Calculating Trimix Best Mix?

[rongoodman]

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?

The rational for using standardized gases is that the standardization itself provides important safety benefits for the diver and his team, outweighing any benefits that the minor tweaks from using a "best mix" might provide. Whether or not you agree with this position is, of course, your choice.

 

[PfcAJ]

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.

The thing is that standard gases greatly simplify your life.

To make any of them, all it takes is putting in the helium and topping with 32% nitrox. Easy.

Within their operational range (50ft increments) the po2 and EAD (and gas density) are within recommended guidelines. Easy.

You can learn the deco because the gases are always the same. Things like "oh 150ft for 25mins? Yeah I'll have about 25mins of deco time on 50%" without even looking at a table or computer. Easy.

If the boat has to go to another site for whatever reason, with standard gases you're not locked in to one depth. Since you're not trying to have a perfect mix, if you're a bit deeper or shallower it's nbd. Easy.

I know I'm doing a 230' dive later this week. No need to tinker with formulas, redo my deco tables, fret over getting the "perfect mix" or any of that. I grab my 15/55 tanks and get on with it.

Diving is complicated enough. Don't make it worse.

 

[Doppler]

GUE "Standard Mixes" are worth understanding.

Hard to see a He dive that isn't well served by one of these mixes.

Keep in mind that unless you are in Cave Country with access to accurate preblended gas or doing your own blending you will almost always end up with mixes that are off by at least several % points on O2 and He. Worrying about selecting the "perfect" mix is a waste of time if you end up with 23/38 or 17/53. As long as you calc the MOD based on the actual O2 and there is "enough" He chances are you are close enough.

Tobin

Yep, using a standardized mix is a superior fix for OC dives to 60 meters or so, but the question was asking about best mix calculations for a TDI course. And I've found having students do the calculations for best mix cements in their minds the benefits of using standard ones...

All that said, "standard mix" for deeper dives is a bit sloppy. Then again, I don't do many OC dives deeper than 60 metres... to damn expensive regardless of what mix you choose.

 

[rongoodman]

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.

Except for the oddball 30/30, all the GUE standard gases can be blended by adding 32%, commonly banked, on top of Helium. Had a different Nitrox blend been more widely used back in the day, I expect the resulting percentages might be a little different.

 

[Diver0001]

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.

Now you're on the right track.

R..

 

[stuartv]

The thing is that standard gases greatly simplify your life.

To make any of them, all it takes is putting in the helium and topping with 32% nitrox. Easy.

Within their operational range (50ft increments) the po2 and EAD (and gas density) are within recommended guidelines. Easy.

You can learn the deco because the gases are always the same. Things like "oh 150ft for 25mins? Yeah I'll have about 25mins of deco time on 50%" without even looking at a table or computer. Easy.

If the boat has to go to another site for whatever reason, with standard gases you're not locked in to one depth. Since you're not trying to have a perfect mix, if you're a bit deeper or shallower it's nbd. Easy.

I know I'm doing a 230' dive later this week. No need to tinker with formulas, redo my deco tables, fret over getting the "perfect mix" or any of that. I grab my 15/55 tanks and get on with it.

Diving is complicated enough. Don't make it worse.

I don't fill my own tanks and it costs me the same whether I request a blend that is "standard" or one that is not. So, using a standard gas is no easier, in that regard. Dropping off my tanks and telling them the mix I want, regardless of a list of standards, and paying the same price for my fill. THAT is easy.

If I'm going out on a boat with potential to get diverted to a different site than what I wanted (which I figure is almost any time I'm out on a boat), then I will allow for that in my plan and choose a mix that will work for any dive site I might possibly end up at. Easy.

What do you do if your gas is good for a 50' increment and you get diverted to a site that just happens to be 10' or 20' deeper than the deepest part of your increment? Say, your gas is for 100 - 150' and you get diverted to a site that is 160' or 170'? Personally, I would have just planned for a max of 160' or 170' in the first place and my gas would still be pretty good if we do get to the actual desired site (< 150'). And I definitely would not choose to use the same gas at 105' that I would use at 145'.

