N2/He Loading numbers

[Abhijit Roy]

Hey everyone!
I got my new garmin and I am curious on how to interpret N2 loading and CNS loading at the end of dive. For example it says my N2 loading at the end of dive is 52% and CNS loading is 13%. What do I understand from it and how should I act on it?

 

[JohnnyC]

Basically the N2 loading is just a way of tracking your tissue loading, as based on whatever algorithm you are using. The manual should explain how best to interpret it.

CNS% is related to your oxygen exposure. As oxygen partial pressure increases, you begin cutting into your allowable O2 exposure. This is just a representation of that. The usefulness of an O2 clock is debatable.

How you act on that information is a personal choice.

 

[rongoodman]

Another thing to think about is that at the end of the dive different tissues in your body are going to incur different amounts of inert gas loading. Those which are highly vascularized and take on gas easily will be more heavily loaded, while others, such as bone or ligaments, will have had time to absorb very little gas during the course of a single dive. Different decompression algorithms model this in different ways. Buhlmann GF, one of the most commonly used in technical dives, postulates 16 compartments, with half-times for N2 loading ranging from 4 to 635 minutes. That means that within 24 minutes(six half-times) of surfacing the fastest compartment will be completely clear, while the slowest will barely show any on-gassing even after a week of 5 recreational dives daily. Shearwater computers have a nice color graph illustrating this process. The big question, of course, is how closely these various models relate to the actual tissues and organs which make up your particular body,

 

[broncobowsher]

CNS will matter a lot more when you get into nitrox (and advanced nitrox) diving. For recreational air dives, I think it is nearly impossible to get into trouble with it. I am sure someone will come up with a special scenario where it can matter, but you will be up against the wall with Nitrogen loading first.

Not sure what Garmin is doing to calculate N2 loading, nor do I really care. What that simplified number means is you have xx% of allowable Nitrogen in your system. Given time it will go down. Dive more, it will go up. How much, how fast? That is where the fancy math happens behind the curtain. Think of it as a number instead of a letter group on the old dive tables. The computer can be a little more precise then just categorizing into a letter.

In short, stay under 100%. The further under 100% the safer you should be. If you are at 85% and are thinking of doing another dive, it would probably be best to sit back and take an extra surface interval and let the numbers work back down.

 

[Abhijit Roy]

Thank you all! Very insightful answers typical of Scubaboard Unfortunately Garmin manual doesn't give enough info around N2/He. I was doing a multi-gas dive with EAN30 as bottom gas and EAN50 as deco. Finished the diving as per the dive plan and all. So, when I came up, it showed me those two numbers. My confusion is what I do with these two. Is there a set number or guideline that I should wait before my next dive (e.g. N2/He at 30/CNS at 10%). Or what is the red line that I shouldn't cross?

 

[JohnnyC]

You have to make your own determination about doing a repetitive dive with inert gas loading. That's your call.

Same for the CNS%. Although if you're doing accelerated decompression, you should probably be more familiar with CNS limits and oxygen toxicity issues.

 

[Nick_Radov]

Thank you all! Very insightful answers typical of Scubaboard Unfortunately Garmin manual doesn't give enough info around N2/He. I was doing a multi-gas dive with EAN30 as bottom gas and EAN50 as deco. Finished the diving as per the dive plan and all. So, when I came up, it showed me those two numbers. My confusion is what I do with these two. Is there a set number or guideline that I should wait before my next dive (e.g. N2/He at 30/CNS at 10%). Or what is the red line that I shouldn't cross?

Garmin dive computers use the Bühlmann ZHL-16C decompression algorithm. The N2/He numbers indicate an estimate of the residual inert gas remaining in various tissues of your body. If you do a repetitive dive before those compartments are cleared then you will have a shorter NDL, or have to do more deco. Note that the Bühlmann algorithm doesn't really account for bubble mechanics so it might underestimate DCS risk on repetitive diving in some circumstances. The Wikipedia page has a decent overview and references to more details if you want to dig into decompression theory.

Bühlmann decompression algorithm - Wikipedia

en.wikipedia.org

The CNS% indicates how close you are to a theoretical limit where a seizure becomes a real risk. The research in this area has been limited and not consistently reproduced so I don't take the specific number on the dive computer very seriously. But in general if you're doing repetitive dives you don't want to push the oxygen exposure. Better to use lower oxygen mixes (within reason) and just do a little more deco.