Oxygen on surface

[jale]

Do any dive planners try to model this re: residual inert gases during intervals?

You can find specific tables for that. Maths have been done

 

[justinthedeeps]

Subsurface will model surface O2.

Up to a ~50 minute pre breath it shows a 3 minute increase in NDL at 100'. That stays same up to about ~200 minutes, after that it starts to reduce NDL. I'm not sure what is up with the model there.

Presumably there will be more of an effect if this is programmed in during a surface interval between dives? Lemme dust off my Subsurface and have a look..

 

[dmaziuk]

Subsurface will model surface O2.

Up to a ~50 minute pre breath it shows a 3 minute increase in NDL at 100'. That stays same up to about ~200 minutes, after that it starts to reduce NDL. I'm not sure what is up with the model there.

The model works on delta-P and if you're breathing pure O2, you're off-gassing nitrogen. Do it long enough and your PN2insp will be approximately 0. When you switch to nitrogen-containing mix, the rate of nitrogen absorption will be driven by PPN2/~0. Without pre-breathing O2 the rate is driven by PPN2/.79 (edit: brain fart).

Someone would have to run the numbers to be sure, but in principle it may be possible to on-gas fast-er enough to actually reduce your NDL.

 

[rjack321]

Hello, question for the people who are trained to dive with 100% oxygen.
Do you use oxygen on the surface before the actual dive to preventively reduce nitrogen tissue saturation?

You are just racking up the CNS and OTUs for very little value.

Do you use oxygen after the dive in addition to regular decompression, e.g. on a liveaboard?

You're much better off extending the 10ft stop

 

[inquis]

in principle it may be possible to on-gas fast-er enough to actually reduce your NDL.

No, pre-breath cannot have a shorter NDL. This is because you have less N2 to start the dive after the pre-breath. Some time after starting the dive, you will have built back up to 0.79 atm (and yes, that absorption rate is faster than the normal initial rate). At that point, your gradient is exactly what the normal dive started with, but that has already used up some of its NDL.

 

[dmaziuk]

No, pre-breath cannot have a shorter NDL. This is because you have less N2 to start the dive after the pre-breath. Some time after starting the dive, you will have built back up to 0.79 atm (and yes, that absorption rate is faster than the normal initial rate). At that point, your gradient is exactly what the normal dive started with, but that has already used up some of its NDL.

Yeah, you're probably right: because of the log curve it's the "Achilles and the turtle" race and the faster one will never catch up.