Oxygen CNS% accumulation debt/ PO2 / repetitive diving

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

iainwilliams

Contributor
Messages
177
Reaction score
0
Location
Hobart, Tasmania, Australia
# of dives
Hi - I have a question regarding partial pressure of oxygen, EAN mixes and repetitive diving.

We know that the NOAA CNS % exposure table recommends a maximum PO2 of 1.6 atmospheres. We also know that EAN mixes have been calculated on the basis of equivalent air depth (EAD), and NOT the PO2 100% which at 1.6 atmospheres is 6m / 20 feet.

QUESTION: If I use an EAN mix above (less than) 1.6 atmospheres on a dive (say to 30 meters/100 feet) then at the safety stop (6-4 m) decide to breath 100% O2 for say 5-10 minutes; will I be risking a lowering of CNS tolerance and possible convulsion? I know what CNS% level I am from the mix, depth and bottom time (from the table or computer) – but will the extra 100% oxygen increase my CNS too much?

I know that I can do this dive on normal air, swim to the safety stop and outgas on 100% oxygen (I do this regularly to offset against DCS).

Furthermore, will this add to my CNS debt at the surface during my surface interval, if I decide to do a repetitive dive? I’m pretty sure it will. If I am using nitrox tables, say EAN32, and off-gas at 4 m on 100% oxygen for 5 minutes thereabouts, how should I calculate my surface time interval? How much do I add? Or is the added CNS debt acquired from my 100% oxygen spell hardly worth the worry?

I hope this makes sense. Thank you. Iain
:confused:
 
It really sounds like a couple of questions.

Let me first suggest that you limit your bottom mix to no more than 1.4 and consider lowering it for extended stays or larger incurred deco obligations. 1.6 for recreational limits (say 130 and above) isn't worth the trade off for potential problems in my opinion.

The best answers to your questions can be found on the 'Ask DR Deco' portion of Scuba Board. There esteemed experts, such as Dr. Bruce Weinke, have laid all this out in a format that I couldn't being to reproduce here. They cover both the CNS calcs and the OTUs.

Good luck.








Originally posted by iainwilliams
Hi - I have a question regarding partial pressure of oxygen, EAN mixes and repetitive diving.

We know that the NOAA CNS % exposure table recommends a maximum PO2 of 1.6 atmospheres. We also know that EAN mixes have been calculated on the basis of equivalent air depth (EAD), and NOT the PO2 100% which at 1.6 atmospheres is 6m / 20 feet.

QUESTION: If I use an EAN mix above (less than) 1.6 atmospheres on a dive (say to 30 meters/100 feet) then at the safety stop (6-4 m) decide to breath 100% O2 for say 5-10 minutes; will I be risking a lowering of CNS tolerance and possible convulsion? I know what CNS% level I am from the mix, depth and bottom time (from the table or computer) – but will the extra 100% oxygen increase my CNS too much?

I know that I can do this dive on normal air, swim to the safety stop and outgas on 100% oxygen (I do this regularly to offset against DCS).

Furthermore, will this add to my CNS debt at the surface during my surface interval, if I decide to do a repetitive dive? I’m pretty sure it will. If I am using nitrox tables, say EAN32, and off-gas at 4 m on 100% oxygen for 5 minutes thereabouts, how should I calculate my surface time interval? How much do I add? Or is the added CNS debt acquired from my 100% oxygen spell hardly worth the worry?

I hope this makes sense. Thank you. Iain
:confused:
 
Originally posted by iainwilliams
QUESTION: If I use an EAN mix above (less than) 1.6 atmospheres on a dive (say to 30 meters/100 feet) then at the safety stop (6-4 m) decide to breath 100% O2 for say 5-10 minutes; will I be risking a lowering of CNS tolerance and possible convulsion? I know what CNS% level I am from the mix, depth and bottom time (from the table or computer) – but will the extra 100% oxygen increase my CNS too much?

I know that I can do this dive on normal air, swim to the safety stop and outgas on 100% oxygen (I do this regularly to offset against DCS).

Furthermore, will this add to my CNS debt at the surface during my surface interval, if I decide to do a repetitive dive? I’m pretty sure it will. If I am using nitrox tables, say EAN32, and off-gas at 4 m on 100% oxygen for 5 minutes thereabouts, how should I calculate my surface time interval? How much do I add? Or is the added CNS debt acquired from my 100% oxygen spell hardly worth the worry?

