Air Consumption?

[narced_1]

I am thinking about getting into tech...GUE. IANTD, TDI...who ever...it doesn't really matter at this point (let's see who argues about the "best" agency)

the really question is what kind of air consumption rate should one be expected to have for tech diving. 20 psi sac....10, 5? also, are there any techniques for getting SAC lower? what are they?

don't just say get in good shape and stay that way...the should be a given, this is a physically demanding activity. I am talking about things while actually diving that will reduce air consumtion. If the numbers come back and i need to. right now i can easily do 20 PSI SAC no problem....12 if i try and about 3-6 at rest....Thanks for any reply that this may get


Narced_1

 

[Uncle Pug]

Originally posted by narced_1
the really question is what kind of air consumption rate should one be expected to have for tech diving. 20 psi sac....10, 5? also, are there any techniques for getting SAC lower? what are they?

Narced_1 you don't say what size tank you are using....
GUE would probably want you to rate your SAC in cf/m....
It is common to think of a resting SAC of .5 and working SAC of .7 as a good starting point.....
Depending on what size tank you are using you can then convert that to psi/m....
No matter how good your SAC is now....
I can guarantee that if you take a GUE course your SAC will be somewhere around 1~2 when Mr. Murphy & Mr. Darwin start having their way with you...

 

[narced_1]

Uncle pug

those rates are with an aluminum 80....3000 PSI...what formula would i use to get that .5-.7 rate? how does the math work? what exactly is the .5-.7 rate. thank you very much.


Narced_1

 

[roakey]

Surface Air Consumption:

SAC = CFM / ((depth / 33) + 1)

Cubic feet per minute:

CFM = CFU / minutes

Cubic feet used:

CFU = cylinder cubic feet * (start PSI – end PSI) / “working” max PSI

So for an example, let’s say with your AL80 full you moderately motor around at 50 feet for 10 minutes and you’re down to 2300 psi.

First, figure out how many cubic feet you used: 80 * (3000 – 2300) / 3000 = 19cf

Now how many CFM: 19 / 10 = 1.9

Now for your SAC you need to “back out” the pressure you’re under: 1.9 / ((50 / 33) + 1) = .76 SAC

This is how many cubic feet of air per minute you would breathe at the surface. So now you can calculate how long a cylinder will last at any depth. Say you’re going to 99 feet. First calculate how many ATMs that is: (99/33)+1 = 4, multiply by your SAC: 4 * .76 = 3 CFM. Then 80 CF /3 CFM = 26 minutes. UNDER OPTIMAL CONDITIONS (And also you’d drain your cylinder, not a good idea).

The reason you measure SAC in CF/min @ 1ATM is that as you get into technical diving the AL80 is about the least used cylinder for back gas and having buddies with dissimilar cylinders is quite common (both pressure AND volume), so you can’t use PSI/MIN as a measurement.

As long as we’re talking about dissimilar cylinders, note that if someone’s a hoover it may make sense to get a larger cylinder for recreational diving, but it doesn’t for technical diving (in this case I’m referring to an overhead and you’re using the rule of thirds).

If Uncle Pug and I go cave diving and UP has a SAC of .5 and I have a SAC of .75, but I have 99 cf cylinder and UP has a 66 (I’m making up the sizes to keep things simple) we’re good to go, right? After all, I breathe 50% faster than he does but my cylinder holds 50% more gas!

Watch what happens if we’re following the rule of thirds: We head into Telford and we both hit thirds at the same time, which means I’ve breathed 33 CF of gas and UP has breathed 22 CF of gas. If I suffer a catastrophic gas loss at this point, the two of us need 22+33 = 55 CF of gas to get us out, but UP only has 44 CF of gas left. We’re both dead, unless he can swim faster then me, and then I’m dead .

I don’t like either option.

You base the rule of thirds on using 1/3 of the smallest amount of gas that ANYONE’s carrying and then figure out how many PSI that corresponds to in the other cylinders.

So if UP and I want to live in such an emergency, we look at the smallest amount of gas, which is his at 66 CF and decide to turn when 22 CF of gas is used. Now I have to figure out how many PSI 22 CF corresponds to in my 2250 PSI working pressure 99.

First I figure out how many PSI that 22 cf is: 22/99*2250 = 500 psi. Then I subtract that from my starting pressure: 2250-500=1750. So at 1750 I turn the dive, and there will be enough gas in UPs cylinder to get us both out if my gas supply fails.

So what does all this teach you? You should strive for everyone in a team to have identical cylinders to make things easy, that’s what!

Roak

 

[Uncle Pug]

Originally posted by roakey
Surface Air Consumption:

<<snipped & saved to good stuff>>

So what does all this teach you? You should strive for everyone in a team to have identical cylinders to make things easy, that’s what!

Roak

Thanks Roaky... the only thing I hates worse than the math is the splainin' of it.... (great job btw)


And the moral of the story is as Roaky sez...standardize!

The reason we figure SAC in cf/m is because you will be dealing with different size cylinders even if you standardize. Shane and I have 104s for back gas but we use 40s for deco and then there is the 80 stage.

Once you have your SAC figured you must then convert it back to psi/m for your tanks because that is what your SPG reads. This might seem like a convoluted way of doing things but it is really not that bad and allows you to manage different sized cylinders.

But you say, "Hey all I ever dive is my AL80." Fine, then you don't have to refigure everytime... you know what your psi/m is for that tank. Just don't forget how to figure the cf/m because if you ever switch tank size you will need to refigure the psi/m from your cf/m SAC.

Oh... and when you give your SAC.... do it as cf/m and everyone will think you are a caver or tech diver.

 

[narced_1]

Thank you for the great explanations....by my calculations i am at about .4 SAC which according to UP is not too bad....Thanks again for the good explanation of the math.....it will be put to good use.


Narced_1

 

[Divesherpa]

Do you guys breath while you are down there?

I guess I'm doin' something wrong!

 

[narced_1]

even better yet uncle pug....if i do it in kg or bar....everyone will think i am european


Narced_1

 

[miked]

Roakey,
Great explanation!
It was clear, concise, and understandable!!
You really do like the science/math end of this activity. Bravo!
I've run into too many divers that are at the opposite end of that spectrum. (e.g are diving nitrox 32 and can't determine the % of nitrogen- no joke- sadly)
Again-well done!!
Mike

 

[Blargh]

Originally posted by narced_1
even better yet uncle pug....if i do it in kg or bar....everyone will think i am european


Narced_1

*cough* litres *cough*