Recreational NDL times ?

[kylehorn]

I don't want to jump off topic but I want to clarify something I've been wondering from reading this thread. Maintaining a contant PP02 would afford some more NDL time (assuming it's a multi-level dive) because the rebreather would be mixing a higher 02 concentration to meet the PP02 you set as you ascend?

 

[DepthCharge]

I don't want to jump off topic but I want to clarify something I've been wondering from reading this thread. Maintaining a contant PP02 would afford some more NDL time (assuming it's a multi-level dive) because the rebreather would be mixing a higher 02 concentration to meet the PP02 you set as you ascend?

Correct, PPO2 stays constant, FiO2 changes to keep PPO2 constant.
Later,
John

 

[Big Stig]

I think I am starting to get a better grip of this as I work through some of the training materials. here is an example to 60ft.
On OC Air to 60ft NDL = 55 mins
On Nitrox 36% (EAD 42ft) = 140 mins
Best mix Nitrox for 1.2 PPO2 is a 43% mix this would get you to same as a rebreather set at 1.2 mix (EAD34ft.) NDL time 205mins.
Give or take a bit of rounding in my math

 

[HIGHwing]

So the CCR diver grabs two Aluminum 13s. In one tiny tank he fills the diluient with EAN32, the other is filled with 100%. Bailout is either EAN32 via an AL30, or perhaps onboard diluient with a BOV (maybe).

How much gas does the OC diver need?

 

[1.3]

So the CCR diver grabs two Aluminum 13s. In one tiny tank he fills the diluient with EAN32, the other is filled with 100%. Bailout is either EAN32 via an AL30, or perhaps onboard diluient with a BOV (maybe).

How much gas does the OC diver need?

Purchase vplanner and you can switch between OC and CCR and see your associated gas requirements -

Simply and assuming 3L onboard dil + O2, irrespective of depth, for a CCR you are time constrained for your primary onboard gas likely to 180 minutes in water due to O2 exposure at a PPO2 of 1.3... but...
1. You need enough off board bailout and mixes to cover a bailout at your target depth and ascent. You are still carrying OC gases for OC bailout but you likely will not use them.
2. Multilevel dives and lots of up and down will burn through dil so may need additional offboard dil.

 

[Big Stig]

So the CCR diver grabs two Aluminum 13s. In one tiny tank he fills the diluient with EAN32, the other is filled with 100%. Bailout is either EAN32 via an AL30, or perhaps onboard diluient with a BOV (maybe).

How much gas does the OC diver need?

Need to know a few more items before you could calculate that. Like the SAC rate of the open circuit diver

 

[Doppler]

So the CCR diver grabs two Aluminum 13s. In one tiny tank he fills the diluient with EAN32, the other is filled with 100%. Bailout is either EAN32 via an AL30, or perhaps onboard diluient with a BOV (maybe).

How much gas does the OC diver need?

Not sure why a CCR diver would use EAN32 as a diluent on this dive... cell verification at depth would be difficult... better he/she uses air as diluent. More conventional and a dil flush results in a projected reading of about 0.84 across the cells (30 metres = 4 bar X 0.21).

As to gas volumes needed. I don't work CCR in imperial so I think a 13 is a 2 litre cylinder. The average metabolic rate we use in classes etc. for rough calcs is 1.5 litre of oxygen per minute (this is depth independent). A 2 litre cylinder at 200 bar holds 400 litres... divide that by 1.5 and you have approximately enough gas for about 133 minutes.

As to the original question... NOT A LOT of advantage... except warm, moist gas, optimal mix to help manage decompression obligation, no noise no bubbles (well, with a CCR). Would it be worth it... depends on the dive, the location, availability of supplies et al. Sometimes OC is the right tool for the job... sometimes not.

 

[1.3]

Not sure why a CCR diver would use EAN32 as a diluent on this dive... cell verification at depth would be difficult... better he/she uses air as diluent. More conventional and a dil flush results in a projected reading of about 0.84 across the cells (30 metres = 4 bar X 0.21).

You are correct.

CCR Diluant can never exceed a PPO2 of 1.6 (or lower) for your target depth else you would have no way to flush your loop PPO2 down (if it gets too high).

Dil is always going to be Normoxic (16%-21%) or Hypoxic (10%-15%) adding helium to mitigate nitrogen narcosis and lessen decocompression obligations (faster ongassing/offgassing) while ensuring PPO2 of the Diluant does not exceed 1.6. A simple NDL dive to 100' on CCR would likely be performed with Air as Dil slinging a big 80CF tank of air/nitrox for bail out.

 

[DepthCharge]

You are correct.

CCR Diluant can never exceed a PPO2 of 1.6 (or lower) for your target depth else you would have no way to flush your loop PPO2 down (if it gets too high).

Dil is always going to be Normoxic (16%-21%) or Hypoxic (10%-15%) adding helium to mitigate nitrogen narcosis and lessen decocompression obligations (faster ongassing/offgassing) while ensuring PPO2 of the Diluant does not exceed 1.6. A simple NDL dive to 100' on CCR would likely be performed with Air as Dil slinging a big 80CF tank of air/nitrox for bail out.

You are talking strictly recreational RB diving correct?
Later,
John

 

[1.3]

You are talking strictly recreational RB diving correct?
Later,
John

LOL - Sorry - No. Air for dil. The recreational rebreathers will self check FO2 on the pre dive test and fail if its out of spec. Real rebreathers will perform a O2/dil gas test as part of their standard predive tests.