Cheaping out on Trimix - travel gas

[rjack321]

(Not on for an argument!)

If you've a fair bit of deco to do, for example if you're doing 40 mins at 60m/200' this would give you roughly 40 x 2 = 80 mins of deco (60m's about 2:1, for every min at 60, it's 2 mins of deco). Of that deco time, roughly half will be at 6m/20', so about 40mins.

If we did the runtimes for a full deco on 50% (as you'd be doing on a bailout) this would be a lot slower than if you were doing a deco with 50% and 80% (obviously).

Turns to Multideco... for ease I'll use an OC dive to 60m/200' for 40mins on 18/45...
GF50:80, SAC = 15 litres/min bottom and 12 litres/min for deco
(showing only the last 3 stops).

50% only
12m 7 (70) Nitrox 50 1.10 ppO2
9m 12 (82) Nitrox 50 0.95 ppO2
6m 64 (146) Nitrox 50 0.80 ppO2
Surface (148)
OTUs: 140
CNS: 50%
2093 ltr Nitrox 50


60% only
12m 7 (78) Nitrox 60 1.31 ppO2
9m 11 (89) Nitrox 60 1.14 ppO2
6m 63 (152) Nitrox 60 0.96 ppO2
Surface (154)
OTU's this dive: 155
CNS Total: 56.8%
1838 ltr Nitrox 60


50% + 80%
12m 7 (70) Nitrox 50 1.10 ppO2
9m 10 (80) Nitrox 80 1.51 ppO2
6m 47 (127) Nitrox 80 1.28 ppO2
Surface (129)
OTUs: 174
CNS: 67%
574 ltr Nitrox 50
1147 ltr Nitrox 80


50% + 100%
12m 7 (70) Nitrox 50 1.10 ppO2
9m 12 (82) Nitrox 50 0.95 ppO2
6m 42 (124) Oxygen 1.60 ppO2
Surface (126)
OTUs: 181
CNS Total: 112% <Over 100>
846 ltr Nitrox 50
825 ltr Oxygen


Thus:
50% alone uses a full ali80; little over for contingency.
60% alone is slower than 50% (as breathing 8 mins more 18/45)
50% + 80% is 21 mins "more efficient" than 50% only.
100% is most efficient but blows the CNS limit - nearly double the 80% for no real time benefit. Balances the 50% and Oxygen usage.


(Or have I misunderstood?)

*checks notes* the OP is not diving CCR

The guy who is "obviously" not here for an argument sure seems to be trying to make one, but for the life of me I can't understand how it relates to the question about 32% travel gases, stages, or even what someone doing 20-30min 21/35 dives might consider.

 

[Wibble]

Apologies. Got a thing going on about planning at the moment.

 

[JonG1]

Apologies. Got a thing going on about planning at the moment.

Start a new thread Wibs, been doing something similar with Ben from your course, u thinking bout Malin?

 

[Wibble]

Malin's just been cancelled - covid restrictions in Ireland meant that we’d have to quarantine for 14 days! The wrecks will still be there next year.

And well spotted, the bout of acute planningitus was probably brought on by Malin

 

[Whitrzac]

Taken straight from the TDI trimix manual...
"The key to preventing CNS oxygen toxicity is keeping P02s with acceptable limits. Most open-circuit tech divers choose to stay within a PO2 of 1.4 ATA for the working portion of any dive, and 1.6 ATA during decompression"

 

[Scared Silly]

So, as it happens to be, I'm currently a newbie Trimix-diver.
Due to the cost and hassle involved in getting a trimix fill, I usually get a 21/35, which is topped up with air after the first dive, to get more dives out of the fill.

My local divesites includes a bit of swimming in the 10-30 meters range both before/after the deep part of the dive, thus I've been playing with the thought of bringing a stage with ie. EAN32 to use as "travel gas". Normally I'd have 1 stage with 50% for limited deco, so now considering bringing two stages.

I dont intend on including the stage with EAN in gas planning, just utilise it to save on the helium.

Any thoughts appreciated!

Assuming a 1.4 PO2. My thoughts is that you will be on the descent / swim for probably 2-3 minutes on the way down before going to mix. On the way up you would be on EAN32 for perhaps again 2-3 minutes from 33m (MOD for EAN32) to 18m (MOD for EAN50) where you switch to your 50%. So how much gas are you going to really be saving??? Do the calculations and figure out the costs. My guess it is would be minimal.

 

[ChuckP]

So why would I use 80% when I can get the same total deco time from 50% which is much easier to blend than 80% and using your logic, you can get on it at 70ft instead of 30? There is a reason 50% is the standard deco gas for those types of dives.

At the risk of being eviscerated - my reasons for 80 verses 100 or 50 as starting point when paying to fill a tank

1 - 100% gets you 2200psi, 80% gets you 3100psi here
2 - I typically have 15 minutes or less deco with first stop rarely deeper than 30'
3 - I dive open ocean and use 15' as my last stop
4 - I easily transfill my "80%" tank from a hp 120 of 36% - I transfill until it's a high 40 number then will pay to have it refilled - I can get 4-6 dives from one "paid" fill
5 - I love the precision of tech diving, hitting my gas changes and planned theoretical run times, perfect stops - but I'm in no hurry to get my head above the water line and neither are my friends - once deco is complete, I'll hang there on back gas with you till your done, who cares!!
6 - I dive in Cozumel, the water is warm, your group is close by - ehhh- relax, finish your deco...
7 - It's illogical to pay for a fill that you really do not need - these are 15 minute or less deco dives that are done all the time.

I understand standard gases when you're talking 30 minute or longer hang times but as long as your gases are close to each other for shorter hang times, ehhh....

 

[Alekseolsen]

Assuming a 1.4 PO2. My thoughts is that you will be on the descent / swim for probably 2-3 minutes on the way down before going to mix. On the way up you would be on EAN32 for perhaps again 2-3 minutes from 33m (MOD for EAN32) to 18m (MOD for EAN50) where you switch to your 50%. So how much gas are you going to really be saving??? Do the calculations and figure out the costs. My guess it is would be minimal.

I'm quite frequent closer to ~10 minutes swim each way - but the stage have been "cancelled" from my thoughts already due to the other answers here in the thread.

 

[rjack321]

I'm quite frequent closer to ~10 minutes swim each way - but the stage have been "cancelled" from my thoughts already due to the other answers here in the thread.

10mins x 4ata x 16 L/min = 640L each way
1280L total of 21/35 is 450 L of helium

How much are you saving here? And maybe you should look at a scooter to turn that 10min swim into a 2 min swim?

 

[Wibble]

One could take an ali80 bottom stage cylinder and bottom breathable nitrox back gas. The bottom stage would then be used purely on the deep segment until nearly empty then either ascend shallower continuing the dive on back gas or end the dive — or switch to the backgas and accept the additional narcosis for the rest of the deep phase.

Mainly depends upon how comfortable you are with multiple stage cylinders.