Calculating Trimix Best Mix?

[tbone1004]

21/35 starts at 100ft. If you're using EAN28 to 130 that's fine, that's your call. Too much nitrogen for me, though I would certainly never be concerned about NDL's if diving below 100ft...

Regarding the conservatism. I'm assuming this is based on the pO2's being low. The pO2's are low because these gasses were designed for LONG dives where your O2 clock is a very limiting factor, pushing pO2's of 1.4 are not something you want to do for multi hour dives. That said, that is why I pointed out that 21/35 can be used quite easily down to 190ft as long as you are OK with the penalties associated with higher pO2's and pN2's.

I said nothing about the best gas for repetitive cave dives being the best for single warm water dives, though aside from cumulative O2, there is nothing else that would factor in for me being single vs. repetitive. I pointed out a limit where the gasses can be used that is higher than what the WKPP has said they are good for, 21/35 good to 190 instead of 150, 18/45 good to 230, etc. The point was that there is no reason to go to PP blending for best mixes when you can go 1atm deeper on the same gasses with no real problem.

for me, I feel narcosis coming on at 100ft on ean32, when working, in a cave. Not cool. Caves are volatile environments where best mixes don't work because you are almost never at a fixed depth for long. The Standard Gasses work well enough by mimicking the pO2's and pN2's of EAN32 at 100ft. up or down an atmosphere is not a huge deal, so if you are OK diving EAN32 from roughly 80ft to 112ft, then diving 21/35 from 130-190, or 18/45 from 150-230 is not the end of the world. If doing single dives, then an extra 5-10 mins of deco isn't going to break you and for me is not worth the hassle of pp blending where a lot of shops don't have boosters set up to boost O2 high enough to actually get the best mix in there.

So to you @stuartv what gasses would you choose for a dive to 150ft, 200ft, and 300ft in the cold dark waters of NJ vs. the warm clear waters of the Bahamas?

 

[stuartv]

I said nothing about the best gas for repetitive cave dives being the best for single warm water dives, though aside from cumulative O2, there is nothing else that would factor in for me being single vs. repetitive. I pointed out a limit where the gasses can be used that is higher than what the WKPP has said they are good for, 21/35 good to 190 instead of 150, 18/45 good to 230, etc. The point was that there is no reason to go to PP blending for best mixes when you can go 1atm deeper on the same gasses with no real problem.

for me, I feel narcosis coming on at 100ft on ean32, when working, in a cave. Not cool. Caves are volatile environments where best mixes don't work because you are almost never at a fixed depth for long. The Standard Gasses work well enough by mimicking the pO2's and pN2's of EAN32 at 100ft. up or down an atmosphere is not a huge deal, so if you are OK diving EAN32 from roughly 80ft to 112ft, then diving 21/35 from 130-190, or 18/45 from 150-230 is not the end of the world. If doing single dives, then an extra 5-10 mins of deco isn't going to break you and for me is not worth the hassle of pp blending where a lot of shops don't have boosters set up to boost O2 high enough to actually get the best mix in there.

So to you @stuartv what gasses would you choose for a dive to 150ft, 200ft, and 300ft in the cold dark waters of NJ vs. the warm clear waters of the Bahamas?

You didn't directly say that. But, from what I have heard so far, using standard gases means that, for a given depth range, there is a specific gas that is "standard". Nobody yet has indicated to me that the "standard" gas would vary for any reason other than a different target depth. So, it's not a direct statement, but, as I understand it, recommending to use standard gases directly implies that the same gas is good for warm, clear, drift dives and for cold, dark working dives at the same depth.

You did say that you might use 21/25 to 190, instead of 150. But, if you're diving to 180 on 21/25, then you're no longer really using a standard gas, right? If you showed up to a team dive to that depth, you would have a different gas than everyone else, right? (Yes, I know that IRL you and the team would sort all that out ahead of time and you wouldn't ACTUALLY show up with 21/25 when everyone else has something different) Are the RD tables even still valid if you go that far out of the recommended depth range on a standard gas?

