10 versus 20 foot last stop

[tbone1004]

@ofg-1 if you're diving 150-300ft it shouldn't matter since you should be on some sort of accelerated deco gas. At 150ft, I'm only using a 50% bottle and I will stop wherever I need to based on conditions. That gets you out faster than just using O2, and if you need to stay below 20ft, you're still doing much better than on backgas.
If diving nitrox, the difference can actually be quite large. 150 minute dive at 100ft on EAN32 with gf50/80. 10ft last stop, 289 total minutes. 20ft last stop is a 332 min run time. Adds 43 mins to your total deco by not going up there

Get on EAN50 and it only drops to 257 mins with a 20ft last stop and 245mins with 10ft. At that point, I'll stay at 20ft. If you can do it on O2 instead of EAN50, then you have 224min run time with last stop at 10ft or 20ft, doesn't matter.

On that dive, if you stay on EAN32, the 10ft stop saves 43 minutes of deco which is probably worth fighting it. If you do it on EAN50 it saves 12 minutes, probably not worth it to go up to 10ft, but it does cut your run time by 75 minutes so you want to be on EAN50 if you can. If you can do it on O2, it doesn't change so by all means, stay at 20ft and take it with you since it cuts your run time by 108 minutes. That's a LOT of deco that you're saving by getting on O2 instead of Nitrox.

On a 60min run time
EAN32-20ft-100mins
EAN32-10ft-93mins
EAN50-20ft-87mins
EAN50-10ft-85mins
100%-78mins

Depending on your personal GF's, particularly the GF-low you can invert the benefit of O2 vs EAN50 depending on your bias of deep stops. If you go to 150ft, then it definitely flips over and the benefit of EAN50 as your sole deco gas becomes very apparent.

 

[beester]

@ofg-1 if you're diving 150-300ft it shouldn't matter since you should be on some sort of accelerated deco gas. At 150ft, I'm only using a 50% bottle and I will stop wherever I need to based on conditions. That gets you out faster than just using O2, and if you need to stay below 20ft, you're still doing much better than on backgas.
If diving nitrox, the difference can actually be quite large. 150 minute dive at 100ft on EAN32 with gf50/80. 10ft last stop, 289 total minutes. 20ft last stop is a 332 min run time. Adds 43 mins to your total deco by not going up there

Get on EAN50 and it only drops to 257 mins with a 20ft last stop and 245mins with 10ft. At that point, I'll stay at 20ft. If you can do it on O2 instead of EAN50, then you have 224min run time with last stop at 10ft or 20ft, doesn't matter.

On that dive, if you stay on EAN32, the 10ft stop saves 43 minutes of deco which is probably worth fighting it. If you do it on EAN50 it saves 12 minutes, probably not worth it to go up to 10ft, but it does cut your run time by 75 minutes so you want to be on EAN50 if you can. If you can do it on O2, it doesn't change so by all means, stay at 20ft and take it with you since it cuts your run time by 108 minutes. That's a LOT of deco that you're saving by getting on O2 instead of Nitrox.

On a 60min run time
EAN32-20ft-100mins
EAN32-10ft-93mins
EAN50-20ft-87mins
EAN50-10ft-85mins
100%-78mins

Depending on your personal GF's, particularly the GF-low you can invert the benefit of O2 vs EAN50 depending on your bias of deep stops. If you go to 150ft, then it definitely flips over and the benefit of EAN50 as your sole deco gas becomes very apparent.

Tbone... I guess you are just exaggerating runtimes to make a point. But seriously 150' bottom times on a 30m profile? Are there many people doing that kind of dives? In any case if you are doing those kind of dives (rather shallow but long) I would be very wary of my slow tissues and and would add conservatism to the profile (and for sure O²).

On rather shallow north sea dives (work dives, netcleaning wrecks), we have seen some issues. A 60' at 30m with 32% is from my point of view a bigger dive than a 30' at 45m with tx (even tho the depth would suggest differently).

I think Devon said it best. In a diffused gas model, M-value drives deco ceiling and the gas that you breath in combo with ambient pressure drives deco time. If the gas you breath is air or backgas (no deco gas) the only way to drive the gradient up is to go shallower. So by all means go up after your computer (or the plan) clears the 6m stop. If you are using a deco gas depending on the gas it will still have a difference, but with O² there is from deco point of view no difference, the Fpo² is already at max so being shallower is not going to push the gradient more than the 100% it already is.

 

[tbone1004]

@beester those were actual times for first dive that were coming out of deco planner using GF50/80. All very real numbers, and not abnormal dive times in Florida caves. They're long dives, but they're not abnormal, especially on CCR's.

 

[beester]

Hi Tbone, stand corrected. Was thinking in context of wreck diving in sea... obviously that 30m depth range is typical Florida caves. You must have been using a breather or stages a lot of stages :-D

 

[MegDiver792]

My comp is set for 10ft, but we usually deco around 14-17 ft. Much shallower and my RB solenoid is firing like a mad man.

 

[kensuf]

My comp is set for 10ft, but we usually deco around 14-17 ft. Much shallower and my RB solenoid is firing like a mad man.

Do an O2 flush and lower your setpoint.

 

[MegDiver792]

Do an O2 flush and lower your setpoint.

why in the world when doing over an hour of deco would I lower my SP?

 

[tbone1004]

why in the world when doing over an hour of deco would I lower my SP?

First, you're probably going to hit your CNS limits by then if you believe in abiding by them. Getting to 10ft asap is ideal even if it means a lower setpoint. pO2 doesn't matter for deco efficiency. Lack of fN2 and fHe matter for deco efficiency as that is what creates the gradient across your tissues.
You could deco on 20/80 O2/Ar and it would be as fast as 100% O2 because it's still 0% N2 and He in the mix. Just easier/cheaper to do it on pure O2 without the lovely effects of argon narcosis and density.

Second, when you are on deco your body is pumping nitrogen and/or helium into the loop as you offgas which is lowering the fO2 in the loop as you exhale via addition of inert gases in addition to the O2 you are consuming. That means that you can't ever achieve 100% O2 in the loop during deco without almost constant O2 flushes in the loop. If you have your setpoint close to the ppO2 of 100% at that depth, i.e. 1.6 at 20ft, 1.5 at 16ft etc. then it is essentially impossible to maintain that setpoint as you can't get the fO2 required in the loop. Set it to something where you have an fO2 of 80% or 90% and then the breather will at least be able to attempt to keep up. Still have to do O2 flushes on a somewhat regular basis as your body keeps dumping inert gases into the loop, but if you set it to the equivalent pO2 of a 100% fO2, it is physically impossible to maintain that setpoint for any length of time

 

[kensuf]

why in the world when doing over an hour of deco would I lower my SP?

Because if you do an O2 flush then you only have oxygen in the loop, but if you're trying to run a 1.3 set point and you're at 10', your solenoid is going to be firing like crazy.

 

[MegDiver792]

yup, which is why we stop around 15ft, running 1.3.

I only ever blow the CNS clock on 4 hour dives.