80% or O2 DECO

[JeffG]

Where's JeffG?

Right here.

 

[JeffG]

I also prefer 80% simple because it is easier to blend a full tank of 80% when an oxygen booster is not available.

Really? 100% is pretty easy to blend. You only need a booster to get more gas in a tank, but the % of the gas never changes. You would need a booster to get the correct % when mixing 80% when the pressure of the supply tank is down.

 

[JeffG]

That item may be debated by people who have more letters after their name than I do but the benefit of 80% can be seen when using v-planner or such software and combining it with a deep deco mix such as 32% you will find many occasions where the combination will result in less deco time.

100% O2 has no N2...thats good. 80% allows the diver to get on a high O2 content gas deeper/quicker.

Deco models change/evolve, whos to say v-planner has got it right. I'll go with no N2 every time.

 

[amascuba]

This tired old rant has been dragged out whenever this question is asked and although it is called bakers dozen really only addresses 2 issues with 80% and therefore should be called the bakers 2. First it wrongly assumes that people choose 80% over O2 because they are unable to hold their bouyancy in open ocean diving and that is just not true! Second it assumes that every tech diver believes in a "magical" process called oxygen window and that unless you are hitting a full 1.6 PPO2 at 20ft you are not getting a good washout. That item may be debated by people who have more letters after their name than I do but the benefit of 80% can be seen when using v-planner or such software and combining it with a deep deco mix such as 32% you will find many occasions where the combination will result in less deco time. I also prefer 80% simple because it is easier to blend a full tank of 80% when an oxygen booster is not available.

I just ran four profiles using mv-plan and hlplanner. Mv-plan uses a bulhmann model and I have the gradient factor set to 30/85. hlplanner uses the vpm-b model and I have the conservatism set to 2.

All dives were 240ft dives w/ 30min. bottom time using a bottom gas of 15/55.

The first dive w/ mv-plan uses 32% and 80% as deco gases and it's run time was at 124minutes with the stops starting at 140ft.

The second dive w/ mv-plan used 21/35, 50%, and 100% O2 as deco gases (which to my knowledge would be the proper DIR deco gases for this dive.) and it's run time was 119minutes with the stops starting at 140ft.

The third dive w/ hlplanner uses 32% and 80% as deco gases. It's run time was 115min with stops starting at 170ft.

The forth dive w/ hlplanner uses 21/35, 50%, and 100% as deco gases. It's run time was 117min with stops starting at 170ft.

They all have pretty similar run times. IMHO though I would consider the dives with 21/35, 50, and 100 cleaner. That's only because the higher % of O2 and the lower % of N2 helps flush the body of residual N2. What do I know though. I'm a newbie at this stuff.

 

[JeffG]

The second dive w/ mv-plan used 21/35, 50%, and 100% O2 as deco gases (which to my knowledge would be the proper DIR deco gases for this dive.) and it's run time was 119minutes with the stops starting at 140ft.

21/35 is for 150 ft, not 240. 240 would be 15/55

 

[amascuba]

21/35 is for 150 ft, not 240. 240 would be 15/55

21/35 for deco. I was using 15/55 for bottom gas.

I had to go double check my post to make sure that I put that earlier.

 

[Meister481]

I read gentiles handbook and in it he made a good arguement for 75%. You can get on it sooner in the plan and the OLF is less if you wish to make a second dive.

Same could be said for carrying 50, because you carry 50 you must really have a bouyancy issue. Whatever.

 

[JeffG]

21/35 for deco. I was using 15/55 for bottom gas.

I had to go double check my post to make sure that I put that earlier.

Sorry, I missed it myself. Saw it the second time though.

 

[H2Andy]

i've heard from a lot of DIR guys that 80% is cheaper, and with the money they save, they buy more cigarettes

also, it's a lot safer to smoke around 80% than pure O2, so ...

makes sense to me

 

[wedivebc]

I just ran four profiles using mv-plan and hlplanner. Mv-plan uses a bulhmann model and I have the gradient factor set to 30/85. hlplanner uses the vpm-b model and I have the conservatism set to 2.

All dives were 240ft dives w/ 30min. bottom time using a bottom gas of 15/55.

The first dive w/ mv-plan uses 32% and 80% as deco gases and it's run time was at 124minutes with the stops starting at 140ft.

The second dive w/ mv-plan used 21/35, 50%, and 100% O2 as deco gases (which to my knowledge would be the proper DIR deco gases for this dive.) and it's run time was 119minutes with the stops starting at 140ft.

The third dive w/ hlplanner uses 32% and 80% as deco gases. It's run time was 115min with stops starting at 170ft.

The forth dive w/ hlplanner uses 21/35, 50%, and 100% as deco gases. It's run time was 117min with stops starting at 170ft.

They all have pretty similar run times. IMHO though I would consider the dives with 21/35, 50, and 100 cleaner. That's only because the higher % of O2 and the lower % of N2 helps flush the body of residual N2. What do I know though. I'm a newbie at this stuff.

Yeah but let's compare apples to apples. You are throwing in a 3rd deco gas which will of course get you out quicker. Try a dive to 150ft using only 50% and 100% and see what happens. I don't have v-planner here at work so I can't post the results.
OTOH I could just do the dive on my rebreather with a fixed set point of 1.2 and get out faster than all you OC guys but again that's not a fair comparison.