PADI Deep Diver course- gas management

[tursiops]

I made reference to every diving being a decompression dive. Which, of course, went down a rabbit hole

Which is where the comment deserves to be. Unless you wish to redefine "decompression dive" as meaning something other than its common definition...i.e., a dive with mandatory deco stops.
The whole point of Haldane and his goats and today's understanding of how our bodies manage dealing with gases under pressure is that we CAN withstand a certain amount of ambient pressure reduction without having DCS. Haldane thought that might be a factor of 2. Today we think it applies to the N2 in our gas only so the 2 got dropped to 1.58 (twice the 0.79 faction of N2 in air). And there have been various refinements on that. But we could not dive unless some amount of ambient pressure reduction could be tolerated without DCS. This thread is about NDL diving, not deco diving. The bubble you are so concerned about are there...but they do not seem to be terribly important, especially in NDL diving.

 

[tursiops]

How about responding to the post I wrote where I used your suggested parameters and my own thinking to argue a point in good faith and in a courteous manner, instead of just spewing inaccurate insults?

OK.

By the way, the GUE protocol calls for a 10m/min ascent rate up to half depth (for NDL dives), and a 3m/min ascent from there. Calculating with 3m/min from the bottom just allows for a suboptimal ascent rate (because it might be hard to do a gas sharing ascent with a stressed diver with perfect timing), and/or a higher SAC rate, and/or more time to execute the gas share and stabilize.

3m/min is silly, and 10m/min is also silly below 60 ft, and not really necessary about 60 ft. Below you use the PADI RDP to try and prove a point bout Safety Stops, but that same RDP allows 18m/min.

So let's say we're calculating with 10m/min. We better use at least 30L/min (1cuft) for a stressed diver, since we have no other conservatism

Now you are adding back in some more conservatism. Do you really need to do that on every parameter in the calculation? How about just calcuting wnt is needed, and then say more than that, like maybe 20% 50% Whatever.

there was just a poster saying he had a gas consumption of 1cuft/min as a beginner without any emergencies.

I hope you are not going to use extreme cases and argue they apply to everybody.

And while a safety stop can be skipped in an emergency, it increases the risk of DCS enough that it is "required" on any dive below 100ft on the PADI rdp table, so I see no reason to PLAN to skip it just because of an OOG or loss of gas.

I'd much rather see the SS skipped in an "emergency," than plan for for it when gas supplies might be low. The extra risk from skipping the SS is minor compared to the extra risk of actually running out of gas.

Total minimum gas from 36m with safety stop: 1110L / 11.1L = 100 bar (1500 PSI). And that requires a constant ascent speed while sharing gas of 10m/min, without losing control of the ascent, or requires them to stay very calm and breathe less in an emergency.

And what is the number with 18m/min, no rapid breathing of BOTH, and so SS?

n any case there is not a lot of margin of error in a stressed situation.

How much margin are you comfortable with?

Even without the safety stop it requires 1100 PSI. So saying 1000 PSI is "more than enough" is quite telling.

Only for the slow ascent and very high breathing rate of both divers.

And I would like to see them do a shared gas ascent from 36m to 5m at 10m/min and make a precise stop at 5m to execute a safety stop.

I don't much care about the ascent speed, so long as it is below 18m/min, which has worked fine for decades and (probably) millions of NDL dives, and the extra risk from a skipped SS is hypothetical.

Try using rsingler's rock bottom ascent calculations. Here is a scenario based on that:

Scenario: you and your buddy are swimming along at 100 ft when suddenly his regulator starts severely free-flowing. What do you do? (assumes 0.7 cuft/min RMV, factor 3 stress factor; no time at depth to sort things out)
(1) immediately begin to ascend, at 60 ft /min up to 60 ft depth, while sharing the air in your tank.
(1a) If he can breath off his bree-flowing regulator, as he learned in OW class, he should, to salvage some of the gas he is losing.
(2) You both try and breath normally as much as and as soon as possible.
(3) Skip the safety stop, and ascend to the surface at 30 ft/min from 60 ft depth on up.
(4) Check your pressure gauge when you near 15 ft depth; if you still have 500 psi, you can both do a safety stop, which will take about 150 psi each.
NOTE: it should not take more than 850 psi to make this ascent, plus 300 psi for the safety stop.

