Gear config, Long primary

[novadiver]

Kinda off topic, but let's do the math....

Figure a stressed breathing rate of 1 cft/min for 2 divers, given an OOA at 100ft....

Plan on at least a minute to handle the emergency at the bottom, then we go 30ft/min to our first deep stop at 70ft and then go 10ft/min from there to the surface. So, our total ascent time is at least:

1 @ 100ft
1 min to 70 ft
7 mins from 70-0 ft

9 minutes total ascent time.

The 1 minute at 100 ft will require 2 cft/min * 4 ATM = 8 cft
The 1 minute to 70 ft will require 2 cft/min * ave 3.5 ATM = 7 cft
The 7 minutes from 70-0 will require 2cft/min * ave of 2 ATM * 7 minutes = 28 cft

So, we require a rock bottom of 8 + 7 + 28 = 43 cft just to get off the bottom, which leaves us only 34 cft for the descent and bottom portion of the dive. That's not much at 100 ft, especially if you end up on a dive that requires thirds (like in a shipping channel where you must return to the anchor point).

Given a bottom sac of .6, that gives us basically 14 minutes at depth....not worth it to me when I've got an NDL of around 40 minutes.

I personally, wouldn't go below 80 ft or so on a single 80, especially in cold water.

Ndl of 40 min at 100 fsw ? what mix is that?

 

[detroit diver]

Ndl of 40 min at 100 fsw ? what mix is that?

32 PERCENT

 

[rjchandler]

14 min is a long time in 40F water.

 

[novadiver]

32 PERCENT

My plan shows your over the ndl by 10 min. at 1.29 ppo. can you post your calculations please?

 

[rjchandler]

100,40,32 on VPM-B gives 8+min of deco.

Of course we are getting way off topic now and that doesn't addresss your point that a dive to 100ft on an AL80 is going to be awfully short for some people.

 

[novadiver]

100,40,32 on VPM-B gives 8+min of deco.

Of course we are getting way off topic now and that doesn't addresss your point that a dive to 100ft on an AL80 is going to be awfully short for some people.

by pushing this dive into deco , gas management needs to change , and an al80 will not do. I hope your young and heathy because here comes the bubble shower!!!

 

[GreenDiverDown]

Yah,

I use different numbers than you do...but the same principals.

For one, I don't use a figure of 1cuft per diver from bottom to top. I use as a general rule - and this might change depending on my buddy's particulars - .6 working .4 resting...for me...and 1 working .6 resting for my OOA buddy. And I think that is conservative.

I don't agree that in an OOA situation that consumption rates should sky-rocket. Both divers should be trained and have enough experience that the situation should be handled relatively calmly. Instead of..."aye aye aye I'm out of air...puff puff puff...what do I do...puff puff puff..." it should be more like..."dang it...how'd I do that...dude I need your hose...ah...that's good...let's go." Been there ...done that...consumption rates didn't change.

My ascent time is less than what you figured, too. More like 7 minutes*...including all stops and an ascent rate of 30. This of course might be different depending on the dive.

I put all these numbers into my equations and round conservatively...I'm pretty comfortable with this as a starting point. In fact this is what I teach my students. I have made some excel tables, laminated them and I pass out in my AOW class.

I also like the rule of thumb that JimC mentioned...it's pretty much in the ballpark and an easy one to remember.


*edited to correct total ascent time

 

[rjchandler]

by pushing this dive into deco , gas management needs to change , and an al80 will not do. I hope your young and heathy because here comes the bubble shower!!!

No one was suggesting to do 40 min at 100ft. on an AL80. Soggy's point was that he beleved he wuold have 40 min available at that depth if he use 32%( i assume - that is the GUE standard) so why should he use a tank that limited him to a shorter time.

 

[cornfed]

Ndl of 40 min at 100 fsw ? what mix is that?

It's based on the "rule of 120", which GUE covers in their classes. It's a pretty good rule of thumb but after playing with if for a while I've found I need to tweak it a bit at the deeper depths to get something I'm comfortable with.

 

[cornfed]

I know thats not the way we determined RB in my DIR-F.

How was it presented in your class?