triox vs. trimix?

[GLOC]

Originally Posted by big_si

Yep, that's exactly how I have V-planner set up... Gives a warning if more than 2/3 of back gas is used or more than 1/2 deco gas. Don't plan for any bottom time more than that.

Is that 2/3 used on your lost deco gas plan?

[Brian of Aquanauts]

I think the safety factor that's given by the 1/3rd in backgas is somewhat misleading, as for some dives it works and others it doesn't. I prefer to calculate out the actual gas needed.

For instance on a 72m with 3 deco gases, you don't actually need a 1/3rd to get to your first 21/35 deco bottle, i worked out that i could get 2 divers up to 57m with a stressed breathing rate in about 40-50bar. But on the same dive with 2 stages, your first bottle is at 21m and that's almost half your gas supply away if you're taking a second diver up there. Both of the above examples were for twin 12s. If you dive twin 18s then a third is overkill.

So a blanket rule seems to put you at risk sometimes and be too cautious at others. Better to actually have a plan than blindly follow a rule that only works sometimes.

[big_si]

Originally Posted by GLOC

Is that 2/3 used on your lost deco gas plan?

2/3 on normal plan.

2/3 warning limits are set for lost gas plan too but some of that reserve 1/3 would be used for deco'ing rather than kept as a reserve for a buddy.

40m 30 mins

With 50% deco gas
Dec to 40m (2) Nitrox 28 15m/min descent.
Level 40m 27:20 (30) Nitrox 28 1.39 ppO2, 36m ead
Asc to 21m (31) Nitrox 28 -10m/min ascent.
Stop at 21m 1:06 (33) Nitrox 50 1.54 ppO2, 10m ead
Stop at 18m 1:00 (34) Nitrox 50 1.39 ppO2, 8m ead
Stop at 15m 1:00 (35) Nitrox 50 1.24 ppO2, 6m ead
Stop at 12m 3:00 (38) Nitrox 50 1.10 ppO2, 4m ead
Stop at 9m 4:00 (42) Nitrox 50 0.95 ppO2, 2m ead
Stop at 6m 21:00 (63) Nitrox 50 0.80 ppO2, 0m ead
Surface (69) Nitrox 50 -1m/min ascent.

Off gassing starts at 25.6m

OTU's this dive: 77
CNS Total: 29.6%

2725.7 ltr Nitrox 28
670.4 ltr Nitrox 50
3396.1 ltr TOTAL



Lost gas
Dec to 40m (2) Nitrox 28 15m/min descent.
Level 40m 27:20 (30) Nitrox 28 1.39 ppO2, 36m ead
Asc to 21m (31) Nitrox 28 -10m/min ascent.
Stop at 21m 0:06 (32) Nitrox 28 0.86 ppO2, 18m ead
Stop at 18m 2:00 (34) Nitrox 28 0.78 ppO2, 16m ead
Stop at 15m 3:00 (37) Nitrox 28 0.70 ppO2, 13m ead
Stop at 12m 5:00 (42) Nitrox 28 0.61 ppO2, 10m ead
Stop at 9m 6:00 (48) Nitrox 28 0.53 ppO2, 7m ead
Stop at 6m 44:00 (92) Nitrox 28 0.45 ppO2, 5m ead
Surface (98) Nitrox 28 -1m/min ascent.

Off gassing starts at 25.6m

OTU's this dive: 54
CNS Total: 21.8%

3866.8 ltr Nitrox 28 (more than 2/3 used)
3866.8 ltr TOTAL

[Scubee]

Originally Posted by GLOC

Here's is one for those who dive with a lost deco plan that Si/Weazlez posted. How many people limit their bottom time based on their lost deco plan?

Not meant to be a ruck, but if you are going to work out a lost deco plan, surely you should limit your BT to make sure you can do it on BG.

That is logical, and it is certainly the way I work out the FUBAR plan. It is how I was taught and the logic behind having that plan somewhere is unarguable.

[GLOC]

Originally Posted by big_si

2/3 on normal plan.

2/3 warning limits are set for lost gas plan too but some of that reserve 1/3 would be used for deco'ing rather than kept as a reserve for a buddy.

Thanks. In this example what gas requirement would you dive?

[big_si]

Originally Posted by GLOC

Thanks. In this example what gas requirement would you dive?

It would be the plan shown in my last post.

[GLOC]

Okay, being thick....

[IanDennis]

Originally Posted by Brian of Aquanauts

I think the safety factor that's given by the 1/3rd in backgas is somewhat misleading, as for some dives it works and others it doesn't. I prefer to calculate out the actual gas needed.

For instance on a 72m with 3 deco gases, you don't actually need a 1/3rd to get to your first 21/35 deco bottle, i worked out that i could get 2 divers up to 57m with a stressed breathing rate in about 40-50bar. But on the same dive with 2 stages, your first bottle is at 21m and that's almost half your gas supply away if you're taking a second diver up there. Both of the above examples were for twin 12s. If you dive twin 18s then a third is overkill.

So a blanket rule seems to put you at risk sometimes and be too cautious at others. Better to actually have a plan than blindly follow a rule that only works sometimes.

I agree- 1/3rds maybe insufficient and inadequate in certain conditions. Its a rough guide- far better to plan out your actually gas requirements prior to the dive and build in sufficient reserves to get you and your team mate to the next available gas source. Blindly following anything is a pretty poor solution - far better to encourage divers to plan their dive and dive their plan

[Steve S]

Originally Posted by Howard Payne

Nope we're stuck to each other like Gary Glitter to a Ladyboy

''Nothing is more fatal to happiness or virtue, than that confidence which flatters us with an opinion of our own strength, and, by assuring us of the power of retreat, precipitates us into hazard."
Samuel Johnson April 14, 1759

Safe diving,
Steve

[Howard Payne]

I prefer:

"The true measure of a man is how he treats someone who can do him absolutely no good" Samuel Johnson

I've always treated you well haven't I Steve?

*runs off and hides behind the sofa*