stage tanks and OOA procedure

[limeyx]

Had a friend that did it once in freshwater in a non-critical situation just to see if it worked; it did.

I almost had to do it once for real in saltwater but luckily for me I got things sorted out just before I made the switch. That was the end of my "second stage should be hand-tight" experiment. A few weeks later JJ posted on Quest that "second stage hand-tight" was no longer recommended.

You mean something in DIR wasn't perfect and had to be changed? OMG I am going to DIE! maybe I am doing it all wrong now. I have to go & check my bible.

 

[limeyx]

That exactly describes what I was taught in my NAUI Advanced Nitrox class as well ...

... Bob (Grateful Diver)

This is for sure one way to do it, but it then requires you to carry a 7L or AL80 of 50% for any reasonable Tech1 dive.

usually we leave say +5 mins deco gas in an AL40 of 50% and reserve enough backgas to cover normal deco with the loss of one bottle.

 

[limeyx]

I am assuming that increasing the deco by a factor of 1.5 is only applicable to the loss of a 50% deco bottle? (and comes from the fact that 50% halves deco times compared to a typical backgas?)

Presumably if you lost an O2 bottle then 2 divers could alternate backgas and the one bottle of O2 if the deco time was doubled? (deco on O2 is about 1/3 deco on backgas)

You are saying that deco on 100% O2 alone is about 1/3 of that on 50% ?
So if I did a 150 for 30 mins, then on 50% I need 30 mins of deco from 70 feet, but with 100% I would only need 10 mins at 20 feet?

That sounds quite low to me. However, you do breathe the 100% much shallower than the 50% so an AL40 will go further, BUT since you dont have 2 second stages on the deco bottle, you still need to extend the deco times because while one breathes the 100%, the other is on backgas and so not decoing efficiently.

And it would be quite normal to use 100% O2 and no 50% for a deco dive in the 100foot range, where stopping at 70 feet doesn't do much for your offgassing.

 

[ianr33]

You are saying that deco on 100% O2 alone is about 1/3 of that on 50% ?

No.

I was saying that deco on O2 is about 1/3 the length of deco on a backgas such as (gasp!) air

 

[limeyx]

No.

I was saying that deco on O2 is about 1/3 the length of deco on a backgas such as (gasp!) air

I still don't really agree. From my understanding, you dont get to 1/2 the time (for 50%) and then 1/2 it again (for 100%) because you only use the 100% for 2 stops, not 5


Not that I treat decoplanner as my bible, but:

100 feet for 60 mins gives:

air deco: 68 mins
50% deco: 37 mins
100% deco: 36 mins

Now, the 100% deco case has a bunch of "air" deco from 70-30 feet (18 mins) where you cannot use the 100%. So if you used 50% and 100% deco (overkill) you would reduce total deco time to about 18+18/2 = 27 mins, which is closer to 1/3 of 68.

I dont have great experience in this area though.
Generally for 100% deco, I would use exponential deco from 65% ATA to 20 feet, and then do 1/2 the air deco on 100% (I think -- memory is a bit rusty here)

which is essentially what deco planner is telling me.

However, since you breathe the 100% at 20 and 10 feet, I can easily believe that the volume of the gas you need is pretty low (way way lower than 1/3 of the air deco) and this is a "good thing" because it allows you to reserve less back gas for emergencies (essentially moving your rock bottom depth deeper)

 

[Soggy]

This is for sure one way to do it, but it then requires you to carry a 7L or AL80 of 50% for any reasonable Tech1 dive.

usually we leave say +5 mins deco gas in an AL40 of 50% and reserve enough backgas to cover normal deco with the loss of one bottle.

Not really. I do 25-30 mins at 150' on a single al40 of 50% without violating 1.5x, but depending on the water temperature, you're right...back gas deco won't kill you.

 

[limeyx]

Not really. I do 25-30 mins at 150' on a single al40 of 50% without violating 1.5x, but depending on the water temperature, you're right...back gas deco won't kill you.

Then you have a way better SAC than me for deco (I am still new to this)
generally my rule of thumb is 1 min "usable" deco per 100 PSI (so 3000 gets me 30 mins with some reserve but not 1.5x)

Add to that that I usually get an exact 3000 psi fill that is down to about 2700 in the water.

I guess if you assume avg depth = 30 feet on deco, then

30 mins deco, 26 cft gas, 2 ATA average will give you a deco SAC of 0.4 ish which should be doable.

 

[Soggy]

Then you have a way better SAC than me for deco (I am still new to this)
generally my rule of thumb is 1 min "usable" deco per 100 PSI (so 3000 gets me 30 mins with some reserve but not 1.5x)

You're absolutely right.

That is the basic rule I use as well and my post dive analysis has shown me that (with a good fill, which is a good point) I can do a dive to 150' for 30 minutes and maintain my reserve +/- 10 ft and my time is +/- 5 minutes. If it's especially cold or I think conditions will on deco (as they often do), I'll cut another 5 minutes off my bottom time just to be safe.

Below 150' (or heck, even at 150') I tend to bring an O2 bottle now, though it's been a few months since I've done dives in that range. Most of our dives here are 90-120' or below 160'.

 

[rjack321]

Not really. I do 25-30 mins at 150' on a single al40 of 50% without violating 1.5x, but depending on the water temperature, you're right...back gas deco won't kill you.

I'm also saving 5 minutes of fudge in an AL40, so 30 mins max deco on the 50%. Water temp is typically in the low 50s, SAC = 0.4 or thereabouts

1.5x deep stops sharing gas (each does a 75% stop) then go to 2x time on backgas is how I'd approach this. This has been a good discussion. We did not cover the sharing issue very well in class, it was more along the lines of "just do double time."

 

[limeyx]

You're absolutely right.

That is the basic rule I use as well and my post dive analysis has shown me that (with a good fill, which is a good point) I can do a dive to 150' for 30 minutes and maintain my reserve +/- 10 ft and my time is +/- 5 minutes. If it's especially cold or I think conditions will on deco (as they often do), I'll cut another 5 minutes off my bottom time just to be safe.

Below 150' (or heck, even at 150') I tend to bring an O2 bottle now, though it's been a few months since I've done dives in that range. Most of our dives here are 90-120' or below 160'.

Righto. I am on for the NAUI Trimix1 class with AG next year which will give me the training for 2 deco gases, so this will all play into that.

Rjack: Yes, I agree with the better coverage of this needed (maybe the new T1 classes go over it)

In my opinion the t1 class needs to be 1 day longer with 2 deco lecture days.
The second deco lecture would cover contingencies and allow the students to form more questions.

On the "new" day, the diving would be the same as day 4 (100 feet triox) but with "simulated" 25 minutes of deco (i.e. do the deco even though you dont have the obligation).

the first time I really did that was on the experience dives and I think it would be useful to have practiced it a bit first.