CNS O2 exposure

CNS O2 exposure

The Oceanic computers track O2 exposure per 24 hour period using the NOAA table and give no credit for surface intervals. It is entirely possible to reach 80% or even to exceed 100% when one is diving 4 or 5 dives per day on nitrox, especially multiple days.

I'd be interested to hear if other computers give surface interval credit and whether this credit uses the 90 minute half-life often stated for oxygen

scubadada:

CNS O2 exposure

The Oceanic computers track O2 exposure per 24 hour period using the NOAA table and give no credit for surface intervals. It is entirely possible to reach 80% or even to exceed 100% when one is diving 4 or 5 dives per day on nitrox, especially multiple days.

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The O2 loading just drops off completely 24 hours after the dive.

What difference does it make (as far as the computer is concerned) if you dive 4,5 or 1000 days in a row?

O2 loading decreases gradually on the Shearwater Predator. Dunno what half life it uses.

I've never come any where near O2 limits on recreational trips. Stick to a few simple guidelines ( No more than 32%, pO2 no more than 1.4,No deco)
and CNS O2 is a complete non event.

ianr33:

The O2 loading just drops off completely 24 hours after the dive.

What difference does it make (as far as the computer is concerned) if you dive 4,5 or 1000 days in a row?

O2 loading decreases gradually on the Shearwater Predator. Dunno what half life it uses.

I've never come any where near O2 limits on recreational trips. Stick to a few simple guidelines ( No more than 32%, pO2 no more than 1.4,No deco)
and CNS O2 is a complete non event.

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Crappy answer. Four dives at pO2 of 1.0 for 1:15 per dive over 24 hours is 100% of O2 esposure. It makes a difference whether you're credited for surface intervals or not. I'm finding most divers are not aware of their O2 exposure. If Shearwater credits for surface interval, they may be in the minority.

Thank you for bring this characteristic on Oceanic computers up. Recently I was diving pretty hard on Nitrox in Cozumel (mainly drifting without a lot of physical exertion) and I did 4 dives one day on a 32 x 36 x 32 x36 spread. On all of the dives I was riding the depth limits moderate to heavy for a total bottom time of 315 minutes. The next morning my VEO was showing near 100% on the O2 limit so I switched back to 21%, we then dove Punta Sur and I went into DECO (my fault) and had to abort and do a 10 minute DECO stop, not a big deal but it spoiled my dive. Had I known Oceanic did not give a 90 minute O2 half-life credit I would have run another bottle of 32 and enjoyed the dive. After reading this post I found another post with a lot of good info http://www.scubaboard.com/forums/advanced-scuba-discussions/429659-oxygen-exposure-limits.html
Any additional information on this subject would be welcome.

scubadada:

CNS O2 exposure

The Oceanic computers track O2 exposure per 24 hour period using the NOAA table and give no credit for surface intervals. It is entirely possible to reach 80% or even to exceed 100% when one is diving 4 or 5 dives per day on nitrox, especially multiple days.

I'd be interested to hear if other computers give surface interval credit and whether this credit uses the 90 minute half-life often stated for oxygen

Click to expand...

I was taught to use a 90 minute half-life for O2 exposure. Given that, assuming you're staying inside your NDL and practicing a rational surface interval it is nearly impossible to exceed your O2 limit. I was unaware that Oceanic computers behave in this manner, sounds like a limitation, particularly if the computer locks you out if you exceed 100%

scubadada:

Crappy answer. Four dives at pO2 of 1.0 for 1:15 per dive over 24 hours is 100% of O2 esposure.

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It's also a deco dive. Even the first one. Or at least it is in my universe.

Dec to 75ft (1) Nitrox 32 50ft/min descent.
Level 75ft 73:30 (75) Nitrox 32 1.01 ppO2, 58ft ead
Asc to 20ft (76) Nitrox 32 -30ft/min ascent.
Stop at 20ft 12:10 (89) Nitrox 32 0.49 ppO2, 10ft ead
Surface (89) Nitrox 32 -30ft/min ascent.

Off gassing starts at 43.3ft

OTU's this dive: 76
CNS Total: 25.0%

154.5 cu ft Nitrox 32
154.5 cu ft TOTAL

What size tanks are you using to get 75 minutes at 75 feet ?

I stand by my crappy answer.

ianr33:

It's also a deco dive. Even the first one. Or at least it is in my universe.

Dec to 75ft (1) Nitrox 32 50ft/min descent.
Level 75ft 73:30 (75) Nitrox 32 1.01 ppO2, 58ft ead
Asc to 20ft (76) Nitrox 32 -30ft/min ascent.
Stop at 20ft 12:10 (89) Nitrox 32 0.49 ppO2, 10ft ead
Surface (89) Nitrox 32 -30ft/min ascent.

Off gassing starts at 43.3ft

OTU's this dive: 76
CNS Total: 25.0%

154.5 cu ft Nitrox 32
154.5 cu ft TOTAL

What size tanks are you using to get 75 minutes at 75 feet ?

I stand by my crappy answer.

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Hi ianr33, thanks for the reply. Correct, you are in your universe with your assumpitions, and I am in mine. You should take care in generalization.

I dive EAN36 to an average depth of about 60 feet for 75 minutes. My CNS exposure is 25%. I'm diving the DSAT algorithm without conservative factor. This is not close to the NDL of 2:04. I make these dives with an AL80. My average SMRV is 0.30-0.33. I surface from my drift dive with about 500 psi, or slightly less.

It's not difficult to make 4 dives like this per day, 2 in the morning with a reasonable SI and 2 in the afternoon after coming in for lunch. Throw in 1 or 2 slightly deeper dives and you may increase your oxygen exposure. Add a 5th dive and the exposure is increased. Multiple days do add to your exposure because you are adding additional dive time onto the previous day prior to clearing the 24 hour period.

Realizing my problem with oxygen exposure, I have switched from EAN36 to EAN32 for my usual local dives. Some of my dives are now deco limited and my total dive time is somewhat decreased, but I rarely approach 100% of my CNS oxygen exposure. There's an important lesson here, everyone diving nitrox should be aware of oxygen exposure and its basis, not only the partial pressure limits, but the exposure over time.

I'd still like to know if some computers give oxygen surface interval credit. It's my impression that this is generally not taken into account for recreational diving.

Good diving, Craig

Do no dive computers give surface interval credit for oxygen exposure?

Thanks, Craig

scubadada:

Do no dive computers give surface interval credit for oxygen exposure?

Thanks, Craig

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As mentioned in post #2, Shearwaters do give credit. From the Petrel's manual: "The CNS percentage is calculated continuously, even when on the surface and turned off." I know the Atomic Cobalt also gives SI credit for CNS clock, though the less detailed manual doesn't mention that. Like post #2, I have no idea what the half-life used is for either one.

Mares Puck increases the CNS % number during the dive, and decreases it during SI.