MOD marking for 21/35

[rjack321]

There's seems to be wide range of opinions on the MOD of 21/35 both depending on what its used for and other reasons. I am thoroughly confused about how 150ft/45m was actually chosen since it doesn't really align with a "standard" ppO2 of 1.2 (155ft), 1.4 (187ft) or a narcotic MOD of about 4ATAs (170ft)

 

[Bismark]

Are you getting the Ratio deco 150' 1:1 ratio number mixed up with the derivation of the MOD numbers? I thought the MOD of 21/35 was 190 based on PPo2 considerations and END and the ratio deco numbers were something else where 150 was the useful set point?? Kind of two separate systems that overlay on top of each other??

 

[jjones9527]

My guess is the MOD is based on the most conservative factor, which is in this case the PPO2 of 1.2ATA of 155. Its probably just been rounded down to 150 to make for a nice clean number, and add that little bit more conservatism. I use 190 to mark my 21/35, however I see that GUE has some new stickers out for 100, 150, 200 etc to denote the working depth of a stage bottle.

 

[RTodd]

I have never seen tanks marked as other than 20, 70, 120, 190. If you are using it as bottom gas, the 190 label still tells you what is in it and the depth you absolutely can't breathe it past. Your plan should dictate the max depth for the dive. Marking a bunch of bottles for operational MOD and others for deco mod seems to be asking for trouble.

 

[rjack321]

Like I said there's definately some confusion. No I'm not confused about ratio deco setpoints.

When I did Tech1 a few years ago it was my understanding that 21/35 is MOD 190 which is obviously a working ppO2 of 1.4 but exceeds the max END of 4ATA.

More recent students seem to be being told its MOD 150ft for reasons I don't totally understand. It seems to be a shallower rounding of a max ppO2 of 1.2, in line with the recent revision of 30/30 from MOD 120ft to MOD 100ft.

 

[jjones9527]

Yeah, I still use 120 for my 32 and 30/30, and 190 for 21/35 even though I am using them as bottom gases. I had a thread here a little while ago regarding the same issue.

http://www.scubaboard.com/forums/dir-practitioners-zone/208771-mod-markings-stages.html

We have about a dozen or so Tech1/2 guys and have been discussing the issue. Some are for the change, some against, and some like me on the fence.

 

[amascuba]

I have never seen tanks marked as other than 20, 70, 120, 190. If you are using it as bottom gas, the 190 label still tells you what is in it and the depth you absolutely can't breathe it past. Your plan should dictate the max depth for the dive. Marking a bunch of bottles for operational MOD and others for deco mod seems to be asking for trouble.

That has always been my thinking too, but the 120 label still confuses me because it could be 32%, 30/30, or 35/25...

 

[rjack321]

I guess it all comes down to whether you accept a ppO2 of 1.2 as a max or not.

Historically the recommendations have been max 1.2 except for 32% which got 1.3 (100ft) and 30/30 which got 1.4. Tech1 dives were advised to maintain average ppO2 of 1.2 (even on 30/30) but allowed excursions to 1.4. This seems to have been gradually revised to a max of 1.2

Oh and various opinions on doing deep deco at a ppO2 of 1.4 or not.

 

[rjack321]

Yeah, I still use 120 for my 32 and 30/30, and 190 for 21/35 even though I am using them as bottom gases. I had a thread here a little while ago regarding the same issue.

http://www.scubaboard.com/forums/dir-practitioners-zone/208771-mod-markings-stages.html

We have about a dozen or so Tech1/2 guys and have been discussing the issue. Some are for the change, some against, and some like me on the fence.

Whoops forgot about that discussion...

 

[PfcAJ]

Here's how I was taught and what I do. I've included corresponding ppo2's and END's assuming Oxygen is as narcotic as Helium.

32% = 100 (ppo2 = 1.3) (END = 100)
30/30 = 120 (ppo2= 1.4) (END = 74)
21/35 = 150 (ppo2= 1.2) (END = 86)
18/45 = 190 (ppo2= 1.2) (END = 90)

For longer dives or dives with increased workload, decrease ppo2. For instance, if I was spending the majority of my dive in the 120 range, I would most likely up it to 21/35. You not only get a lower ppo2, but the increase in Helium will make for a clearer dive.

Clearly, using more advanced gasses requires divers to think about their choices, not just blindly follow a set of numbers. Keep in mind, these numbers are the MAXIMUM. There is really no reason to push a gas to its edge.

Deco gasses beyond 100% and 50% are past my level of training, so I will not comment on them.