Which first, 21 or EAN?

[Greg G.]

I'm basic Nitrox certified and on a two-tank dive I most often use standard air on the first dive, then EAN on the second dive. My reasoning behind that is the first dive is usually the deeper dive, and EAN is not a deep dive gas, although my usual dive profiles are still well within the MOD of the EAN. Also I figure that breathing EAN on the second dive will help me to off-gas and help prevent post-dive fatigue. I use EAN mostly to help guard against DCS hits and prevent post-dive fatigue. It doesn't extend my bottom time any because my air consumption is such that I use up my air long before I ever worry about bottom time.

However, on a recent two-tank dive, I got into a discussion with another diver who was told to do the opposite, use the EAN on the first dive in order to reduce the amount of nitrogen build up. Who's right?

 

[sky50960]

Well first what is a deep dive to you?, and what EAN (%?) are you using?

I always use nitrox if diving between 20 and 40 maters (max ppO2=1,6 when diving in good conditions).

While performing a deep dive regarding nitrogen uptake kinetics (exponential )that is when you are going to built up a lot of nitrogen and that is when nitrox will surely have a benefit and also increases the security when performing successive dives. That you use nitrox or air for the following dives after a deep nitrox dive, there always will be a little bit more security regarding DCS.

While diving shallow depth (0 to 18/20 meters) the benefit of nitrox is not so important ( regarding DCS). If my successive planned dives (even the deepest) are between 0 and 20 meters i might not be using nitrox.
If you push the diving tables in there far end limits (many successive dives) you better use nitrox.
Remember a majority of decompression accident in sport divers (regarding statistics) are after performing more than 2 successive dives.

You said that : “ Also I figure that breathing EAN on the second dive will help me to off-gas and help prevent post-dive fatigue”

Nitrox doesn’t help you off-gassing while performing a second dive. During this second dive you still will built up nitrogen but less than if on air.
As our DrDeco likes to say : " off gassing continue after surfacing."
If you breathe Nitrox you will reduce the off –gassing stress while ascending and during surface intervals and that might lead to less fatigue.


To me :
-The best way to use nitrox is: always use it when possible.
-If I can not use nitrox all the time I will use it for my first deep dives (I believe that the biggest benefit of nitrox is between 18 and 40 meters ( for EAN32%))
The shallow dives (0 to 18 meters) will be with air.
-If performing deep (>40metres) dives use nitrox as decompression gas.
-When using air and nitrox for successive dive you always can make mistakes in the type of gas. I like homogeneity and don’t like to mess around with nitrox and air.

All what is said here is true if you use Nitrox(EAN32%) on air tables.


Nitrox an me = :1st:

Fa

 

[Greg G.]

My usual dive profile is 70-80ft on the first dive and 30-50ft on the second dive, using EAN36. Without giving me a lot of technical mumbo-jumbo, is it better for the purposes of preventing a DCS hit and post-dive fatigue to use the air or nitrox first?

Originally posted by sky50960
Well first what is a deep dive to you?, and what EAN (%?) are you using?

Nitrox an me = :1st:

Fa

 

[blacknet]

Hello,

Think you forgot one thing for the eanx on the first and air on the last dives. The o2 clock If you saved the eanx for the last dives your o2 clock may already by full and the eanx may push it over the limits and cause an ox tox issue (or so I was told in my training could be wrong tho)

Ed

 

[Dr Deco]

Dear Greg:

I have never thought about his aspect before. My guess might be that EAN on the first of two dives is better than the reverse. That is just a guess. Again, that is assuming you are diving the EAN on air tables.

Now for those who are interested, here is the technical mumbo jumbo behind the answer. Whenever there are gas bubbles formed during a decompression, they can act in several ways.

• - they serve as a “sink” for the inert gas in the tissue; the nitrogen collects in the bubbles rather than moves into the capillaries where it is transported to the lungs;
  - the bubbles can also form in the capillaries and block the blood flow in that tissue thus also reducing inert gas washout;
  - the bubbles formed in the tissue, while formerly a “sink” now become a “source” of inert gas in the tissue on the next dive.

We see that is this process occurs on the last dive, little happens since there is not additional gas loading when diving since it is the final dive of the day. IF it is not the last, then those bubbles participate in the dives that are subsequent. This “carryover” is not in the computers or table. Now, in virtually all cases, few gas bubbles are formed and there is not a problem. But, should one exercise heavily at depth, or perform strenuous movements at the surface following that dive, then those excessive numbers of bubble might form.

Dr Deco

 

[sky50960]

Well this is a pertinent question.
I used to be confronted with the same question.
I just GUESS that it would be better to use nitrox for the first deep dives because they will load you with more nitrogen.
But I don’t have any scientific reference.
It seems like there is not a lot literature out there about that question.

 

[Greg G.]

That's what the other diver said she was told also, that using EAN on the first dive would result in less of a nitrogen load going into the second dive. I could try it both ways and keep track of what my 'puter says about bottom time on the second dive. It's programable so I can switch from one mix to another. As I said before, I usually don't pay much attention to the bottom time left because I'm still an air PIG!

Originally posted by sky50960
Well this is a pertinent question.
I used to be confronted with the same question.
I just GUESS that it would be better to use nitrox for the first deep dives because they will load you with more nitrogen.
But I don’t have any scientific reference.
It seems like there is not a lot literature out there about that question.

 

[blacknet]

Hello,

OR you could just run it on the pc software for deco planing

I was told that you do eanx first to help minimize the risk of ox tox.

Rather be an "air pig" than a co2 retainer due to improper breathing.

Ed

 

[Greg G.]

I don't do deco dives, so don't have the software. I've got some buddies who do deco dives, I could ask them.

Originally posted by blacknet
Hello,

OR you could just run it on the pc software for deco planing

I was told that you do eanx first to help minimize the risk of ox tox.

Rather be an "air pig" than a co2 retainer due to improper breathing.

Ed