Calculating Trimix Best Mix?

[Colliam7]

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So, I gather that there is not really a compelling argument to calculate FHe in a trimix blend one way or the other? Whether you set a target ppN2 and calculate from there or whether you set a target for (ppO2 + ppN2) doesn't make enough of a difference to drive the industry to adopting a universal standard, the way it has for calculating a Nitrox Best Mix? Or is there and the industry just hasn't "gotten there" yet?

There is not (yet) a compelling argument. I believe that is because there are not enough data to support one (and only one) compelling argument. FWIW, I adjust my O2 to stay at 1.4 or below for my target depth, and my combined O2 and N2 to stay at and END of 100 at depth. Others may choose differently.

The UTD tables that kevrumbo linked come close to what I dive.

 

[stuartv]

You don't calculate the FHe. You just work out what % of the gas it needs to be.

What is the difference between calculating the Fraction of Helium and working "out what % of the gas it needs to be"?

You have to start by looking at your target depth. You set the ppO2 and ppN2 target pressures based on (usually) 1.4 for oxygen and let's say 3 for N2 (this can vary from diver to diver)

I just use Dalton's Law... I have two constants: a set limit for partial pressure of oxygen (let's say 1.3 bar) and a set limit for partial pressure of nitrogen (let's say 3.1 bar

Right. So both of those are examples of the method TDI teaches. You are determining your FHe by choosing a limit for FN2 - not a limit for (FO2 + FN2).

My personal limits put my combined PO2 and PN2 at 4.0 in an overhead environment, with 1.2 being the PO2 and 2.8 being the PN2.

And this is an example of the second method given in the NOAA manual. Not a specific limit for FN2, but rather a limit for the combined (FO2 + FN2).

@Diver0001 and @Doppler use a formula that does not assume O2 is narcotic. @kensuf uses the formula that assumes O2 and N2 are equally narcotic.

Is there some other method for determining Trimix Best Mix? (obviously, they all start by first determining FO2, based on target max depth and personal choice of max ppO2)

My point in the OP was that these 2 methods are the only 2 methods I've found. One is taught by TDI and the NOAA Diving Manual. The other is not taught by TDI but IS also included in the NOAA Diving Manual.

My statement inviting discussion was that there does not appear to be a compelling reason to use either method. And, as they both produce results that are pretty close and narcosis is a very fuzzy subject, scientifically, I gather that the diving community is okay with that (i.e. thinks it's fine to use whichever method the diver chooses).

 

[tbone1004]

why do our formulas not assume O2 is narcotic? You have personal pO2 limits, and pN2 limits, the sum of those is your total narcotic load. You can choose to increase or decrease either or both of those limits, but your maximum pN2 and maximum pO2 should be your total narcosis limit if you believe it. Either way, standard gasses are worthwhile, best mix is too much of a hassle to deal with.

 

[Kevrumbo]

IMHO, Gas Density and Work-of-Breathing are the critical parameters for both Open & Closed Circuit UBA, along with choosing the optimum bottom mix for a working depth with a total density of 5 g/L or less to minimize CO2 Narcosis. At 6 g/L and above, CO2 retention cycle and Hypercapnia become concerns stemming from any increased physical activity & exercise at depth:

Gas density guidelines

The optimum bottom mix can be easily selected simply by using the recommended GUE/UTD Standard Gases for the appropriate working depth. . .@stuartv, use whatever best mix formulas you need to pass your TDI courses, but the state-of-the-art strategy has always been the use these standard gases to mitigate work-of-breathing and the more dangerous effects of CO2 build-up with physical exertion:

FKD - Standard Mixes

 

[Diver0001]

What is the difference between calculating the Fraction of Helium and working "out what % of the gas it needs to be"?

I'm surprised it took someone so long to notice that. I was starting to think nobody reads my posts LOL

At the time I wrote it I was thinking about it in terms of partial pressures of helium in the mix versus % of helium in the tank. Granted, in all practical terms they're virtually the same thing. I almost went back to edit that but I left it to see if anyone would notice

R..

 

[Diver0001]

Right. So both of those are examples of the method TDI teaches. You are determining your FHe by choosing a limit for FN2 - not a limit for (FO2 + FN2).

Right. Don't make it more complicated than it needs to be. If you feel too narced using a setpoint for N2 of 4 then use 3. If you still feel too narced using 3 then use 2.5 ..... This is one of those things that every diver will just work out as they go. 3 and 4 are common set points. Try them and see how it feels. If they don't feel right then dial it in to what feels right for you.

I personally don't see the point in making any of the math the least bit more complicated than it needs to be.

R..

 

[stuartv]

why do our formulas not assume O2 is narcotic?

The whole process starts with the premise that you are planning a dive and you are planning to limit your narcosis to the equivalent amount that you would experience by using air at some shallower depth.

So, let's take an example. I'm planning a dive to 50m and I want to limit my narcosis to the level I would experience if I were diving on air to 30m. Additionally, I want to limit my O2 exposure to a ppO2 of 1.4 bar.

