bottom time ridiculously short to compensate for that level of SAC.


So the ONLY case for hyper elevated SACs is C02 hits.


Now there is a new issue of Narcosis. C02 is 20 times as narcotic as Nitrogen.

When Howard had his C02 hit on the Audacious at 65m he was scootering with two Al80s and he became disorientated to the point his buddy could not coax him back to the shot line. Howard repeatedly just stopped moving or scootered off into the blue.

When his buddy realized the situation was serious and decided to ignore the shot and go u, Howard did a rapid ascent and became separated from his buddy. (teem bailout any one)

In Howard own words he came too at about 15m and went OC and back down to do his stops. He used a lot of gas but he made it on two AL80s and a drop tank.

IMHO what saved his arse was the rapid ascent.

He was so narked on C02 he wasn't going to be able to manage his gas or his deco so having reduced the narcosis by going from 68m to 15m, he became clear headed enough to go down and do his deco. But because he was breathing heavy he emptied his 80 of 18/45 a lot shallower than he would normally have done. A low viscosity high helium mix allowed him to clear his retained C02.


For me a serious C02 hit has one cure and that is to get shallow fast and breath a high helium mix.

In getting shallow fast you negate the need for monster bailout tins as gas consumption is depth related.


I see my big problem not as one of gas volumes but one of being mentally able to cope with the narcosis enough to realize I need to go up.


After 8 years diving CCR i have found my tolerance for narcosis is now tiny. I used to dive 50-60m on air and feel in control. But recent activity on OC air to 30m left me feeling very odd indeed. I have a machine that makes the cost of He negligible so i am often diving max SACs of 10-15m. Suddenly being on a 30m EAD I found my skills had left me and i felt uncomfortable (and this was in blue water).


So my 2p is avoid C02 If the gas consumption and 02 tox doesn't get you the Narcosis probably will.


In order to avoid C02 theres obvious issues\ with scrubber packing and run times but I don't believe most C02 hits are scrubber related. I believe WOB work load and stress are the bigger issues and most hits are retained C02 not inspired Co2


One way to reduce stress and workload is limit the equipment carried. QED carrying multiple large tanks for unreasonably high SAC calculations is likely to cause the very thing your planning against.

ATB

Mark

 

[matt]

I think the real problem with 50-60lpm is that it cannot be carried for long dives or deep dives.

Matt.

 

[Mark Chase]

I think the real problem with 50-60lpm is that it cannot be carried for long dives or deep dives.

Matt.

Do the maths on a 45min bottom time at 50m and you will find you cant plan 50-60lpm even this little dive


At 50m a 10ltr blown to 250 bar would last 6.5mins


Asume 2mins at 50m to sort out SMBs and start the ascent

Thats 720ltrs

6m/min ascent to 21m is 5min avg depth 35m 276lpm avg is 1380ltrs

Toatal gas required 2100 ltrs


Switch to 50% at 21M and do 50mins deco.

3 @ 21 = 558 ltrs
4 @ 18 =672 ltrs
5 @ 15 =750ltrs
6 @ 12= 792ltrs
7 @ 9 = 798 ltrs
25@ 6 = 2400ltrs


Total deco gas required 5980 ltrs

So two Al 80s blown to 235 bar wont be enough at 5405ltrs so one has to be 80% to speed up the deco and spread the gas load onto the 6 & 9m stops.


So planning for 60lpm on a simple 45min dive to 50m you need an AL80 of 21/35 and two AL80s of 50% and 80%


Thanks but ill stick to my two 7s. @ 250 bar.

ATB

Mark

 

[Peter L]

Sorry to throw sand on both of your contributions but my guess is it's 3 minutes of high ventilation or bust, you either have it in hand or you have stroked out at the end but my guess is that you start at 80 and end at 40LPM and if you don't make it you don't make it. I think however a breathing average of 50LPM for the first 5 minutes is fair but for both your points, your ability to survive is limited by fitness, gas availability and the role of the dice.

I would like however a tool that helps planing that my first deep bail out is not sufficient for the return to deco depths...

 

[matt]

Hey Mark - I agree that lugging countless cylinders is a stress load that makes things worse not better.

So for bail-out planning you may as well ignore CO2 hits, if you get one then shucks, game over, turn out the lights as you pass.

That said then 2xRMV based on full cylinder volume is probably OK, so around 30RMV.

Which is more likely - fully flood or hypercapnia? (I don't know...)

Matt.

 

[matt]

Sorry to throw sand on both of your contributions but my guess is it's 3 minutes of high ventilation or bust, you either have it in hand or you have stroked out at the end but my guess is that you start at 80 and end at 40LPM

So the first bit at 60...

and if you don't make it you don't make it. I think however a breathing average of 50LPM for the first 5 minutes is fair but for both your points, your ability to survive is limited by fitness, gas availability and the role of the dice.

I have not found an article yet on this...please point me at it!

I would like however a tool that helps planing that my first deep bail out is not sufficient for the return to deco depths...

Sounds like a job for XLS...

Matt.

 

[Mark Chase]

Sorry to throw sand on both of your contributions but my guess is it's 3 minutes of high ventilation or bust, you either have it in hand or you have stroked out at the end but my guess is that you start at 80 and end at 40LPM and if you don't make it you don't make it. I think however a breathing average of 50LPM for the first 5 minutes is fair but for both your points, your ability to survive is limited by fitness, gas availability and the role of the dice.

I would like however a tool that helps planing that my first deep bail out is not sufficient for the return to deco depths...

How did you get to 80? Thats the question I always ask

My average working SAc on OC is 12-15. If i were to hit 20lpm id know all about it

If i didnt have a good reasion to be at 20lpm id bail on to OC at that point and god forbit I hit 30 id be heading up fast.