As for calculating my deco, I just look at my computer. That's even easier than doing arithmetic! If my computer dies, I look at my other computer. If that one dies, too, I look at my wet notes. Still easier than doing arithmetic when I might be a little narc'ed. If 2 computers die and I lose my wet notes and my buddy, well then I guess I'll have to wing it.

Calculating Best Mix is easy. Why would I want to complicate my diving by having extra deco because my gas was actually good for another 40' of depth beyond what I was going to do? Carrying gas for a 200' dive when I'm only going to 160' seems like a pretty major compromise (as in, a lot of extra deco and possibly NEEDING to carry an AL80 of deco gas, when an AL40 would have been just fine).

Why would I want to complicate my diving by memorizing tables and calculating deco in my head during a dive? EASY is planning it ahead of time, using PC software, then staying within the planned limits and following a dive computer for the actual deco.

Anyway...

I think I understand now about how people in the real world are determining their trimix mixes.

 

[PfcAJ]

I don't fill my own tanks and it costs me the same whether I request a blend that is "standard" or one that is not. So, using a standard gas is no easier, in that regard. Dropping off my tanks and telling them the mix I want, regardless of a list of standards, and paying the same price for my fill. THAT is easy.

If I'm going out on a boat with potential to get diverted to a different site than what I wanted (which I figure is almost any time I'm out on a boat), then I will allow for that in my plan and choose a mix that will work for any dive site I might possibly end up at. Easy.

What do you do if your gas is good for a 50' increment and you get diverted to a site that just happens to be 10' or 20' deeper than the deepest part of your increment? Say, your gas is for 100 - 150' and you get diverted to a site that is 160' or 170'? Personally, I would have just planned for a max of 160' or 170' in the first place and my gas would still be pretty good if we do get to the actual desired site (< 150'). And I definitely would not choose to use the same gas at 105' that I would use at 145'.

As for calculating my deco, I just look at my computer. That's even easier than doing arithmetic! If my computer dies, I look at my other computer. If that one dies, too, I look at my wet notes. Still easier than doing arithmetic when I might be a little narc'ed. If 2 computers die and I lose my wet notes and my buddy, well then I guess I'll have to wing it.

Calculating Best Mix is easy. Why would I want to complicate my diving by having extra deco because my gas was actually good for another 40' of depth beyond what I was going to do? Carrying gas for a 200' dive when I'm only going to 160' seems like a pretty major compromise (as in, a lot of extra deco and possibly NEEDING to carry an AL80 of deco gas, when an AL40 would have been just fine).

Why would I want to complicate my diving by memorizing tables and calculating deco in my head during a dive? EASY is planning it ahead of time, using PC software, then staying within the planned limits and following a dive computer for the actual deco.

Anyway...

I think I understand now about how people in the real world are determining their trimix mixes.

If I get diverted to a dive site that's 10ft deeper I just dive because my po2 and end is fine for something 10 or 20ft deeper. It's not on the edge. If you're planning your best mix for a site you're not going to be at then you aren't doing best mix anymore.

Knowing your deco before you hit the water is important regardless of how many computers you have. "Hey I know I'll have 40mins of deco if we do this dive but I only have enough deco gas to do 30". Very important if you end up headed to a different site than what you planned on.

I think if you run some numbers you'll see that you really won't need to change what size deco tank you're carrying. A 40 is marginal at best for most dives sub 140' anyways.

Knowing the deco doesn't complicate anything. Not knowing the deco does. If you're about not knowing and being reliant on computers go nuts.

 

[tbone1004]

one thing to remember as you start going deeper is the depth increments required to increase the total pressure grows. I.e. a 10ft depth change at 50ft is a helluva lot bigger than the one at 50m. You have calculated your MoD of whatever mix it is to be close enough and if that is a pO2 of 1.2 or 1.3, then you can easily go 10% deeper without being concerned.

regarding the mix. What you aren't getting is that instead of calculating the pN2 based off of an END with air, some of us are calculating it based off of an END with EAN32. By using equivalent depths based on EAN, the O2/N2 ratios are closer to what I think is ideal. I get narc'd generally around 100ft if I'm working hard and it is dark, i.e. a cave. This is all in fresh water btw, so you'll be a few feet off in the salty stuff.

