NACD Intro - 58cf Limit - Why?

Jax · Mar 18, 2012

Okay, so maybe I suck severely at the search function, but I cannot find out why . .

" . . .must begin the dive with an air supply of at least 58cf."

58???

From where came that odd number?

It seems incredibly low for someone that can go out on the main line.

Everything else makes sense . . . but 58 cu ft . . . :idk:

lv2dive · Mar 18, 2012

Hey Jax... just to run the numbers bc I know you love those numbers <G>

58 cubic ft on double aluminum 80's would be about 1160 psi.... meaning, depending on exactly how close one was to 58 cu ft and or how one's SPG was reading, diving sixths on 1160 means that the dive would either be a 100 or 200 psi penetration.

Why would you want to? Depending on the cave, especially in MX - you could get some time out of 1-200 psi! Especially if the line is already in. It's a lot easier to stay in the water for that second dive on sixths with 104's/130's, I'll grant you that.

I don't know WHY NACD came up with that exact cubic foot amount, but that is an explanation for why some may consider doing a dive with that amount of gas.

Jax · Mar 18, 2012

Thanks, Kate . . . I figured all that. . .

I'm just trying to figure out why they said 58. Why not 60?

And 58 seems pretty darned low! (That's for Cavern and Intro, BTW) So, why so low???

Just doesn't pass my common sense filter. I would think a limit more like 90cf. . . .

Dive-aholic · Mar 19, 2012

A steel 72 has a tank factor of 2.9 which means 2.9 cubic feet per 100 psi. 2.9 x 20 psi = 58 cubic feet. The standards were written when 72s were the most common cylinders.

jago · Mar 19, 2012

But why were there 2.9 - 58 cf (~8.2 - 164 L) tanks ? Usually water-volume or air volume is a multiple of 12 in metric or funny unit.

Jax · Mar 19, 2012

Dive-aholic:
A steel 72 has a tank factor of 2.9 which means 2.9 cubic feet per 100 psi. 2.9 x 2000 psi = 58 cubic feet. The standards were written when 72s were the most common cylinders.

jago:
But why were there 2.9 - 58 cf (~8.2 - 164 L) tanks ? Usually water-volume or air volume is a multiple of 12 in metric or funny unit.

Jago, I think you misunderstood. What Dive-aholic is saying is that the steel 72's at 2000 psi is 58 cu feet.

So, you don't go for a dive without a minimum of 2000 psi in your 72cf tank, and you are limited to use one third of that tank, or 660 psi, for the entrance, and the rest for the exit.

I think he meant to type, 2.9 x 20 = 58cf. Otherwise, it would have been .029 * 2000psi = 58cf.

cavemn · Mar 19, 2012

Jax:
From where came that odd number?

Maybe you could actually take a cave class and learn the limits.

jago · Mar 19, 2012

I didn't understood. I'm just asking why there isn't any volume ( water capacity or 2000psi air capacity) which would be a multiple of ten, in metric or imperail units.
You use S40, S80, not S82 or S39.

VooDooGasMan · Mar 19, 2012

I guess I will give dive aholic a thanks and a likes for answering the ? before it goes to page 2.

Displaying the math right or wrong, 2000 psi for a single small tank is reasonable for a cave dive to be done.

The reason I say this is look at how many cave divers die who carry a whole lot of CF, yes even over the weekend, I would beg to say it might be safer to cave dive with a single tank as you can only go so far.

Jax · Mar 19, 2012

jago:
I didn't understood. I'm just asking why there isn't any volume ( water capacity or 2000psi air capacity) which would be a multiple of ten, in metric or imperail units.
You use S40, S80, not S82 or S39.

Oh, dear Jago . . .

You were so right calling them "funny units". If you look at this tank specification page, you will see that we use a LOT of odd numbered capacities.

We also use s6, s13, LP72s, LP104s, 119s . . . . Nothing as nice as metric.

Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

NACD Intro - 58cf Limit - Why?

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