Dives to 160-165 ft

[Joe]

A bunch of us were sitting around planning dives last night and the subject of standard mixes came up.

When planning dives in Deco Planner to 160 the ppo2 for 21/35 is 1.23 and for 18/45 the ppo2 is 1.06. I know for the working part of the dive we keep the ppo2 to a max of 1.20. But for this depth the 21/35 gets you out of the water faster. Is the minimal increase in the ppo2 that big of a deal? Also the difference in END, CNS and OTU's is minimal. Are there any reasons, other than the pp02, why 21/35 is not an acceptable gas for a dive to this depth?

[Jay Roy]

21/35 range is 130' to 160' with an MOD of 190
18/45 range is 170' to 200' with an MOD of 220
So diving to 165' on either gas would be a matter of choice but splitting hairs really...Who plans to 165 and not just 170 or 160?

BTW, why was I not invited to this dive planning party? ;)

[Chantelle]

If I am doing this correctly, and you do the calculations, MOD of 21/35 at PP02 of 1.4, is 186 fsw. If you do it for 1.2, it's 155 fsw.

What calculations were you using, Joe?

[Joe]

Thanks Jay.

I told John we always plan for 10 ft increments anyway so 160 or 170. We were talking about doing "G" and the St James etc and got on the subject of gases. I liked the 21/35 schedule better and that is where the questions came from. I agree we are splitting hairs with this example.

We are going to continue the planning Sunday after the dive if your interested.

21/35 range is 130' to 160' with an MOD of 190
18/45 range is 170' to 200' with an MOD of 220
So diving to 165' on either gas would be a matter of choice but splitting hairs really...Who plans to 165 and not just 170 or 160?

BTW, why was I not invited to this dive planning party? ;)

[Jay Roy]

If I am doing this correctly, and you do the calculations, MOD of 21/35 at PP02 of 1.4, is 186 fsw. If you do it for 1.2, it's 155 fsw.

What calculations were you using, Joe?

Looks right to me...

[Jay Roy]

Thanks Jay.

I told John we always plan for 10 ft increments anyway so 160 or 170. We were talking about doing "G" and the St James etc and got on the subject of gases. I liked the 21/35 schedule better and that is where the questions came from. I agree we are splitting hairs with this example.

We are going to continue the planning Sunday after the dive if your interested.

I would use 21/35 on those dives...You will probably never hit 160' on those wrecks, and if so, the average depth is going to be a fair bit less...I just took a quick look at my last dives on those wrecks and St James was max 163' and G was max 165'
You could call it 150, 30 min bottom time, 30 mins deco...Go back, have lunch at the Barcelona Cafe :)

[Joe]

Deco Planner. I don't think Deco Planner considers O2 narcotic. When you run the numbers through you get 21/35, pp02 1.23, MOD 187ft at a depth of 160.

If I am doing this correctly, and you do the calculations, MOD of 21/35 at PP02 of 1.4, is 186 fsw. If you do it for 1.2, it's 155 fsw.

What calculations were you using, Joe?

[Joe]

Yepp I had 160-165 on the St James. Thats probably where we got the 165ft from.

I would use 21/35 on those dives...You will probably never hit 160' on those wrecks, and if so, the average depth is going to be a fair bit less...I just took a quick look at my last dives on those wrecks and St James was max 163' and G was max 165'
You could call it 150, 30 min bottom time, 30 mins deco...Go back, have lunch at the Barcelona Cafe :)

[Oren]

Gas ranges changed recently according to GUE:
21/35 100-150
18/45 150-200

At least, this is according to the Tech1 course I was able to sit in recently.
What you do, is, of course, entirely up to you and the team.

[Steve B]

Gas ranges changed recently according to GUE:
21/35 100-150
18/45 150-200

At least, this is according to the Tech1 course I was able to sit in recently.
What you do, is, of course, entirely up to you and the team.

Oren has it right here. Our END limit is 100 feet and we should not surpass 1.2 PPO2 on the working portion of the dive. This translates to using a 100 MOD label on a stage that is used as bottom gas for EAN32, 30/30 would be 120 MOD, 21/35 would be 150 MOD, 18/45 would be 200 MOD so on so forth.

The MOD are different when using these standard gases for deco purposes.
Generaly, we use 1.6 PPO2 for deco gases above 100 feet and 1.4 PPO2 for deco gases bellow 100 feet. Some dives may require changes. Deep deco gases may require a lower PPO2 du to CNS and OTU's which does not really affect us as we don't du these 25 hr epic dives like they do in Wakulla.

Steve

[Kevin]

There's always a tradeoff, but t's better to spend a few more minutes on deco rather than push the CNS limits. The deco time difference between 18/45 and 21/35 is negligable anyway.

Many of us gave up 21/35 for 18/45 years ago just to cut down on the number of deicated cylinders we have ready to go and simplify the logistics of weekend travel planning.

[Joe]

Thanks for the replies guys. :cool: