MOD marking for 21/35

[rhlee]

Just as, if not more, confusing if I bring a stage of 32% its marked exactly the same as a stage of 30/30.

I've heard this many times, and I've never understood it. As I understand it, MOD markings are just that, MOD, and not any indication of mix. Since we (should) analyze on-site, and I'm hard-pressed to see how bottles containing both gasses would be pressed into service on the same dive, there shouldn't be any confusion introduced by labeling both 120 (or 100, or whatever).

Or, that's how I see it anyway...

 

[MattL]

I do 21/35 at 150 if its backgas and 190 if its deco. this is allso what it saysing in my crouse work hand out.

0-100fsw 32%
0-100fsw 30/30 this is new it was at 120
100-150fsw 21/35
160-200fsw 18/45
200-250fsw 15/55
deco gas are
0-20 o2
20-70 50%
70-120 35/25
120-190 fsw 21/35
this is what i have so i hope thats some knid of help

 

[limeyx]

Because we are thinking divers and not mindless koolaid drinkers.

Then this "thinking" is killing me

This is an area where the thinking needs to be done by a group of people, out of the water (GUE) and communicated (with reasoning) to all GUE divers, so we all follow the same procedure.

Relating it to "being a thinking diver" gives us nothing -- part of DIR is having common sensible procedures, not all thinking up our own MOD's for various gases.

 

[limeyx]

So, if both are there to extend bottom time, why should we have two standards for marking the bottles? I

Because we "tolerate" different max PPO2 in each case.

Bottom it is (1.2 max) (was: 1.2 average)
Deco: 20,70,120 foot are 1.6, deeper are I think 1.4 for "safety" (or maybe just not wanting to add a new gas

So we may have to have different schemes for them as we assume we are more "at rest" on deco (of course, with currents, swells etc. this may not be the case)

 

[rjack321]

I've heard this many times, and I've never understood it. As I understand it, MOD markings are just that, MOD, and not any indication of mix. Since we (should) analyze on-site, and I'm hard-pressed to see how bottles containing both gasses would be pressed into service on the same dive, there shouldn't be any confusion introduced by labeling both 120 (or 100, or whatever).

Or, that's how I see it anyway...

Yes but one is a MOD based on 1.2 and the other is a MOD based on 1.3. I would never mark a 32% bottle 120, but there's historical precedent to do so for 30/30. Supposedly the new MODs are a max of 1.2 ppO2 but there are obviously GUE/DIR divers not following those recommendations.

 

[rjack321]

Then this "thinking" is killing me

This is an area where the thinking needs to be done by a group of people, out of the water (GUE) and communicated (with reasoning) to all GUE divers, so we all follow the same procedure.

Relating it to "being a thinking diver" gives us nothing -- part of DIR is having common sensible procedures, not all thinking up our own MOD's for various gases.

Yes the issue is mostly related to consistency. If I travel someplace I'd be surprised if local folks are doing something different than in my own community. We all don't create "best mixes", why are we dreaming up at least 3 different ways to mark (or not) a bottom stage of 21/35?

I fully understand the logic behind each method. Is the choice then left up to local consensus? Since HQ is supposedly telling me what to put in my left pocket, you'd think they'd have an interest in the much more critical issue of tank marking (if at all).

 

[limeyx]

I've heard this many times, and I've never understood it. As I understand it, MOD markings are just that, MOD, and not any indication of mix. Since we (should) analyze on-site, and I'm hard-pressed to see how bottles containing both gasses would be pressed into service on the same dive, there shouldn't be any confusion introduced by labeling both 120 (or 100, or whatever).

Or, that's how I see it anyway...

How about a cave dive where you dive 30/30 on the bottom for a while, then dip down to 150 for a while. Now you decide you want a 120 foot deco bottle.

The 30/30 and 35/25 may both be marked 120 -- which one do you choose for the deco ? will it make a huge difference? in this case likely not as the mixes are (as you might expect) very similar.

You could also just take 21/35 as a bottom gas and do more deco .. I think the EKPP is dealing with exactly this issue (at deeper depths)

 

[limeyx]

Yes the issue is mostly related to consistency. If I travel someplace I'd be surprised if local folks are doing something different than in my own community. We all don't create "best mixes", why are we dreaming up at least 3 different ways to mark (or not) a bottom stage of 21/35?

I fully understand the logic behind each method. Is the choice then left up to local consensus? Since HQ is supposedly telling me what to put in my left pocket, you'd think they'd have an interest in the much more critical issue of tank marking (if at all).

Heh, you think?

My perspective is, mark everything with MOD of 1.4 (bottom stages) and 1.6/1.4 (deco -- 20, 70, 120 @ 1.6, 190+ @ 1.4)

Do not bring 30/30 and 35/25 on the same dive (I think, since each would be marked as 120 -- or just use 25/25 and mark 130)

Now you know based on END/PPO2 what depth you "could" breathe each bottle and not get too narced/tox.

Now when you plan the dive, you choose the appropriate mix, which may mean breathing a gas way less than its MOD. For instance, I might choose to dive 18/45 in Lake WA even though it's only 140-150. In sunny Socal, 21/35 will do just fine.

However, the bottle is still marked to a "safe" level.

 

[AndrewJD]

Just to get in on the conversation....

Duct Tape and a big black marker are your friend!

These are the three parameters I learned for bottom gas (of course I'm a heretic by association...)

shallowest - working MOD of 1.2 - where you really want to be diving the gas
middle - END of 100ft
deepest - hard MOD of 1.4

So, according to my trusty calculator,

32/0 - 91|99|111
25/25 - 125|143|152
30/30 - 99|156|121
21/35 156|170|187
18/45 - 187|207|260
etc...

Hmmm, one of these gases isn't like the others... but that's another thread.

J

 

[rjack321]

However, the bottle is still marked to a "safe" level.

See you and I were trained the same way "1.2 average". But recent tox fatalities and the rather hazy guidelines are backing off to a hard MOD of 1.2 for the working phase.

I guess I wouldn't be in such a quagmire if I didn't feel diving 21/35 to 160ft was relatively safe. Marking it 190 is rather too generous on END and marking it 150 is a bit excessively conservative on ppO2.

170 would be fine with me except that there's too great a risk of screwing it up with a 70 bottle.

And the last choice...
I'd happily leave them unmarked except I have a variety of stages kicking around and I'd hate to grab the wrong one and discover it at a dive site when the analyzer told me of my error. Or to have a 21/35 and a 32% stage in the boat for a multidive trip and have the tape come off.

Mostly though this seems to be a remarkably unstandardized chunk of DIR which could benefit from some learn-ed discourse.