Standard Mix reasoning

[b1gcountry]

Apologies if this has been asked before, but I've been reading the Tech 1 manual, and I had a couple questions about recommended mixes.

GUE recommends a Max END of 100', or 4ata, correct?

They basically consider anything not helium narcotic, so they combine PPO2 and PPN2 and limit the total to 4ata.

They also recommend a maximum PPO2 of 1.4 on the working part of the dive.

The recommended mix for 110-150' is 21/35. This mix will have a MOD (1.4ata ppO2) of 187'.
(1.4/.21 - 1) * 33

It also had a END of 100' at 170'
(4ata / (1-.35) - 1) * 33

So if narcosis is acceptable down to 170', and O2 % good down to 180+, why the recommendation for a 150' max depth?

I'm trying to reason out their logic with their recommendations, and it just doesn't work out.

Tom

 

[rainman_02]

It makes it "simple."

<100 - 32%
100-150 - 21/35
150-200 - 18/45
...

 

[rjack321]

One of the more recent grads might be able to help you more...

While max ppO2 has historically been 1.4, average has been 1.2 This has recently been racheted down to a max of 1.2.

There's nothing wrong with more helium, you're not saving anything (deco time or otherwise) by pushing 21/35 at 170-180ft, might as well use 18/45.

 

[ppo2_diver]

They have also done more research into deco theory and found that a lower ppo2 during the bottom phase of the dive makes for more efficient deco. I know I read this somewhere, but I can't find it. Also a much bigger concern than narcosis is CO2 toxicity. More helium, less risk of CO2 tox.

 

[johnkendall]

Apologies if this has been asked before, but I've been reading the Tech 1 manual, and I had a couple questions about recommended mixes.

GUE recommends a Max END of 100', or 4ata, correct?

They basically consider anything not helium narcotic, so they combine PPO2 and PPN2 and limit the total to 4ata.

They also recommend a maximum PPO2 of 1.4 on the working part of the dive.

The recommended mix for 110-150' is 21/35. This mix will have a MOD (1.4ata ppO2) of 187'.
(1.4/.21 - 1) * 33

It also had a END of 100' at 170'
(4ata / (1-.35) - 1) * 33

So if narcosis is acceptable down to 170', and O2 % good down to 180+, why the recommendation for a 150' max depth?

I'm trying to reason out their logic with their recommendations, and it just doesn't work out.

Tom

The deeper we go, the lower the PO2 for the bottom. So in the T1 range we are looking at a PO2 max of 1.2, as we go deeper that can be pulled down to 1 or lower if circumstances require it.

HTH

John

 

[b1gcountry]

These are the standard mixes as outlined in JJ's Tech 1 pdf. END and ppO2 are the values at the max depth for the mix. MOD is the depth where the ppO2 will reach 1.4ata, and 100' END is the depth where END reaches 100'.

1-100' 32%
END: 100'
ppO2: 1.28 ata
MOD (1.4 ppO2): 111'
100' END: 100'

110-150' 21/35
END: 86'
ppO2: 1.16 ata
MOD (1.4 ppO2): 187'
100' END: 170'

160-200' 18/45
END: 94'
ppO2: 1.26 ata
MOD (1.4 ppO2): 223'
100' END: 207'

210-250' 15/55
END: 94'
ppO2: 1.28 ata
MOD (1.4 ppO2): 275'
100' END: 260'

So I guess the only one that is really conservative is the recommendation for 21/35. I guess I can buy that so it's easier to remember.

Tom

 

[rjack321]

So I guess the only one that is really conservative is the recommendation for 21/35. I guess I can buy that so it's easier to remember.

Really conservative is not diving

You have to remember that standard mixes other than 30/30 are made by transfilling with 100% He and topping with 32% (since that's what was/is commonly available in FL). Other agencies/groups/divers use alternative "standard mixes" and they are predicated on different assumptions. For instance an easy to make standard CCR dilutent is 10/50 (50% He topped with air), and 18/50 is commonly used by other OC divers (50% He + a 36% top)

Best way to approach this is create the mix via 100% + EAN32 and then think about what ranges you'd use it in.

BTW, you forgot the deco gases...
02, 20ft
50%, 70ft
35/25, 120ft
21/35, 190ft

 

[lamont]

18/50 is commonly used by other OC divers (50% He + a 36% top)

18/50 fits with in GI3s "at least that much helium" guideline and will deco sufficiently similarly to 18/45 that it wouldn't matter. Somewhere in between those gases and 18/82 the high level of helium might affect decompression and it wouldn't necessarily be compatible with 18/45 any more, but I'd consider 18/50 to be well within the fuzzy lines.

YMMV.

 

[rjack321]

18/50 fits with in GI3s "at least that much helium" guideline and will deco sufficiently similarly to 18/45 that it wouldn't matter. Somewhere in between those gases and 18/82 the high level of helium might affect decompression and it wouldn't necessarily be compatible with 18/45 any more, but I'd consider 18/50 to be well within the fuzzy lines.

YMMV.

Me too

The key thing to realize with standard gases is that mixing is roughly linear 35%, 45% etc (discounting differences in ideal vs. real gases and compressibility etc.) But ppO2s and ENDs are not following the same slope.

 

[DeepSeaExplorer]

Apologies if this has been asked before, but I've been reading the Tech 1 manual, and I had a couple questions about recommended mixes.

GUE recommends a Max END of 100', or 4ata, correct?

They basically consider anything not helium narcotic, so they combine PPO2 and PPN2 and limit the total to 4ata.

They also recommend a maximum PPO2 of 1.4 on the working part of the dive.

The recommended mix for 110-150' is 21/35. This mix will have a MOD (1.4ata ppO2) of 187'.
(1.4/.21 - 1) * 33

It also had a END of 100' at 170'
(4ata / (1-.35) - 1) * 33

So if narcosis is acceptable down to 170', and O2 % good down to 180+, why the recommendation for a 150' max depth?

I'm trying to reason out their logic with their recommendations, and it just doesn't work out.

Tom

I took Tech 1 some time ago when 21/35 was the standard mix for 180ft. I felt a little buzzed at that depth on 21/35. It was even more noticeable after trying 18/45 and then switching back. 18/45 is definitely the right mix for that depth. There's no real issue with the PO2, just lack of helium. On the other hand, a limit of 150ft for 21/35 is way too conservative in my opinion. This is one of those odd inconsistencies I've run into from time to time.

When I took Tech 1, we were told we would be certified to dive the limits of 21/35 which was stated as 180ft - it all made sense. Then, when I got my card it said I was certified to dive Triox to a depth of 150ft. I was like, "What the hell is 'Triox' and why am I being limited to 150ft?" At that time, everyone just ignored the posted speed limit and went on with what they had already been doing. Divers certified for Normoxic were diving to the limits of 18/45 and no one objected. Eventually, an edict was issued that you were only allowed to dive to the limits of your GUE certification regardless of whatever other certifications you held, including instructors.