Gas Math

cocoajoe · Dec 25, 2013

Taking IANTD Deep Diver course in preparation for Advanced Nitrox and Deco procedures. To me, manual is not very well written and leaves a lot to be desired. Anyway, instructor sent me a question(s) that I can't answer.

Q: Dive plan is 32% for 20 minutes. Don't exceed 1.4 PPO2. How deep can I go?
RMV is .7 and I want to know how much gas I will need.
Use 1/3s, 1/2 plus 200 or rock bottom

I know how to calculate SAC, RMV, find ATA, MOD, rock bottom rule of thumb, IANTD 1/2 +200, 1/3s but I don't know how to answer this. I found 111' as MOD for 32% by just trying several depths on my calculator but this is like hunt and peck and doesn't allow for exposure (20 minutes)

Help please

Superlyte27 · Dec 25, 2013

You can go 112' at 1.4
If your sac is .7 at the surface, at 112' you are at 4.39ata. .7 x 4.39 = 3.08cu'/min. X 20 minutes means you need 61.5cu'. If you are diving 1/3'rds you need to multiply that times 1.5 = 92.27cu' of gas.

rongoodman · Dec 25, 2013

1.4/.32=4.38ATA (4.38-1)*33=111'
4.38*.7=3.07 ft3/min*20min=61.5 ft3(actually a little less, since you're going to take 2 min down and at least 4 min up, plus your safety stops, so you're not spending the whole 20 min @ 111'.)
If you call Rock Bottom 35 ft3, you're going to need at least 95 ft3 tanks to do the dive.

(RB here assumes 1 min to sort things out at the bottom, 2 stressed divers with an RMV of 1 ft3/min, 4 min ascent with average 2.2 ATA, and a 3 min stop at 15'.)

You're coming close to NDL limits here too, so a delay at the bottom is not only going to use more gas there, but could put you into deco, sending your RB requirements sharply higher.

rx7diver · Dec 26, 2013

cocoajoe:
Q: Dive plan is 32% for 20 minutes. Don't exceed 1.4 PPO2. How deep can I go?
RMV is .7 and I want to know how much gas I will need.
Use 1/3s, 1/2 plus 200 or rock bottom

Cocoajoe,

Your sets of answers will be different depending on whether the dive will be in fresh water or salt water. Did your instructor indicate which? Also, did he/she specify descent and ascent rates?

Safe Diving,

rx7diver

Diver0001 · Dec 26, 2013

cocoajoe:
I found 111' as MOD for 32% by just trying several depths on my calculator but this is like hunt and peck and doesn't allow for exposure (20 minutes)

No need for hunt-and-peck. Here's how you do it:

1) For the MOD you first work it out in ATA's and then convert it to depth, which will depend on if you are in fresh or salt water. We'll work with fsw (feet of salt water)

2) To start, you have to realize that 32% is an oxygen content (pprO2) of 0.32 on the surface. your MOD is 1.4 so to find out how many ATA's you're allowed, you just divide 1.4 by 0.32 (= 4.375).

3) now you convert that to depth. Converting ATA to fsw is done with this forumula: depth = (ATA * 33) - 33 so your MOD = (4.375 * 33) - 33, which equals 111fsw once you round it off.

If you're working it out for fresh water then use 34 instead of 33. That's it.

Make any sense?

R..

MXGratefulDiver · Dec 26, 2013

... all the info you need to answer the questions can be found here ...

I found the IANTD Deep Diving gas management (Chapter 8) to be a bit wonky and overly complicated ... but I give them credit for at least having that topic in their book. Some of the Deep Diver material I looked at didn't even mention it, which is why I ended up writing my own.

... Bob (Grateful Diver)

kwinter · Dec 26, 2013

Superlyte27:
You can go 112' at 1.4

As an instructor, you should know better. In a class setting, it is all about the strict textbook answer. The question clearly said "don't exceed 1.4". 112 FSW would indeed be 4.39 ATA, giving a pO2 of 1.406. Fine for a real world answer, but incorrect in a class setting. You can't round, you must truncate. 111 FSW max to avoid exceeding 1.4 max pO2.

Gas Math

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