With EAN32, that is a pO2 of 1.3, and pN2 of 2.7. Total pressure of 4.
If we look at the standard gasses, one is 21/35 and good to 150ft. pO2 is 1.1, pN2 is 2.5 Total pressure of 3.6
next up is 18/45 and good to 190. pO2 is 1.2, pN2 is 2.5. Total pressure of 3.7
Last is 12/70 and good to 300. pO2 is 1.2, pN2 is 1., total pressure of 3.

If you look above, as you get deeper, the total loading of narcotic gasses goes down as you get deeper, not higher. O2 stays roughly the same for decompression efficiency, but the total loading stays roughly equivalent to EAN32 at 100ft, both with O2 and N2. Whether the guys who decided what these gasses were going to be believed in O2 as narcotic or not, they chose the gasses that worked from a blending stand point, as well as a working standpoint. These gasses work in caves, deep, dark, caves. You are often hauling a lot of gear, in a drysuit, in flow, etc. and that is one helluva workload since you have to kick it around. If they work in that environment, then they work in just about any environment, and based on the numbers above, really aren't worth mucking with.

If you subscribe to standard gasses, which I do, then while the cost may be the same in fills since you really are just paying for the helium, you save a lot of money in maintenance by not having to O2 clean all of your tanks and valves. That adds up FAST. The total pressures in the gasses above are no higher than diving to 100ft on 32% so there is no need for any more helium than that and adding more O2 helps a bit on decompression, if you are diving in the shallow limits of those gasses, but not enough in my opinion to warrant O2 cleaning everything.

You brought up going to deeper depths than the original mixes. Fine, no problem. Your limits of 21/35 are 150ft and the total pressure is 4, but pO2 is only 1.1. If you allow pO2 to hit 1.4 and pN2 to hit 3, total pressure of 4.4 *equivalent to ean32 at 112*, then you can take 21/35 to 190ft, and 18/45 down to 230 *roughly an extra atmosphere for reference* If that isn't enough of a buffer for you that is already built into your gas mixes, then I'm not sure what else would be.

 

[stuartv]

You're right that I did not get that some people are basing their END calculation off a comparison to EAN32 at 100', instead of air.

But, I think you missed one of my points. It was stated that the standard gas mixes are given in 50' increments. You said 21//35 is good down to 150'. I'm not sure how shallow you would use that mix, but even if I were going to 130, I would much rather use EAN28 than, one, have the increased deco (or decreased NDL) from using 21%, and, two, pay for that much Helium.

What you're saying about standard gases - at least for dives in the <200' range, is coming across as each one has a sweet spot where it's good, and anywhere else it is so conservative that I'm doing extra deco (or having reduced NDL) for no reason.

And you're saying that the best gas for repetitive dives over multiple days, where you're working and it's cold and dark is also the best gas choice for a nice, easy, drift dive in warm, clear water, with no repetititve dives planned. And the same gas is best for both JUST and ONLY because they happen to have the same planned max depth. That makes no sense to me. Maybe I'm missing something. But, if I were planning a nice, long drift dive in the Caribbean at 130', I would be perfectly comfortable with a ppO2 of 1.4. But, if I were planning multiple dives per day, for multiple days, in cold, dark water, where I'm working at 130', then, yeah, I would choose a gas that will give me a ppO2 of 1.2 or less.

I've had a fair bit of training now that says when you're in dark or cold water and working, you should be more conservative on things like your ppO2 compared to easy, warm dives. If you agree with that, then how can you say the same gas is best for both kinds of dives?

 

[rjack321]

+1 for standard gases. +1 for treating oxygen as equally narcotic as N2 and the helium fraction being completely non-narcotic (whether those are perfectly true or not is unimportant)

At most, standard gases make a 5min difference in deco. Getting to a site and finding its 20 or 30ft deeper than planned (or a cool turtle swims by 20ft below you on a wall) they are priceless.

As someone without a regular tech diving buddy you'd be well served to use standard gases like 25/25, 21/35, and 18/45, EAN50 and O2 for deco. Perhaps adding EAN30 for places like NC which balk at helium. But showing up with something like 26/20 and EAN70 for deco is going to exasperate what buddies you do find.