 

[steinbil]

I hope you are not going to use extreme cases and argue they apply to everybody.

I didn't argue that it applied to everybody. But if there are beginners with a consumption of 1cuft/min in non-emergencies, it seems unreasonable to me to estimate a lower than that consumption for emergencies as a baseline. What we're discussing in this thread is training standards for training of deep recreational dives - so the baseline standards for training should be applicable to less experienced divers with high consumption, and it's also good to plan for high gas consumption if you have an unfamiliar buddy. OF COURSE these calculations can be adapted to for more experienced divers when they have significant experience in various environments and know how they respond to failures underwater, and I'm not arguing against that at all, but in the case of training standards for divers they need to start with a conservative baseline.

And what is the number with 18m/min, no rapid breathing of BOTH, and so SS?

I'm not comfortable with 18m/min ascents, especially while gas sharing. Maybe I would have a different opinion on this if I wasn't diving in freezing water with a drysuit and very heavy undergarments. I'm also not comfortable with ascending that quickly for reasons of DCI risk - maybe that is misplaced with NDL dives, but I like having the same ascent rates for NDL dives and deco dives.

I'd much rather see the SS skipped in an "emergency," than plan for for it when gas supplies might be low. The extra risk from skipping the SS is minor compared to the extra risk of actually running out of gas.

Gas supplies are not low before the dive starts (which is when you should make a gas plan). If you plan to have enough gas to complete the SS, there is no extra risk of running out of gas.

NOTE: it should not take more than 850 psi to make this ascent, plus 300 psi for the safety stop.

And there you prove my point. 1150 PSI with a safety stop. The whole reason I wrote up the examples, was to argue that it's silly to claim that "1000 PSI is more than enough". Clearly it is not enough for many divers according to many different ways of calculating minimum gas. Saying "You could get by with 1000 PSI, if you are confident you can calm your breathing in an emergency and your SAC is not very high, and you are comfortable planning to skip the required safety stop." is very different from saying "1000 PSI is more than enough". I still claim that there is very little reason to PLAN to skip a safety stop – why accept a greater risk of DCS in an emergency than any other dive?

PS.
Maybe some of these differing views come from the different types of diving that we do. I can understand that people are more comfortable with less conservatism when they're diving in a group led by a professional dm, in warm water, wetsuits etc., than I am diving with a single buddy or in a team of 3 in cold, murky water with drysuits and heavy undergarments. But unless the training specifies that you're only certified for certain conditions and are mostly a "supervised diver", I think it should cover autonomous diving in more challenging conditions.

 

[tursiops]

And there you prove my point. 1150 PSI with a safety stop. The whole reason I wrote up the examples, was to argue that it's silly to claim that "1000 PSI is more than enough".

850 psi gets you to the surface. 1000 more is more than enough. For this scenario. There are an infinite number of scenarios where you would need more than 1000 psi. and there are an infinite number of scenarios where you would need less than 1000 psi.
It was an emergency....they got to the surface. Maybe they need to go to a chamber. It is NOT an everyday event...it is an extremely rare event....you cannot plan for everything!
We do not have hospitals on every street corner, which is where car accidents are most likily happen.
Why don't you plan for TWO diers out of gas, at depth, at the same time?
Because it is extremely unlikely.
If you are quite afraid of extremely rare events, don't dive.

 

[KenE]

And there you prove my point. 1150 PSI with a safety stop. The whole reason I wrote up the examples, was to argue that it's silly to claim that "1000 PSI is more than enough".

I believe the biggest difference here is the amount of gas the OP stated. The 1000psi is what remained after reaching the surface. The calculations are based on sharing air, do the calculations based on not sharing air and you will see that 1000psi is more than enough. Even with high stress a novice diver should be able to surface from 100ft with 1000psi or most 100cuft tanks are insufficient for use at this depth. Maybe my math is wrong, but based on my high consumption rate and my stress consumption rate, it would be very easy to ascend from 100ft with 1000psi in an 80cuft tank.