I hope this makes sense. Thank you. Iain
:confused:
Iain,

First of all, Maddiver is correct in stating that 1.4 is the recommend PO2, and the 1.6 is considered the ABSOLUTE RECOMMENDED Limit. The advice to check with some of the areas in the board that deal with this type of question specifically is sound also.

Having said this, I will take a stab at answering your question anyway.
First we need to know the best mix for your statement:
If I use an EAN mix above (less than) 1.6 atmospheres on a dive (say to 30 meters/100 feet)

Fg = Pg / P
Fg = 1.4 / ((100/33)+1) = 1.4 / 3.03+1 = 1.4 / 4.03
Fg = 34% EANx

Pg = Fg x P
Pg = .34 x ((100/33)+1) = .34 x 4.03
Pg = 1.37 PO2

So now we know the best mix for this dive is EANx 34 and it will yield a P02 of 1.37 at 100’. Next, we need to know how long this dive is. I am going to use 25 minutes as that is max allowable from IANTD Table for EANx 34.

To calculate the percentage of our O2 clock used we will divide Time at P02 by NOAA max single exposure limit.
CNS02 = 25 / 150 = 17% of O2 clock used on this dive.

A safety stop at 15’ for 10 minutes on 100% O2 yields the following:
Pg = Fg x P
Pg = 1 x ((15/33)+1) = 1.45 PP02
CNS02 = 10 / 120 (using the 1.5 PO2 table) = 8.3% of O2 clock used at safety stop

Therefore this dive uses (17+8.3 = 25.3) 25.3 % of your total CNS02 clock for the day. You should never exceed 100% of your O2 clock, and it is recommend you stay under 90% to leave yourself a cushion, so you have about (90 – 25.3 = 64.7) 65% of your 02 clock remaining for subsequent dives for this day.

So, to answer your question, the use of 100% O2 will help to decrease the amount of nitrogen, but it WILL add to your CNS02 clock, a factor that should be considered when planning subsequent dives for that day. For the purpose of this illustration, I am going to assume that a SI does NOT credit anything back to your CNS02 clock as that just adds more complexity into it. You have a greater safety factor basing your clock on no SI credits.

Now, for the disclaimer... The above information is used to illustrate an example and *MAY* contain errors. It is not intended to teach or be used as dive planning, it is merely for discussion purposes. Before embarking on ANY dive it is recommended that someone recieve proper training and not dive beyond their training or experience.
 
and the calculations. However, I infer that you will not remain using the same profile(s) as you posted and will no doubt begin seeking deeper dives and/or longer deco obligations. As pointed out by John, it is a rather benign issue with the dive you use as an example and are not in danger from violation of the tables (as you know tables are theory and not the distinct line that defines your specific physiology).

This is why I sent you to read over on the other area. Many divers frequently exceed the CNS and OTU limits as published and have no measurable adverse effects. See reference to WKPP etc. There is a lot of discussion surrounding air brakes and CNS clock resetting. I **HIGHLY ** recommend you read all of this plus research other areas before cautiously implementing it into your dive table calculations.

Originally posted by Cave Diver
Iain,

First of all, Maddiver is correct in stating that 1.4 is the recommend PO2, and the 1.6 is considered the ABSOLUTE RECOMMENDED Limit. The advice to check with some of the areas in the board that deal with this type of question specifically is sound also.

Having said this, I will take a stab at answering your question anyway.
First we need to know the best mix for your statement:
If I use an EAN mix above (less than) 1.6 atmospheres on a dive (say to 30 meters/100 feet)
 
Originally posted by maddiver
This is why I sent you to read over on the other area. Many divers frequently exceed the CNS and OTU limits as published and have no measurable adverse effects. See reference to WKPP etc. There is a lot of discussion surrounding air brakes and CNS clock resetting. I **HIGHLY ** recommend you read all of this plus research other areas before cautiously implementing it into your dive table calculations.


Excellent advice.
 
Many thanks for taking the to reply! I agree with all your comments. 1.4 is definitely safer. I also like your idea regarding zero surface credit for CNS if doing another dive that day - added conservatism here also. The profile I mentioned was not meant to be diciphered as a an actual profile. I'll re-list the question on the section you mentioned - I was not sure which area was the right one! Dive safe-and THANK YOU:)
 
https://www.shearwater.com/products/peregrine/
http://cavediveflorida.com/Rum_House.htm

Back
Top Bottom