RD lost deco gas planning

[Rainer]

1) Extending the shallow time by 1.5x on backgas is now assuming that you were getting a 50% time advantage from the EAN50 up there (shallow). The time advantage is derived from the average over all stops the gas is used for. EAN50 is only "used for" the 70-30ft segment - Tech1 level EAN50 dives are actually the exception. You've taken the exception and made it the rule that the 50% time advantage is derived from the entire 70-surface segment while that is not assumed for deeper dives.

I understand perfectly well how the 50% advantage works. In any case, since I *am* doing T1ish dives only, of course I'm most concerned with lost deco gas to cover such dives!

The point is, once both are on backgas (i.e. after 60'), it really seems you ought to be extending shallow stops. The 50% is still advantageous over the backgas even at 50'. I do agree with Nick, though, that over-staying 50-30' isn't doing much. Personally, however, I can't see any reason not to camp out at 20' for awhile (assuming no O2). If not 2x, then at least 1.5x. I'm already planning gas reserves for this, so that's not an issue.

I just don't see doing 1.5 x time at 70' and 60', then treating the rest of the deco as normal (where 50'-surface is all on a high helium back gas). I do see the point, though, of trying to squeek some greater use of the bottle by emphasizing the 70' and 60' stops.

Thanks for the discussion!

2) Streetdoctor's profile has more "O2 window" emphasis than I've ever seen. Limey and I did a profile like that once and I felt like crap. He thinks he was fine but was asleep on the couch so I don't know how he knows Maybe is was because of the shape, maybe because it was the 2nd dive of the day. In any case, I don't think its fair to say UTD emphasizes the O2 window more or less than GUE anymore, it depends on the circumstances now.

Yeah, we never even discussed such a profile. I will say, though, that the BAUE people I've done dives with do a similar profile. Our dives were all 25 minutes (on a single 50% bottle) and they all shape 5-2-2-2-2 for the 70-30'. 20-10' is "normal" (e.g. 8-4). I can't remember feeling any different.

 

[Sloth]

The GUE Tech 1 standard was to not double the deep stops but hand off the EAN 50 reg every 10 feet during the deep stops and then double the shallow stops and switch off every 5 mins.

I think we talked about this a little as to why you dont take advantage of the o2 window on the deeper stops. I thought I remember there being a comment about additional nitrogen on-gassing that you have to worry about when you start doing a fair chunk of time on back gas during the deeper stops. Is the additionally nitrogen loading not an issue since we are talking about tech 1 level dives?

 

[Rainer]

I think I covered the GUE T1 standard in the other thread. What we were taught was to hand off the reg every 10' from 70-30' (1.5 x normal deco time) and then do 1.5 time at 20' and 10' (i.e. hand off the reg after each has done half that time).

The GUE Tech 1 standard was to not double the deep stops but hand off the EAN 50 reg every 10 feet during the deep stops and then double the shallow stops and switch off every 5 mins.

There's more N2 in 50% than 18/45...

I think we talked about this a little as to why you dont take advantage of the o2 window on the deeper stops. I thought I remember there being a comment about additional nitrogen on-gassing that you have to worry about when you start doing a fair chunk of time on back gas during the deeper stops. Is the additionally nitrogen loading not an issue since we are talking about tech 1 level dives?

 

[rjack321]

Yeah, we never even discussed such a profile. I will say, though, that the BAUE people I've done dives with do a similar profile. Our dives were all 25 minutes (on a single 50% bottle) and they all shape 5-2-2-2-2 for the 70-30'. 20-10' is "normal" (e.g. 8-4). I can't remember feeling any different.

Until a couple days ago this is how I "thought" GUE tech1 deco was being taught. A little O2 window (the 5 @ 70) then linear up to 20, then reversed 20-10ft stops from deco planner.

The GUE Tech 1 standard was to not double the deep stops but hand off the EAN 50 reg every 10 feet during the deep stops and then double the shallow stops and switch off every 5 mins.

I think we talked about this a little as to why you dont take advantage of the o2 window on the deeper stops. I thought I remember there being a comment about additional nitrogen on-gassing that you have to worry about when you start doing a fair chunk of time on back gas during the deeper stops. Is the additionally nitrogen loading not an issue since we are talking about tech 1 level dives?

Meh, I know someone who blew off ~18mins of Tech1 level deco, went on surface O2 and didn't get clinically bent. At this level I really doubt the backgas time at 70ft is all that significant one way or the other from an N2 loading perspective. Likewise the lost deco contingency plan in general. The goal here is to get out "good enough" with the resources you have at hand. If you can't do that, you're diving too aggressive IMHO. Getting out completely, run a marathon afterwards, clean is not realistic or likely.

 

[rjack321]

There's more N2 in 50% than 18/45...

And its a wash with 21/35. One good reason not to use EAN50 only for longer dives where you really have accumulated substantive N2 loads.

 

[Rainer]

Until a couple days ago this is how I "thought" GUE tech1 deco was being taught. A little O2 window (the 5 @ 70) then linear up to 20, then reversed 20-10ft stops from deco planner.

When I did the course, we were shown s-shaped and linear, where the latter was same time across all five stops (with caveat that you always do at least 3 min at the gas switch). Seems definition of linear has changed (i.e. do loads of time at gas switch, then linear).

 

[Sloth]

I think I covered the GUE T1 standard in the other thread. What we were taught was to hand off the reg every 10' from 70-30' (1.5 x normal deco time) and then do 1.5 time at 20' and 10' (i.e. hand off the reg after each has done half that time).



There's more N2 in 50% than 18/45...

haha good point. I wish i did a better job of taking notes when we had side bar discussions in my tech 1 class

 

[ScubaInChicago]

Streetdoctor's profile has more "O2 window" emphasis than I've ever seen.

When has Streetdoctor's made any comment in this thread, or even in the new tek threads, other than to say thanks?

That's twice

 

[rjack321]

When has Streetdoctor's made any comment in this thread, or even in the new tek threads, other than to say thanks?

That's twice

The Chicago Street, that's it yeah

 

[TSandM]

Man, I thought I had a firm toehold on this stuff, and you guys are confusing the dickens out of me.

EAN50 is only "used for" the 70-30ft segment - Tech1 level EAN50 dives are actually the exception.

Don't you stay on 50% all the way to the surface if you're doing a 21/35 Tech 1 dive? Or am I misunderstanding this statement?

I thought the idea was to add enough time to the gas switch depth to allow one complete circulation time (which is said to be 2 minutes, although what I know about circulatory mechanics would suggest that is probably longer than necessary). We didn't talk about adding any more than that, either in the Rec Triox classroom work or in my UTD T1 class.

As far as lost deco gas goes, it puzzled me that we could do the time deep on the 50% and then go back to a normal schedule on backgas. Maybe it's because the deco we're doing is already pretty well padded? It's been interesting to me, running V-planner on +3 on my Liquivision for kicks and giggles, to see that even when I fail to remember to gas switch the computer, I clear the deco obligation before the end of the dive. So V-planner thinks we could lose our deco gas and just do pretty much our schedule on backgas and be okay . . .