I beleive a BOV helps with this decision. It makes the decision to switch less final. Going off loop seems so drastic and theres always the feer that you may not be able to get back on loop. Going OC via the BOV is less drama so you tend to do it with less hesitation.


Id like to think by bailing early id have a good chance of avoiding monster SACs but I have never had a C02 hit so its hard to say anything for sure.

ATB

Mark

 

[GLOC]

Lots of Stuff about the Dude's Malin Head Ascent

Mark,

I have spoken to Howard about this. His recollection which I use as a case study for Human Factors in Diving Incidents was different to what you recount.

He didn't have a CO2 hit but was working hard and was still having head games from his CO2 hit 3 weeks previously. He bailed onto his Al80 as he thought he was having a CO2 hit and made the rapid ascent when he realised that he was going to run out of gas. He didn't use the reg that his buddy offered to him. He ran out of gas approximately 30m and changed to 50% knowing he was going up quickly. He lost control of the ascent and flooded his loop to stop him hitting the surface (good move).

Sending a bag up to get more gas was a good thing.

AFAIK, he and his buddy have a much more conservative view on buddy skills now.

You keep saying that people can't keep up that level of breathing and yet there are those who have.

The consensus appears that if you are going to ignore CO2 hits, then you can carry enough bailout at depth for you because you are unlikely to have elevated breathing rates. However, if you are going to try and mitigate CO2 hits, then you are not going to have enough gas once you get down much past 60-70m.

Regards

 

[Clare]

Seems an odd discussion from my point of view as on a cave dive gas is the one thing we really would not be short of. If you plan for the loss of a scooter you need to plan for the kind of gas reserves which make loss of RB look easy

Rb80 60 minutes average depth 60m (for example). We took:

36 litres per diver bottom gas (one stage, backgas in 12s)
11 litres 35/25
11 litres 50/25
7 litres O2

I would sometimes dive in 7s so would add on another 80.

Obviously this is allowing for a 30 minute exit to first deco gas not the whole 60 minutes.

On CCR we would take an additional bottom stage each but only drop one O2 bottle as we would each be carrying 7litres O2 anyway.

This leaves us with: 28 litres bottom gas each plus the deco gases above. It is less than in RB80 but still hits the dive requirements.

The gas calcs on the thread above on RMV won't fix the loss of the ability to scooter (a lost scooter is less of an issue than loss of ability as we carry spares and can tow for long distances).

I guess the reason I'm posting this is that the moment you take away the option of a fast ascent, drop bottles or smaller gas requirements, you still find a way of doing it.

 

[lizardland]

That's similar to what I took though I had more bottom gas, 4x12's and a 12 for oxygen rather than a smaller bottle (mainly because that's what I had full at the time). Again none of that was carried, the only gas I took was a pair of 12 stages and a spare scooter.

By comparison, 60-70m cave diving, particularly on CCR, is more an exercise in boredom management next to doing the same depths in the sea.

Sent from my GT-I9100 using Tapatalk

 

[GLOC]

Mark,

If you want to see the breathing rates that are possible, have a look at this paper on the Rubicon Exercise tolerance at 4 and 6 ATA

Seven normal male subjects performed 5-min bicycle exercise ranging from 50-100 percent maximum oxygen uptake at 4 ATA and three were also studied at 6 ATA. At all pressures, the subjects breathed 0.2 ATA O2 plus nitrogen. All subjects were able to perform maximum work at all pressures. No pressure-dependent variations in heart rate, O2 uptake, or CO2 output were noted. At both 4 and 6 ATA, ventilation was decreased at exercise levels greater than 80 percent maximum O2 uptake. The magnitude of the decrease was not great, however, and signified only minor CO2 retention. In some instances exercise ventilation closely approached the 15-S maximum breathing capacity and these subjects noted severe dyspnea, possibly due to dynamic compression of large airways. In three subjects, respiratory frequency was measured as well as minute ventilation; this relationship did not change with depth. Subjects performing heavy exercise at 6 ATA noted disturbances of consciousness, presumably due to N2 narcosis.

Above 100l/min is possible and has been show in this paper, with the added issue of N2 narcosis...

Regards

 

[matt]

Seems an odd discussion from my point of view as on a cave dive gas is the one thing we really would not be short of. If you plan for the loss of a scooter you need to plan for the kind of gas reserves which make loss of RB look easy

At least the lost-scooter is not causing elevated PCO2.

How long does it take to swim back 1 minute of scootering?

Matt.

 

[Clare]

We allow roughly 4 times (depending on volume of gear)

We certainly don't plan for 4 lost scooters and RB failure on the same dive though!

 

[matt]

We allow roughly 4 times (depending on volume of gear)

We certainly don't plan for 4 lost scooters and RB failure on the same dive though!

Thanks. Last foray OT on scooters

So do you normally plan for a tow back rather than a swim at a reduced speed?

Matt.

 

[Clare]

Depends on depth, gear and scooter availability. Short shallow hops (like the Hinckle) we'd swim. Deeper more gear intensive dives where swimming is not an option then spare scooters are needed. However, as we learned when we got burned a few years back, the loss of visibility which prevents you from scootering is far more worrying. So whether you have spares or not you still need gas.

 

[Johan Isaksson]

From my, limited but medical, perspective I'd say that high RMVs can be maintained far longer than what's practical to plan for. Just the task loading of carrying the gas needed for a worst case scenario at depth is enough to make me want to take up gardening.

Personally I'll be a bit more aggressive (2x normal task loaded rmv) on the bailout plan for open water deco-dives. If my RMV goes through the roof I'll start blowing stops to get shallow or steal gas off a buddy.