Trimix

[Jax]

I wasn't attempting to question that. I understood the OP to be calculating a mix based on given dive parameters that include a depth of 90m and a target max ppO2 of 1.4.

In that case, I would like to understand what model or assumptions tursiops is talking about that result in having an option for a calculated mix that has FO2 of 13%. As I said, I don't have training yet in trimix or special calculations for diving as deep as 90m. From what I know so far, I would have expected the answer on the mix to always have FO2 of 14% and the FN2 and FHe to be what is really being calculated.

Like I said, I'm just trying to learn more. Even just a keyword or two to search on or a link to something I can read would be appreciated.

Actually, the PPO2 for a 14% O2 at 195 ft is 1.0, per multideco, assuming an equivalent air depth of 100ft. It gives a 14/39 mix.

Having a target max PPO2 of 1.4 at depth could result in a problem if, for some reason, you go deeper. That should not be the plan.

I recommend the TDI Advanced Trimix book as a primer.

 

[tursiops]

@stuartv, most trimix calculators and all hand calculations assume ideal gases. Under 10 ATM pressure Van der Waal gases are a better assumption, and give the 13/48 result. The VDW calculation drops the 14 to 13, and the END constraint thus causes the 47 to go to 48 so you still only have 39% N2.

Edit: Sorry, I said this backwards. The VDW calc moves the He 47 to 48, which drops the O2 14 to 13 to keep the END constraint. At least, I THINK that is what is going on.

 

[boulderjohn]

Why did that get deleted?

I was not the one who moderated the thread, but I assume it just got swept up with everything else. Your question was fine, and now that you have repeated it, it is back in place.

 

[rjack321]

I assume you are a trimix student working your way through the academic portion of the course materials. Make sure to talk to your instructor about this issue. He or she might have a lot to say about the concept of best mix in addition to what those materials say--I know I would.

He (she?) asked this exact question on a whole bunch of other forums too. Hard to know exactly what the actual plan is and if this is even an OC dive. Or possibly just an academic question

 

[fennel]

14/40 1,4 ppO2 / 10 ATA (90 mt) = %14
5 ATA (40 mt) x 0,79 (%79N)=3,95 ppN
3,95/10 ATA= %39,5

 

[stuartv]

14/40 1,4 ppO2 / 10 ATA (90 mt) = %14
5 ATA (40 mt) x 0,79 (%79N)=3,95 ppN
3,95/10 ATA= %39,5

Isn't the standard for the second number to be the He content?

And if you want END of 40m (or less), then wouldn't you round 39.5% down to 39%?

And if you have 14% O2 and 39% N2, then you'd have 47% He, right?

So, you'd write that as 14/47, right? I.e. what everyone else already said?

 

[CptTightPants21]

Isn't the standard for the second number to be the He content?

And if you want END of 40m (or less), then wouldn't you round 39.5% down to 39%?

And if you have 14% O2 and 39% N2, then you'd have 47% He, right?

So, you'd write that as 14/47, right? I.e. what everyone else already said?

Yes.

 

[fennel]

Isn't the standard for the second number to be the He content?

And if you want END of 40m (or less), then wouldn't you round 39.5% down to 39%?

And if you have 14% O2 and 39% N2, then you'd have 47% He, right?

So, you'd write that as 14/47, right? I.e. what everyone else already said?

Yes my bad, sorry

100-(14+39,5)=46,5 so, 14/47