question about NDL's, need some insight..

[socaljohn]

fantastic explanation.

Movement of gas into the tissues is not a function of respiratory rate, it is a function of the difference between the partial pressures.
As you descend, the partial pressure of the gases you're breathing goes up. As it goes up, the gases move into the tissues faster, until the partial pressure of the gas in the tissue is equal to that of the gas you're breathing. At this point, the tissue is "saturated".
The deeper you dive, the faster the tissues will become saturated, resulting in shorter NDL times.
When you ascend, the partial pressure of the gases you're breathing goes down, causing gas to move back out of the tissues until, again, the partial pressures match. This is "off gassing". If you ascend too quickly, the gases moving out of the tissues form bubbles. This is being bent.
The rate at which you breath has no real impact on these pressures, and therefore no impact on tissue saturation.

fantastic explanation. thanks.

 

[Dirty-Dog]

All above answers are wrong.
SAC is related to stress, workload, thermal stress etc. Why do you have higher SAC?
Let's take workload. It is DCS risk factor. More workload is more inert gas intake. Same with cold.
But I wouldn't worry. NDL for rec divers is set with big margin error and it's very unlikely you will be sick just because of higher air usage.

Here is the proof:
Workload | Decompression Sickness - DAN Health Issues & Diving
Thermal Stress | Decompression Sickness - DAN Health Issues & Diving

Perhaps you should re-read the original question. It wasn't about SAC rates, it was about NDL.

 

[scagrotto]

I noticed on a recent dive that NDL's simply calculate from depth and the minutes/ seconds spent at that depth

My first thought was that you should have noticed that during your OW class. Maybe it's a bit more excusable to not learn it now that most classes don't teach tables, but it strikes me as a very basic concept. I suppose that as long as you pay attention to your computer (and know how to use it) it's not essential to understand the physics of ongassing and offgassing, but I'm still very much inclined to think it's useful to understand the concept.

if someone uses for example 200bar on a single 10 and someone else only uses 50bar from a single 10 (exact same profile)

It may seem to make sense that breathing more gas could result in absorbing more N2, but perhaps you've noticed that after you inhale you've always managed to exhale, too? That's because no matter how much or how little gas you breathe most of it isn't absorbed and there's plenty of N2 (and unmetabolized O2, plus some CO2) left to exhale. The ongassing is limited by the physics and physiology of the process, rather than the amount of available N2.

Perhaps you should re-read the original question. It wasn't about SAC rates, it was about NDL.

Perhaps you should re-read the original question yourself. The OP specifically asked if using more gas increased N2 loading. I'm pretty sure that using more gas is, in fact, about SAC.

 

[dmaziuk]

I'm pretty sure that using more gas is, in fact, about SAC.

Only if you define "using" as "blowing out through your regulator" as opposed to gas getting into your bloodstream. While I'm sure the claim that higher workload causes more gas absorption it technically true, I'd like to see DANs' definition of "high workload". Specifically, how hard do you have to work for how long to get nitrogen loading comparable to 5 minutes at 1 extra atm of pressure.
Which is not the same as increased risk of DCS due to exertion. There's more happening there than just N2 loading.

 

[Dirty-Dog]

Perhaps you should re-read the original question yourself. The OP specifically asked if using more gas increased N2 loading. I'm pretty sure that using more gas is, in fact, about SAC.

What was asked was why a higher SAC rate does not equal a lower NDL.