Ground-up DIR divers and preparation for the real world

[Blackwood]

Could you share that with me!? 60 seconds? I was trying to catch on to rock bottom by reading something I found and... well in 60 seconds I had just gotten into some equations and... perhaps what you could share might get me started. I normally dive a steel 80.

Rock bottom depends only on depth and gas consumption (and time once you enter stage decompression territory).

If you make some conservative estimates (1.0CFM/atm per diver is common), you can establish rock bottom for each 10' depth increment, and you need not re-figure the calculation each dive. Let's say it's 30CF for the dive you're planning. No matter what cylinder you or your buddy have, the ascent starts when either one of you hits 30CF.

The 60-second planning Lynne mentioned is probably something like the following:

"What is the appropriate gas rule?"

• All
• Halves
• Thirds
• Sixths
• something else

That distinction depends on the environment and where you want to begin your ascent.

"Given the gas rule, what's the turn pressure?"

Let's assume we're beach diving. I go out, and I want to come back, but it's not really crucial that I ascend where I descend. That means I can use halves (half of my usable gas going out, half of my usable gas coming back, rock bottom as the remainder.

For your cylinder, you have 50CF usable (80CF total - 30CF rock bottom = 50CF usable). That's 25CF out and 25CF back. So you turn when you hit 55CF (80CF total - 25CF out = 55CF turn). Knowing the relationship between pressure and volume in your tank, you can easily figure out the associated turn pressure.

It may seem overwhelming the first time you do it, but it's really very simple and can easily be discussed in a minute for a familiar dive site and profile.

 

[TSandM]

Thanks, everybody, for helping out before I could get back here to answer the question!

As far as doing the calculations in 60 seconds, I have memorized a couple of data points -- RB from 60 (20 cf) and 100 (40 cf). I rarely go below 60 with somebody I don't know, and I don't go below 100 without helium in my mix (and then I'm not calculating RB for anybody else). So if you have an HP 80 which holds 80 cf at 3442, and we have a 20 cf RB, that's a quarter of your gas, or about 850 psi (round up to 900). Then I do the all available/halves/thirds thing (again, I'm not doing a thirds dive with somebody I don't know). All easy mental arithmetic, and takes about 60 seconds. The explanation may take just a little longer, if the person has never been introduced to a gas reserve concept before.

 

[Peter Guy]

Could you share that with me!? 60 seconds? I was trying to catch on to rock bottom by reading something I found and... well in 60 seconds I had just gotten into some equations and... perhaps what you could share might get me started. I normally dive a steel 80.

In addition to Lamont's and Bob's articles, I'm posted a "quick and easy cheat sheet" that may help:

Air Supply Management Cheat Sheet