Anyway, I think you really hit the nail on the head when you said that these gases are designed for LONG dives. Well, I have probably never done a dive that would qualify as LONG by your definition. And probably won't be until I am more experienced and THEN get full normoxic trimix training and certification.

So, to wrap it up and answer your last question at the same time. For dives to any depth, I would calculate Best Mix, as I've been trained to do, and dive that. I would factor conditions into my calculation in the form of possibly choosing a target ppO2 of 1.2 or even less and also possibly choosing a target END of 100' or even less. But, if it's a warm, clear drift dive, I would almost certainly target 1.4 bar and 130'. Number of dives per day and number of days diving would also be factors in my choices for those two parameters.

I have never been to a shop that banks any kind of standard trimix (that I know of, anyway). OTOH, I have gotten fills at several shops that only do partial pressure blending (except for possibly having banked EAN30 or EAN32). So, I will continue to keep all my tanks O2 Clean. It's NO extra hassle. Just a bit of extra money each time I have them VIP'ed. Also, all 8 of my tanks are actual 3500 psi tanks and, so far, I've never had any trouble getting full fills. Several places pump them to 3800 or so, so they'll still be full when they cool down, as normal practice.

So, if the depth I'm diving dictates a "standard" gas of 21/25, but my Best Mix calculation is 28/20, then the ONLY reason I would have for not choosing 28/20 is if I were diving with a buddy who wanted to choose a different mix - in which case we would discuss it and come to some agreement on a gas that we would both use.

If I go, for example, to dive the Oriskany and logistics work out that their is a suitable standard gas banked that I could use, and I'm diving with a buddy that wants to use that, then that might be a reason to use it. But, that boils down to logistics and cost. It's not that it's actually better or safer in the water.

Anyway... like I said earlier. This thread was for me to understand how real people determine their FHe. I've got that. I'll use standard gases if/when they make sense for me, in whatever situation I'm in at the time. But, I'm not going to go ask for a custom blend of 21/25 just because it's standard, if me and my buddy agree that 25/20 would be better for the dive we're planning. At my current level (about to finish Helitrox), I can't get less than 21% O2 or more than 20% He anyway.

 

[tbone1004]

I don't dive with GUE or the WKPP, so can't comment on them. For my buddies and I, we would all have the same gas mix in the tank. I.e. if we planned the dive to 150-160ft, we would dive 21/35. If for whatever reason we had to go to 180 for a limited amount of time, we would not likely scrub the dive based on that gas mix.

Ratio deco assuming standard gas mixes and 50% deco bottles can account for the depth differences. Read up on it, quite simple. basic run down is with 21/35 and 50% o2, you have 1:1 at 150ft, and +5 mins for every 10ft deeper, and -5 for every 10ft shallower. Their validity if you take them to the extremes is not something I am familiar with so the guys that came up with it will have to talk on that point. RD is a last resort for me and a lot of sh!t has to go wrong before I fall back on it. Not my preferred method of decompressing, I am anything but an expert on the topic, and my choice to believe in the standard gasses has everything to do with convenience and nothing to do with any sort of affinity for GUE/WKPP.

Long diving for me is anything that requires over 300cf of bottom gas. At deep mix depths that isn't all that long, but it's long enough.

Standard gasses do not require trimix being banked, only EAN32, though you can get away with EAN30, 34, whatever is around. You just adjust the depths a little since your final fO2 will be a touch lower or higher. Ratio Deco goes out the window, but I'm not advocating for that, I'm advocating for easy enough blending, and close enough mixes. This was a long paragraph, but I wrote something that made it easy enough to delete. Here is what I think you are hung up on. The books you are reading are all talking about EADs-Equivalent AIR Depths. I don't think air is the ideal gas because the ratio of O2 to N2 is big enough that you are nowhere close to the pO2 limits of the gas by the time you become narc'd out of your mind. Air has a VERY deep MoD. What the standard gasses are trying to do is give you an E-EAN32-D-Equivalent EAN32 Depth. The balance between the O2 and N2 in EAN32 is close to the point that by the time narcosis starts to kick in, you are approaching your pO2 limits. Good balance point. The standard gasses simplify filling and logistics. You only really need 3. will your deco be the most efficient? Of course not, but it's close enough, and you can adjust the fHe in the blends if you want them to be closer to ideal, but you're still blending to an E-EAN32-D, instead of EAD.

