Technical Diving Gas Mix Question

[Technophile]

Hi all,

Hopefully this will make sense to you tec divers. i have never done technical diving and would be interested in knowing the answer, if its not a hassle.

Here is the question a friend asked

What is the best mix for 457 feet - Trimix and the MOD + EAD of the mix. We will compare gas mixes. And your _________ can't have more nitrogen in it than your back gas or else you will experience _________ ___________________.

thanks

 

[Simon-]

Odd question for someone to ask you out of the blue

What END is required to calculate the Tmx for this depth?

 

[BluewaterSail]

Is this a homework question from a course?

 

[Technophile]

I am not sure it this is a homework question, its not for me as i am only rescue diver certified. i a just interested in knowing the answer

 

[Hydreliox]

And your _________ can't have more nitrogen in it than your back gas or else you will experience _________ ___________________.​

That is very open ended. Errrrm, first part 'deco gas' ...... second part 'isobaric counterdiffusion' ? But that is assuming a very high He backgas and using no travel gas?

As for gas choice, ideally 9/67 (pp02 1.34 140m)...MOD 145 @ pp02 1.4 (at this depth work to a lower pp02?...I don't know).

END - 39m

EAD (N2 only) 36m

(NOTE: I used metric for all of these calculations. Using imperial may cause slight variations in results)

 

[boulderjohn]

First of all, I assume you realize that we are not talking about only one gas. There are different gases to be used during the dive depending upon depth. There is actually a formula you can use to determine best mix on any dive using trimix, but few people I know use it. They prefer to use what is called a standard gas, a gas that is commonly used for a range of depths. These are two different philosophies, and which one is best can lead to an interesting debate.

There is a natural hesitation to respond with specifics to your question because people want to make sure they are not abetting a bad idea.

 

[mathauck0814]

For bottom mix at 457 feet, your "best" mix would be a hypoxic trimix with 9% (MOD of 480fsw @ PPO2 of 1.4). The inert components of the trimix will depend on your personal tolerance and END requirements. Let's go with 100 feet, in which case your He content would be somewhere around 80% So you could dive 9/80 "safely" at the max depth. That does not take into account any of the additional gases that such a dive would require.

I certainly hope that this is a hypothetical and you're not asking for advice on how to execute such an extreme dive on the internet...

 

[wedivebc]

Another factor to consider is the answer will depend on the agency`s view on the effect oxygen has on narcosis. Agencies such as TDI and ANDI teach that oxygen has a negligible effect on narcosis to calculate END based on PPN2 only. Agencies like PADI and GUE maintain the oxygen has an equal narcosis effect and so calculate END by 1-fHE

 

[DBailey]

This question is also ask on other sites around the web. Very interesting to say the least.

 

[syntaxerrorsix]

One part math, one part empirical evidence, one part voodoo. The same as any other dive profile.