 

[inquis]

And then of course you have plenty of data on divers getting bent on "no decompression" dives.

I'd like to read any details, because I thought this was quite rare. Do you happen to have a link to any writeups?

 

[boulderjohn]

Here a

I'd like to read any details, because I thought this was quite rare. Do you happen to have a link to any writeups?

I suspect he does not, so I will try to help. According to the NIH, it occurs roughly in 3 cases for every 10,000 dives with what they call "sport divers" (contrasting with commercial divers). The problem with the estimate is that we can't really know the denominator (number of dives), and we really can't be sure of the numerator (diagnosis is inexact, and many people treated for DCS were likely misdiagnosed). Those totals include all recreational divers, including technical divers, so the totals for NDL dives are likely much lower.

A recent European study showed that the majority of people with DCS were diving within established limits (either NDL or decompression dives), which is not surprising, since the overwhelming majority of divers dive within limits. The majority of people who die in Germany are Germans, but that does not mean it is especially dangerous to be a German in Germany.

 

[Blackcrusader]

Last month I was diving with a young man who had completed the PADI deep certificate.
He was asking me if I had done one and I replied I had not. I did not tell him about my bsac sports diver deco training.
He was concerned I was diving deeper than my certificate allows. I told him OW by PADI can dive to recreational depth limit 40m.
He was shocked. He thought certs had a depth limit after training.
I asked did he have DAN dive insurance. He didn't have any dive insurance. He thought his travel insurance would cover him.


I showed him some posts on scuba board back at the dive shop later that day

Anyway I asked him how many liters of air in an AL80 with 210 bar. He didn't know.
I asked him did he know what his sac rate or RMV was. He didn't know what that was.
I asked him what is the emergency ascent rate, he didn't know. He said he did not recall being taught these things in his Deep Cert.
I asked have you been taught a CBA controlled buoyancy ascent using his BCD to bring another diver to to the surface. Nope.
I didn't see him with an SMB or dive computer. He did not own them.
Basically he went diving to deeper depths with an instructor for several dives and passed the written test. He really didn't seem to remember what was in his course.

He did one dive to maybe 32m depth with me but the others were less than 25m. After one dive he asked me why the guide did not check my air very often. The guide and I laugh. I asked why didn't you check yourself you are my buddy we both laughed but he understood the point. I said if you are my buddy you don't wait for the guide to ask you can check yourself. On the next dives he would check.

He asked me how I knew so much about gas management and consumption etc. I told him about BSAC Sports diving courses deco training, and then also by reading Scubaboard forums. Gradient factors and surf GF a lot of good advice on SB.

He saw I have a Perdix and asked what DC he should get. I recommended a Peregrine. I showed him the dive logs and he was like dang, graphs and lots of information on the DC. Told him just upload them so you can study them on your PC.

He was also asking what would be a good course to do. I recommended the TDI ANDP.

 

[MarkA]

Anyway I asked him how many liters of air in an AL80 with 210 bar. He didn't know.

That’s a bit of a trick question. Using metric and American in the same question, instead of 11.1 litre for the size of the cylinder.

 

[grantmac]

GUE use an estimate of 20L/min (0.7cuft), and a conservative ascent rate of 3m/min (10ft) - both because it's hard to control a quick ascent while gas sharing and to account for an actual higher stressed SAC rate in an emergency.

So from 36m:
Consumption - 2 diver x 20L/min = 40L/min
ATA - 36m / 2 / 10 + 1 = 2.8ATA
Time - 1 minute for gas sharing + 12 minute ascent = 13 minutes
Mingas = 40 x 2.8 x 13 = 1456L (51.42cuft)
in pressure of AL80 = 1456 / 11.1L = 131.17 bar rounded up to 140 bar (2000 PSI)

EDIT:
I would challenge anyone with a less conservative gas plan to try practicing gas sharing ascents with your equipment configuration from 36m and time it. Let me know how it goes.

I used 9m/min and a 3 minute stop which I find quite reasonable. If anything people are likely to blow past the stop under stress.