Cheaping out on Trimix - travel gas

[MichaelMc]

You said your bottom gas is 21/35. So make your deco gas, which you are cleared for, be the 32 you want shallow. Use it to hang out a while in the 10-30 meter shallows after. But no 50%.

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

He is not cleared for multiple stages. Just one deco.

 

[Wibble]

You said your bottom gas is 21/35. So make your deco gas, which you are cleared for, be the 32 you want shallow. Use it to hang out a while in the 10-30 meter shallows after. But no 50%.

He is not cleared for multiple stages. Just one deco.

Problem is this removes the redundancy on the bottom gas

The biggest benefit of the next course Normoxic Trimix is the use of multiple stages. You don't necessarily need to use the course knowledge just for deep stuff, it’s having two stages that makes your diving so much easier as you now have redundancy, etc. (e.g. should a deco stage fail, you should be able to deco out on only one)

 

[tbone1004]

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....

not going to eviscerate, just have to agree to disagree. My O2 bottles are usually only filled to 2100psi as well since my Homefill stops around there as well, but I prefer the way an AL80 handles at 2100psi anyway and it at least keeps everything consistent.

 

[KenGordon]

(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?)

The third law of Scuba Board is thou shalt not use 80%. It is just before the bit about how it is impossible to get 250 bar of O2 into a three and a couple before never doing deco without a special deco mix.

I think it might have something to do with not really diving in the sea or in interesting conditions and thinking the world revolves around caves.

 

[flymolo]

The third law of Scuba Board is thou shalt not use 80%. It is just before the bit about how it is impossible to get 250 bar of O2 into a three and a couple before never doing deco without a special deco mix.

I think it might have something to do with not really diving in the sea or in interesting conditions and thinking the world revolves around caves.

I dive 100% in the ocean all the time. Do people really think it can't be done?

80% is not a good gas choice IMO. As others have said you're far better off bringing a 50% to improve your min gas margins, as your next gas is at 70 feet.

 

[ChuckP]

not going to eviscerate, just have to agree to disagree. My O2 bottles are usually only filled to 2100psi as well since my Homefill stops around there as well, but I prefer the way an AL80 handles at 2100psi anyway and it at least keeps everything consistent.

My opinion would probably change somewhat if I tried an 80 for deco and had mainly 80's for that, all my deco bottles are 40's though. My 80's I just keep "normal" mixes in and do take a 80 of 36 and a 40 of whatever is in it on deeper or longer dives - just watch your plans.

We don't do "big" dives, more like what I'll call recreational deco dives and for me in sidemount, that 80 just seems big and bulky as compared to a 40 and way overkill - again, for what we do.

Our dives aren't "square" profiles, very multilevel as we are wall diving.

 

[Marie13]

From hearing people talk, there seems to be a small number on the Great Lakes who like 80%. 50% seems much more popular.

 

[shoredivr]

I have dived with some who like 80%. (Have dived because thanks to the plague there hasn’t been charters running this year till just recently).

I am diving 50% for deco

 

[Wibble]

Sticks neck out...

If I was diving a shallowish dive, say less than 35m/115', I'd use a single deco stage of 80% every time. I have access to ali40 with 100% but hate it as it's too "precise" and of only any use at 6m/20' and above. I also get a sore throat when using 100% for any time. Using the 80% means I can get on to a decent rich deco gas at 9m and just chill out.

(OT) For bailout from deep, again the 80% would be my choice, but I do need an additional 'cooler' deco gas. This will be an ali40 of 50% to go with an ali80 of 80%. Having 3 cylinders (bottom bailout) does get tedious, although a bit of bungee and sidemount magic puts it under control.

(OOT) Woke up this morning thinking that I should do another planning post, this time for the OP's question about gasses to work through the gas plans and the optimum gas for 45m (for price as well as effect). But I'll leave it for now ;-)

 

[boulderjohn]

IMO, a lot of the ado about 80% comes from an otherwise long-forgotten history. During the DIR wars, the chief spokesperson for DIR, George Irvine III, published many articles about his beliefs, and he frequently used highly inflammatory language that both excited and enthused his followers and enraged his opponents. Here is a 2013 post from one of his most devoted followers, Dan Volker. In it he quotes one of George's tamer articles explaining why DIR divers never use 80%. If you want to get the tone, skip right to the end.

So, just who is George Irvine III?????

I also suggest you read my response to it, which is in the next post.