I will not support dive shops that on PP blend nitrox. Mix sticks are idiotically simple and cheap to make and PP blending is too much of a safety risk, aside from the wasted money on O2 that can't be used and the extra cost of O2 cleaning all of the tanks. If they don't have banked nitrox or at least the ability to fill it direct from the compressor premixed then I'm diving air and I'll deal with the decompression.

I would not dive with a buddy with a significantly different gas than I am diving. Wouldn't do it. I'm a firm advocate for solo diving and same ocean diving with my buddies, but no way I'd do a mix dive with a significantly different mix than a buddy.

If EAN36 or 40 was more commonly banked, I would be 100% on board with adjusting my personal "standard gasses" to whatever the EAN36/40 mixes came out to be since the fO2 and fN2 are more balanced, but since EAN32 is the gold standard, then I stick with those. If EAN30 is the only one available, I'll dive a set of "standard mixes" based on that, but you will never see me pp blending He, then O2, then topping off with air into tanks. It's just not worth fighting it.

 

[Jack Hammer]

I've found 18/45 to be pretty versatile and prefer it to 21/35. It allows me to make dives in a wide range of depths with a clearer head than 21/35. The cost difference between the 2 isn't too bad for me and I don't have to deal with the limits of ideal mixes which can cause you to be unable to dive if the planned depth changes or be stuck with a mess because the dive was cancelled. YMMV.

 

[kensuf]

While knowing how to calculate "best mix" is a useful skill, because it helps you to understand managing END's and PO2's, diving a few standard gases has benefits.

One of the nicest things about standardizing gases is you can cut your own custom tables for a variety of depths and times, laminate them, and keep them in your wet notes. If the dive plans change on the fly, let's say you're blown off of Site A and forced to move to Site B, you've got your next set of tables ready to go.

A second benefit is that after you gain some more experience diving those mixes, you'll get to know your deco schedules consistently. Adaptations / deviations are easy.

Keep in mind, the body is an analog system and not digital, nothing is really cut and dried, everything is kind of "fuzzy" and minor deviations due to blending issues are simply noise. Just because your computer says you're cleared doesn't mean you won't get bent, and just because your computer says you have 5 more minutes of deco doesn't mean you will get bent if you are forced to abort. Keeping that in mind, I don't get worked up about minor deviations in the final mix -- the tables I use for 21/35 work just as well if the mix comes out to be 20/37 or 22/32.

 

[stuartv]

A second benefit is that after you gain some more experience diving those mixes, you'll get to know your deco schedules consistently. Adaptations / deviations are easy.

I don't want this to sound like I'm trying to argue with you. I'm definitely not. I'm recognizing my own inexperience and asking someone who is much more experienced and I mean that sincerely.

The question is: If you always dive Best Mix, wouldn't you also get to know your deco schedules consistently? I mean, I feel like, for the dives I've been doing (say, in the 120 - 160' range), I already could plan a dive and write out an ascent plan, based on Best Mix and without looking at any tables or computers first, and end up with something that is pretty close to what Multi Deco would give me (if I were trying to be as close as possible just to test myself) or reasonably more conservative than what MD would give (if I were trying to create something I would actually use in a pinch).