When you assume that only N2 is narcotic, your process is then to say that ppN2 at 30m is 3.2 bar, so I will use a mix at 50m that yields 3.2 bar ppN2. 3.2/6 = 53% N2. 1.4 bar of O2 means 23%. For FHe, 100 - 53 - 23 = 24 % FHe. So, my Trimix Best Mix would be 23/24.

But, if I assume that O2 is also narcotic, then my process is to say that, for air, ppO2 + ppN2 at 30m is 4 bar. Since I'm going to use O2 at 1.4 bar, that means my N2 limit is (4 - 1.4=) 2.6 bar. Which means FN2 is 43%. And my Trimix Best Mix would be 23/34.

 

[stuartv]

Right. Don't make it more complicated than it needs to be. If you feel too narced using a setpoint for N2 of 4 then use 3. If you still feel too narced using 3 then use 2.5 ..... This is one of those things that every diver will just work out as they go. 3 and 4 are common set points. Try them and see how it feels. If they don't feel right then dial it in to what feels right for you.

I personally don't see the point in making any of the math the least bit more complicated than it needs to be.

R..

Yes, I have realized now that all the math is just a fuzzy way of getting you to a reasonable starting point for each person to then develop experience with and refine to their personal taste. It would be a whole lot simpler in the class if they just said "ppN2 for air at 100' is 3.2 bar. When you're calculating your own mix, use 3.2 as your limit for Nitrogen to start then vary it up or down based on your experience, the planned dive and conditions, etc.." Because, clearly, there is no hard science to support calculating and using an exact number. It's not like O2 exposure where we have SOME kind of actual data that was used to develop tables for calculating exposure limits.

 

[stuartv]

The current & easiest strategy to follow is to use GUE or UTD Standard Gases, select the optimum bottom mix with MOD ppO2 of 1.4 ATA and average working ppO2 of 1.2 ATA; an END of 30m/100' (4 ATA) or less; and gas density of 5 g/L or less to mitigate Work-of Breathing -all at the planned average operating depth:

Thanks. I've watched that presentation previously and it is excellent.

So, you're saying that what I should do is let someone else calculate the gas mix I use for me? And that if I'm diving to, for example, 50m there is one mix of gas that will always be the best choice for me to use, no matter whether it's a short drift dive in warm, clear water that is my only dive of the day, or a long, working dive in cold, dark water, with multiple dives planned each day for several days in a row?

That's not how I was trained and not a notion I subscribe to. Using a gas that someone else calculated and tells me is what I should use feels like it should be put into the same general category as Trust Me dives.

Further, I'm not sure where the suggestion that standard gases are "easier" comes from. My "regular" places for getting fills are 4 different shops, depending on where I am. 2 of them bank EAN30, 1 banks EAN32, and the other has Air, O2 and He. So, unless I'm getting air, 30% or 32%, anything I want is a custom blend. For which, they check my tank pressure and current blend, punch the numbers into a computer, punch in my requested target blend, and then fill the tank however the computer tells them. Other than getting banked mixes, how much "easier" can it get? Anywhere I normally go is going to use the same process whether I request a "standard gas" or a non-standard one.

 

[kensuf]

The whole process starts with the premise that you are planning a dive and you are planning to limit your narcosis to the equivalent amount that you would experience by using air at some shallower depth.

So, let's take an example. I'm planning a dive to 50m and I want to limit my narcosis to the level I would experience if I were diving on air to 30m. Additionally, I want to limit my O2 exposure to a ppO2 of 1.4 bar.

When you assume that only N2 is narcotic, your process is then to say that ppN2 at 30m is 3.2 bar, so I will use a mix at 50m that yields 3.2 bar ppN2. 3.2/6 = 53% N2. 1.4 bar of O2 means 23%. For FHe, 100 - 53 - 23 = 24 % FHe. So, my Trimix Best Mix would be 23/24.

But, if I assume that O2 is also narcotic, then my process is to say that, for air, ppO2 + ppN2 at 30m is 4 bar. Since I'm going to use O2 at 1.4 bar, that means my N2 limit is (4 - 1.4=) 2.6 bar. Which means FN2 is 43%. And my Trimix Best Mix would be 23/34.

Hi Stuart,

I think you're making a mountain out of a molehill and worrying too much.

@ 50m:

23/24 = PO2 1.4, PN2 3.1, PHe 1.5, END (just N2 narcotic) of 99', EAD (both O2/N2 narcotic) 115'.

23/34 = PO2 1.4, PN2 2.6, PHe 2.1, END (just N2 narcotic) of 75', EAD (both O2/N2 narcotic) 97'.

IMHO, either way you want to calculate it, the 18' difference between the more conservative method (assuming both are narcotic) and least conservative (assuming just N2 is narcotic) is truly negligible at the END's/EAD's you're targeting and the reduced gas density of either mix will reduce your CO2 retention enough that you will have a clear head.

BTW -- I'd probably get in the habit of dropping your PO2 to 1.2/1.3 maximum for the working portion of your dives and stop using 1.4. Some reasons include reliability of sensors used to verify gas contents (+/- 1% can make a huge difference at deeper depths), increased CNS load when deco'ing at PO2 of 1.5-1.6, and the "**** happens" factor.