When to use limited stash of Trimix on a boat trip? Early or later?

[rongoodman]

If you're gonna clean, it means your tank has been contaminated, how do you want 80% to get contaminated? I wouldn't play with that kind of stuff, pushing non clean gas on top of pure O2...

He's using "clean-up" in the sense of getting rid of inert gases, rather than "oxygen clean".

 

[victorzamora]

If I'm bringing only one 40 of deco gas, and the boat already has 32%, that 40 is filled with 100%. Topping with 32% will allow me to have something in the 100-80% range for clean-up for quite some time given the shallow depths...though if they're not PP blending their 32%, both the bottle and the reg will probably need an O2 cleaning afterwards.

He's using "clean-up" in the sense of getting rid of inert gases, rather than "oxygen clean".

He's talking about O2 cleaning bottle and reg, which means the "previously O2-clean pure-O2 reg and bottle" would be being topped off with dirty gas. Pumping dirty gas into pure O2 is just as scary as pumping pure O2 into a dirty tank.


My concern goes back to: It doesn't sound like OP has much knowledge of Trimix, how to plan or execute a trimix dive with or without deco, and what the implications of said trimix are. First of all, calling it 28/25 is good enough for anybody with training. Second and most importantly, following a rec-only (nitrox-only Suunto) computer blindly on a trimix dive is a little concerning. Not being able to plan repetitive dives is extra scary. OP: I'm not saying you're going to die. But seriously, seek some training out.

 

[Dr. Lecter]

Pumping dirty gas into pure O2 is just as scary as pumping pure O2 into a dirty tank.

Given that the O2 is not going anywhere under pressure in the former scenario--namely through the sharp turns of the valve at pressure--it simply isn't. Is it ideal? Obviously not. Would I really worry about "dirty" EAN32 being used to top off an O2 bottle for one trip? Nope.

 

[victorzamora]

Given that the O2 is not going anywhere under pressure in the former scenario--namely through the sharp turns of the valve at pressure--it simply isn't. Is it ideal? Obviously not. Would I really worry about "dirty" EAN32 being used to top off an O2 bottle for one trip? Nope.

I agree.....just trying to point Ron in the direction of the right quote. As for that line, let's say it's "dirty gas" that could actually cause issues being pumped into the tank. It's not pumping it into the tank that's a concern, it's O2 fills after the fact that are potential sources of problems. However, if the EAN32 is clean (and let's be real, plenty/most professional fill stations are clean enough) there's seriously zero issue.

 

[Dr. Lecter]

It's not pumping it into the tank that's a concern, it's O2 fills after the fact that are potential sources of problems.

Which is why I suggested cleaning after the trip as a (likely overdoing it but you never know) precaution. If there was O2 on the trip, this wouldn't be an issue.

 

[walkonmars]

Hi Folks,

Thanks for the replies. The process provided from RX7diver would work. But I only have a two cylinder helium bank without a booster. I don't want to expend too much from my highest pressure tank because that leaves you less pressure to top off tanks later(cascade fill to pressure needed). I don't intend to use the trimix without the proper computer (I run a Petrel now).
I already blend my own nitrox anyway (I have a gas blending cert.) and a bank of oxygen, air and helium. I can only analyze oxygen right now. I always blend carefully adding oxygen first at a rate of 60psi/min and same with topping off air (60psi/min) until I get to 40% before I allow a standard topping off. I have run dozens of models using various mixes and depths.
I should probably clarify that there are saturated gas benefits from my interest in trimix but my goal now is to have clearer awareness and improved memory when I dive in the 90-110 foot range. We have had this debate in the past and a couple trimix divers have confirmed noticeable improved acuity when dropping into a shallower target (~140ft range or shallower?). I don't recall the exact depths but it was within my non-tech diver range and it started a debate about if helium use as a recreational gas. Some agreed it was a valid option if the cost was not a factor. My thoughts are to use it as a way to better remember details of deeper dives. I suspect the benefits will not be immediately apparent since a deep dive going normally does not raise any additional interest. My concern is that all of my 110-140ft dives are blurry memories. I fairly certain I am slightly 'narced but don't have quantitative proof.
Thanks for your inputs.

