Geezer Gas

[drrich2]

MaxBottomTime:

Maybe they teach it in Advanced Nitrox? I always wondered what people learned in that course...

Richard.

 

[MaxBottomtime]

They teach equivalent air depths in basic nitrox, but that doesn't make it OK to make a rapid ascent from any depth, even one meter.

 

[NAM001]

KenGordon

I would guess you need to reread your nitrox material. Well perhas I should maybe. I have never heard of nitrox mitigating the effects of fast ascent. I hope you did not mean what you posted. I would not use nitrox in any other way than what it was made for. To reduce the amount of N2 being absorbed. There are many ways of looking at this statement you made in regards with its logical conclusion.

1st.. You can ascend faster.... NOt true. Fast ascent does nothing but speed up the off gassing, no matter wht the gas is. Nitrox increases the ongassing time and shortens the off gassing time. Hence longer bottom times OR shorter SI's. Pick one or the other.. cant have both. Si times are shorter not because of the magic of the gas but the amount of A (N2) gas in your tissues. The other eliment is that wth less N2 you have less effects of being narced. When you use nitrox to max bottom time then you are hitting NDL. NDL IS NDL. At least one tissue has saturated, the fastest one. Once full your SI will be the same as if you used air. Similar to you car and a gas pump . some pumps pump fast. but when you are filling with nitrox premium it pumps slower. When your tank is full it still takes the same time at 70mph to empty the tank.

2. Nitrox and EAD tables with dcs. I cant speek to this very much however the name EQUIVILANT air depth. is a device to convert you depth on nitrox to a depth that provides the same N2 effects when on air. You only have these cause otherise you would have to have a table for every mixture from 20-100%. Granted with nitrox you would only have 20-40 and thats only 20 tables. This is one aspect I always asked my self WHY, at the time i took the class. When I got into trimix it became clear the real usefullness of such a table or math process. Can you imagine how many tables would be neeeded for all the trimx variations. The math of EAD is used for END in the trimix world. same concept just different numbers. Being an unedcated type, Im not really sure what youare refering to about increased DCS. I am comfident that when using the EAD table I can now use the air dive table to the corrected depth and if after exceeding NDL be able to use the air table to do the deco that may be needed. Once again I am not sure if this is what you are referencing. One thing for certain , your thoughts have made me recall long past training.

 

[KenGordon]

At a given depth on nitrox the ppN2 is lower. Agreed? Thus when any given tissue becomes saturated, ie the tissue ppN2 reaches the inspired ppN2, there will be less nitrogen in the tissues. This is just the same as being at a lesser depth but on air. Staying longer on Nitrox you never catch up with being on air as the ambient ppN2 is lower. Saturated on nitrox is not the same as saturated on air.

One of the limiting factors in ascent speed is DCS. Decompression algorithms try to limit the rate at which gas comes out of solution and forms bubbles, with less gas in solution to begin with there is less chance of forming bubbles. Just as running for the surface immediately is not as dangerous as running for the surface at the end of a dive. There is just less nitrogen to cause a problem.

I am not saying fast ascents are safe, just less dangerous on richer mixes.

For other claims of wellbeing and lack of narcs I don't see that myself. I dive it because it lets me stay longer for shorter stops and be on average safer while I am there.

 

[GrumpyOldGuy]

At a given depth on nitrox the ppN2 is lower. Agreed? Thus when any given tissue becomes saturated, ie the tissue ppN2 reaches the inspired ppN2, there will be less nitrogen in the tissues. This is just the same as being at a lesser depth but on air. Staying longer on Nitrox you never catch up with being on air as the ambient ppN2 is lower. Saturated on nitrox is not the same as saturated on air.

One of the limiting factors in ascent speed is DCS. Decompression algorithms try to limit the rate at which gas comes out of solution and forms bubbles, with less gas in solution to begin with there is less chance of forming bubbles. Just as running for the surface immediately is not as dangerous as running for the surface at the end of a dive. There is just less nitrogen to cause a problem.

I am not saying fast ascents are safe, just less dangerous on richer mixes.

For other claims of wellbeing and lack of narcs I don't see that myself. I dive it because it lets me stay longer for shorter stops and be on average safer while I am there.

Please explain the physics of why nitrogen saturation from nitrox is different than nitrogen saturation on air? No doubt it takes more time to hit saturation on Nitrox, but once you get there it is the same.

While you are at it, explain how you quantify "safer".

 

[NAM001]

No matter what you are breathing you If you are saturating tissue, are doing it with some gas or another. It all has to com out. some gasses come out easier than others. Some one correct me if needed. Helium off gasses faster. Ascents need to be slower for that faster gas. You will , with nitrox, "catch up" with nitrox.It just takes a while for it to happen. perhaps 20 min instead of 8 min on air. That is why nitrox is used. That is why NDL's are longer with nitrox than air with all else equal. beer or moonshine you still passs out as XXX BAC. it just takes longer to get there using beer.

 

[DaleC]

Saturated on nitrox is not the same as saturated on air.

This is not true. Nitrox simply provides less of a gradient for the on gassing of N. The N is the same element regardless and saturates the same way.
Conversely, nitrox allows a greater gradient for the off gassing of N, when compared to air in the same scenario.

It looks like this:

 

[Steve_C]

(or 500psi, which I understand is the norm on your side of the pond) left in the tank. Which incidentally, is far from enough for a controlled ascent from 20+ meters' depth while donating gas to an OOG buddy, with enough reserves to do a proper safety stop before surfacing.

In some locations, like Keys Reefs, you are talking 7-10 meters and not 20+ Personally I usually start up on deeper first dives with around 700-800 (not counting a pony). Second dives are NDL limited so even more air. Locally in NC most of my buddies dive HP100s or larger and have decent SAC rates. We are always NDL limited especially on dive 2 and often on dive 1.

 

[KenGordon]

Please explain the physics of why nitrogen saturation from nitrox is different than nitrogen saturation on air? No doubt it takes more time to hit saturation on Nitrox, but once you get there it is the same.

While you are at it, explain how you quantify "safer".

Both depth and fraction of nitrogen matter.

Saturation is when the partial pressure of a gas (ppN2) in the tissue reaches the partial pressure of the gas in the lungs (inspired ppN2). So at the surface on air that is 0.79 bar and at 30m it is 3.16 bar. After a time your tissues will stop taking on more nitrogen because it reaches 3.16 bar and the gas going in and coming out is in equilibrium. On 32% at 30m saturation is 2.72. Hence there is less mass of nitrogen dissolved and less to be off gassed on the ascent.

Safer means a lower probability of injury. I can't give a number but I can influence the contributing factors in the right direction. Less dissolved nitrogen is better than more nitrogen.

 

[NAM001]

This is starting to get abovce my head but are you speeking about a single tissue or all tissues when you cal the pont of saturation? I dont believe you ar saturated when yo hit NDL. By the definitions you are citing,,, to come out of saturation you just go deeper brvsudr your tissues can now take on more. Something for me is missing from your position on this.

Both depth and fraction of nitrogen matter.

Saturation is when the partial pressure of a gas (ppN2) in the tissue reaches the partial pressure of the gas in the lungs (inspired ppN2). So at the surface on air that is 0.79 bar and at 30m it is 3.16 bar. After a time your tissues will stop taking on more nitrogen because it reaches 3.16 bar and the gas going in and coming out is in equilibrium. On 32% at 30m saturation is 2.72. Hence there is less mass of nitrogen dissolved and less to be off gassed on the ascent.

Safer means a lower probability of injury. I can't give a number but I can influence the contributing factors in the right direction. Less dissolved nitrogen is better than more nitrogen.