Isobaric Counterdiffusion question

[DanMacLeod]

I'm planning a dive to 275 fsw, shown below, I would like to add an air break after 20 minutes of breathing 100%. I think my best option would be to use my travel gas, 21/35, on the air break, but this brings up the concern about isobaric counterdiffusion. My question is: is isobaric counterdiffusion a real concern? Is there a better option for a travel gas?


Dec to 275ft (3) Trimix 14/50
Level 275ft 11:11 (15) Trimix 14/50
Asc to 190ft (18) Trimix 14/50
Stop at 190ft 0:27 (19) Trimix 14/50
Stop at 180ft 1:00 (20) Triox 21/35
Stop at 170ft 1:00 (21) Triox 21/35
Stop at 160ft 1:00 (22) Triox 21/35
Stop at 150ft 1:00 (23) Triox 21/35
Stop at 140ft 1:00 (24) Triox 21/35
Stop at 130ft 1:00 (25) Triox 21/35
Stop at 120ft 1:00 (26) Triox 21/35
Stop at 110ft 1:00 (27) Triox 21/35
Stop at 100ft 1:00 (28) Triox 21/35
Stop at 90ft 2:00 (30) Triox 21/35
Stop at 80ft 3:00 (33) Triox 21/35
Stop at 70ft 2:00 (35) Nitrox 50
Stop at 60ft 3:00 (38) Nitrox 50
Stop at 50ft 3:00 (41) Nitrox 50
Stop at 40ft 4:00 (45) Nitrox 50
Stop at 30ft 7:00 (52) Nitrox 50
Stop at 20ft 4:00 (56) Oxygen
Stop at 15ft 25:00 (81) Oxygen
Surface (81) Oxygen

 

[PfcAJ]

I'd do it on either that 14/50 or 21/35, no prob.

At 275 that END is a little deep, though.

 

[DanMacLeod]

I'd do it on either that 14/50 or 21/35, no prob.

At 275 that END is a little deep, though.

At 275, 14/50 has an END of 121 feet. I believe PADI recommends not having more that 130 feet END, that's why I thought 14/50 would be a good choice. What gases would you recommend? I'll always listen to suggestions.

thanks
Dan

 

[PfcAJ]

I'd do that on 12/70.

130 end isn't what you want when 275ft deep. Gas goes quick like a bunny + a decline in cognitive function isn't what you want.

Keep that end shallow.

 

[JohnnyC]

I'd do that on 12/70.

130 end isn't what you want when 275ft deep. Gas goes quick like a bunny + a decline in cognitive function isn't what you want.

Keep that end shallow.

This.

I'd do an air break on 21/35 no problem.

 

[rossh]

I'm planning a dive to 275 fsw, shown below, I would like to add an air break after 20 minutes of breathing 100%. I think my best option would be to use my travel gas, 21/35, on the air break, but this brings up the concern about isobaric counterdiffusion. My question is: is isobaric counterdiffusion a real concern? Is there a better option for a travel gas?

Hi,

You are OK at the Oxygen stop. IBCD is not an issue there. You are swapping between 0% inert gas and backgas inerts - both in inspired and exhaled form. Hence its a slowing of both inert off gassing rates, but not a reversal of gradients that is the IBCD problem.

 

[DanMacLeod]

I'd do that on 12/70.

130 end isn't what you want when 275ft deep. Gas goes quick like a bunny + a decline in cognitive function isn't what you want.

Keep that end shallow.

Thanks for the suggestion, always nice to get another opinion.