-T

 

[rx7diver]

Thanks for the replies. The process provided from RX7diver would work.

walkonmars,

I had a couple of free minutes just now, so I *quickly* ran these numbers:

1. S'pose you desire PO2 = 1.4 ata at MOD = 130 fsw. Then this implies FO2 = 0.28.
2. S'pose you desire EAD = 75 fsw (i.e., "1.5 martinis") at MOD = 130 fsw. Then this implies FHe = 0.34.
3. So, your first HP cylinder (the one that is to be dived as is) should be filled with Tri-Mix 28/34/38.

If the boat can provide HP EAN32, then your second HP cylinder (the one that is NOT to be dived as is!) should be filled with Tri-Mix 0.24/0.68/0.08. Then, following the approach I described above, once the first HP cylinder is dove, and then completely emptied, and then equalized with the second HP cylinder (so that both HP cylinders are now half-filled with Tri-Mix 24/68/08), and then both HP cylinders are topped up with EAN32, both HP cylinders will now be full, containing Tri-Mix 28/34/38. (So, you can again use the same Tri-Mix tables you cut for your initial Tri-Mix dive, with either of these two HP cylinders!)

Note: I don't have access to my deco software at the moment, so I cannot tell you what the NDL's are for Tri-Mix 28/34/38.

DISCLAIMERS: You should ABSOLUTELY check my quick calculations!!! And someone who is Tri-Mix certified (I am NOT Tri-Mix certified) can speak to whether any of this theoretical exercise is at all practical!

Safe Diving,

rx7diver

---------- Post added August 1st, 2015 at 01:47 PM ----------

Just spent a couple more minutes with this. A couple of things:

1. You'll almost certainly need either a booster or a mixing stick + compressor to fill your not-to-be-dived-as-is tank of Tri-Mix 24/68/08 before your leave for your trip.

2. I played a bit with my old copy of Abyss. It seems you can get a reasonable amount of bottom time on a 130 fsw no-decompression Tr-Mix 28/34/38 dive only if you switch to EAN50 (say) during your 30 fpm ascent. Of course, *requiring* a second gas complicates things significantly, imho, when considering recreational dive profiles.

2. About my proposed six Tri-Mix dives: S'pose you finish your Tri-Mix 28/34/38 dive with a tank that is one-third full. That is, s'pose you breathed two-thirds of your HP cylinder during your dive. If you top up this cylinder with the boat's EAN32, then your now-full tank will contain Tri-Mix 31/11/58, and dealing with a F02 = 31% mix containing FHe = 11% on your shallower repetitive dive is probably more trouble than it's worth. Maybe, instead, simply dump the 1/3 remaining Tri-Mix 28/34/38 in your tank, and fill your HP tank with the boat's EAN32, and do your repetitive dive using EAN32.

So, maybe six tri-mix dives is not really so practical after all. Maybe, a Tri-Mix 28/34/38 initial dive followed by one (or two) EAN32 repetitive dives per day is a better approach. Six to nine dives, considering.

Safe Diving,

rx7diver

 

[walkonmars]

Thanks for the replies and running the models.

I decided a little bit ago that it would be impractical to dive more than a couple cylinders of trimix on any boat trip. I'm thinking that most boat operators would be wary about letting me mix using my own whip and ask them to top off my tanks afterward without a means of testing the final mix.
My descriptions of trimix using three digits (28/25/47) was a courtesy to those not quite versed in all the trimix details(Although I don't see the need to complain about info that adds no additional confusion). Sorry for the extra digits.
The trimix for me would be most useful on dives with specific, unchanging depths such as a wreck and less useful chasing game and exploring new sites.
My reference to literature ("Recreational Trimix Diving") points to potential narcosis from high O2 as well and it likely demands mixes that closer to normoxic rather than EAN. I will attempt to keep the PP of O2 low (in the 1.0 atm or lower) as well as the PP of N2 below that for air at 80ft. Like the contributors have said, it will demand a mix of not 28/25 but more in the range of 23/30 or 21/35 for dives to the 110 to 130ft range.
My spreadsheet of various mixes and the PP of N2 at various depths will be updated with a column of PP for O2. Next is to model lots of simulated dives using MAC compatible software (I still need to find). Thanks for the comments.

-T

 

[Agility]

if cost is not a factor you should be able to correctly analyze your mix
that includes the He content as well as oxygen.

 

[Centrals]

if cost is not a factor you should be able to correctly analyze your mix
that includes the He content as well as oxygen.

Having a He analyser is very useful in mixing trimix. Some would argue that it is not really necessary but I don't work on approximation on dive plan.