I would not actually do a dive based on some plan I just came up with in my head. I'm just saying that I already (with my limited experience) feel like if I did a dive and EVERYTHING went wrong - my computer died, my backup computer was only functioning as a bottom timer, I lost my wet notes with my written plan, and I lost my buddy, I could come up with my own ascent plan, just based on use of Best Mix and knowing what my depth (current, max, and average), and dive time are. And, of course, after all that goes wrong, I'm not going to hesitate to pad my plan very generously. So, is using a standard gas REALLY a significant advantage in this regard?

Also, isn't repetitive experience reinforcing? So, if you dive repetitively using an ascent plan that you come up with in your head, that will reinforce your ability to do that - even if what you're doing is suboptimal in some way (say, for example, consistently, unnecessarily, overly conservative). OTOH, if you plan ahead and write it out in your wet notes, but then follow your dive computer each time, that would build reinforcing experience of what the optimal ascent plan is (according to your chosen algorithm and parameters). Which knowledge you could then use and pad liberally, if stuff happened and you were forced to come up with an ascent plan on your own.

 

[rjack321]

120 to 160 is a really narrow range. In previous threads you were calculating best deco mixes too, are you really going to do that for a 10ft or 20ft depth difference? Best backgas and best deco mix?

We roughly use
32 down to 100 or 110 (with O2 if a deco dive)
25/25 from 90 to 130 (with O2)
21/35 from 120 to 160 (with EAN50)
18/45 from 140 to 210 (with EAN50 and O2)
They are all easy to repeatably mix by adding He and topping with 32%

Best mix has a bunch of negatives:
riding max allowable ppo2 (which won't work for a 2 hour dive)
has a planned max depth instead of a flexible one (doesn't work so well when you are going out to an unknown target)
is hard for a shop to precisely and repeatably mix (you'll be off by 2 or 3% on O2 and 2 to 5% on He often)
its hard enough to have a set of doubles per standard mix as it is, so when you get blow out you have a custom mix that's hard to reuse, even worse by having custom deco mixes.
matching gases with buddies is a mess
matching ascent profiles with buddies isn't going to go well

I'm pretty sure any of us could come up with a working schedule for 25mins at 130ft using 28/20 using nearly any kind of EANx for deco. That's barely a deco dive. Once you are doing 160 or 190ft for 25, 30 or 35 mins the value of standardized mixes is much more obvious.

 

[Kevrumbo]

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. . .

Do what you have to do to pass your TDI requirements and courses to the satisfaction of your Instructor using Best Mix procedure. But using Standard Gases is better -the motivation is to work it all out smarter not harder. . .

Here's a simple example and strategy, for a novice Open Circuit Tech/Wreck diver with double AL80's (twinset 11L cylinders) on a first time visit to the deeper wrecks of Truk Lagoon, with practical & economical use of only the standard mixes of 21/35 and Eanx50:

At a setpoint 150'/45m for Ratio Deco 1:1 Schedule, my one 40cf/5.5L Deco Bottle of 50% can safely cover a total deco profile time of 20 minutes for my particular deco Depth Consumption Rate. So my pre-dive "insta-profile" depth & bottom time choices, using RD rules (i.e. adding 5min deco for every 10'/3m you go over the setpoint of 150'/45m, and subtracting 5min for every 10'/3m interval less than the setpoint), calculated out like this:

140'/42m for 25min;
150'/45m for 20min [RD 1:1 setpoint]
160'/48m for 15min
170'/51m for 10min
180'/54m for 5min.
(Choose one of the above for a square profile at the particular depth of interest, or stay around the average depths of the 1:1 setpoint).

The Deco Ascent Profile and total time on Nitrox 50% is the same (20min) whichever depth and corresponding bottom time you choose above. The deco stop times in minutes starting at 70'/21m with 10'/3m interval stop depths ascending to the surface has a progression like this: 2,2,2,2,2,10 and finally slow 1m/min (3'/min) ascent to surface. Alternatively, this is one simple and easy back-up deco profile for the novice tech diver's wet notes, if the computer craps out and all that's left is a back-up bottom timer.

IMO, this should be your general framework & overview profile for planning your initial dives on the Oriskany:
Planning multi-level deco dive?