Basic questions about air consumption

[jwalko]

I was reading another thread, and one of the posters was talking about air consumption. Got me thinking, and I came up with the following questions.

What does SAC mean? (I assume it has to do with air consumption)

How do you figure your own SAC?

How does your SAC relate to cubic feet of air consumed?

How do you use this information to estimate your turn around pressure?

I understand that turn pressure is related to depth, but assuming all dives are NDL dives, how much different should your turn pressure be at, say 100' compared to, say 50'? Assuming an ascent rate of 30'/min, it takes you about 2.5 minutes longer to ascend from 100' than from 50'...how do you relate SAC (or do you) to estimate your turn pressure?

I hope these questions make sense. I would like to understand this better.

Thanks;

John

 

[markfm]

Lamont and Roakey did some nice writeups a while back, I pulled them together -- PM sent with the link.

 

[rockjock3]

SAC stands for Surface Air Consumption. This is the rate that you breath air at the surface. It is usually measured in Liters per minute.

There are a few ways to figure your SAC rate. You can use calculations with your starting and ending tank pressures and the depth of your dive for the most common method.

Depending upon the calculation you use it will give you your SAC in either L/min or cubic feet/minute.

You use this information to estimate your turn around pressure because you probably know before the dive,
how deep you’re going,
how much air you have,
your SAC rate.

With these you can calculate how fast you will breath down your air (not including your planned reserve) now chop that in half and that would be your turn pressure.

Turn pressure is important for all diving, but is more a matter of life/death for more technical diving. In diving that you don’t have a direct ascent to the surface (i.e. decompression or cave diving). In normal types of rec diving a turn pressure would be how much air you need to ascend straight to the surface from where you are on at the bottom.

By the way, at a 30fpm ascent rate coming up from 50 feet only takes 1 minute 40 seconds and coming up from 100 feet only takes 1m40s more, not 2.5 minutes longer.


I know I have simplified a lot, but I hope this helps some.

 

[jwalko]

RockJock:

I realized after I posted I had the math wrong. Chaulk to up to trying to figure it in my head after a really, really long day.

Thanks for the answers, this helps me understand this much better.

 

[Meng_Tze]

SAC stands for Surface Air Consumption. This is the rate that you breath air at the surface. It is usually measured in Liters per minute.

There are a few ways to figure your SAC rate. You can use calculations with your starting and ending tank pressures and the depth of your dive for the most common method.

Depending upon the calculation you use it will give you your SAC in either L/min or cubic feet/minute.

You use this information to estimate your turn around pressure because you probably know before the dive,
how deep you’re going,
how much air you have,
your SAC rate.

With these you can calculate how fast you will breath down your air (not including your planned reserve) now chop that in half and that would be your turn pressure.

Turn pressure is important for all diving, but is more a matter of life/death for more technical diving. In diving that you don’t have a direct ascent to the surface (i.e. decompression or cave diving). In normal types of rec diving a turn pressure would be how much air you need to ascend straight to the surface from where you are on at the bottom.

By the way, at a 30fpm ascent rate coming up from 50 feet only takes 1 minute 40 seconds and coming up from 100 feet only takes 1m40s more, not 2.5 minutes longer.


I know I have simplified a lot, but I hope this helps some.

Small corrections RockJock,

Turn pressure is not 1/2 per se. Turn pressure is the pressure agreed to by a team of divers of when they (re)turn. Turn pressure can also be a pressure where the team decides that today we dont go further in one direction, and subsequently take another direction. Or if you want to do two dives on one tank, your turn pressure is calculated for the first dive and for the second dive.

Rule of 1/2's is when it is preferred to return to the point of entry. I.e return to the boat, or a place, but it would not be life threatening if you dont (open water)

Rule of 1/3's is for overhead environments where you have to be back at the point of entry and have gas for your buddy. Where it is a life and death situation if you dont return to the point of entry.

SAC is the breathing rate in l/min or cf/min. Not to be condused with DCR (depth consumption rate, or working rate). SAC is a rate at the surface. This rate is then translated to depth (say 4 ATA), so in this case the DCR is 4XSCR ...... this then results in a time you have before the tank is empty. Subtract what you want to have at the end of the dive (rock bottom) and the apply turn pressure.....

 

[Don Burke]

Here are some formulas that are handy:

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

 

[dive_lover88]

To calculate SAC, for the depth(ATA) we calculate is to the max we have gone to?

 

[The Kraken]

The SAC rate, with respect to diving, should be calculated on the average depth of the dive.

the K

 

[Diveral]

I have a Surface Consumption Rate Calculator in an Excel Spreadsheet and a Procedure in PDF. I tried attaching them but the attachments are not functioning. PM me with your e-mail address and I will send them to you.

AL

 

[H2Andy]

What does SAC mean? (I assume it has to do with air consumption)

surface air consumption. usually expressed in cubic feet per minute, this is the
rate at which you use gas at the surface. average is about .6 cfm (that is,
6/10ths of a cubic foot per minute)

How do you figure your own SAC?

easiest way is to do a dive with a computer than can track your average depth.

note how much gas you started with (say 3000 psi) and how much gas you ended
with (say 1400 psi). then note your average depth, say 40 feet.

so you used 1600 psi at an average depth of 40 feet.

your depth consumption rate (DCR) is 1600/40 or 40 psi/min

SAC = (DCR x 33) / (Depth + 33)

in this case (40 x 33) / (40 + 33) or 18.08 psi / min

How does your SAC relate to cubic feet of air consumed?

excellent question! that's what comes next. you now need to convert your
SAC in psi/min to SAC in cubic foot/ minute (cfm)

you will need to know your tank volume at rated pressure. let's assume an
aluminum 80 with a rated volume of 72 cubic feet at 3,000 psi.

first, we find cubic feet per psi, or 72/3,000 = .024

using your SAC of 18.08 psi/min, let's convert to cfm by multiplying
the psi used by .024 to find out cfm = .43 cfm, a rather nice SAC.

that means that you are using .43 cfm at the surface

How do you use this information to estimate your turn around pressure?

exactly... you're thinking along the right lines here

now that we know your surface air compsumtion, we can predict how much air
you will use at any depth by using the following formula:

SAC x ATM = gas needed in cubic feet per minute at that depth

ATM = (depth/water type) + 1, where fresh water = 34 and salt water = 33

so, to find ATM at 70 feet of salt water (fsw), you would do as follows: (70/33) + 1

or 3.12

and we plug that in so .43 x 3.12 = 1.34 cfm. thus, you will use 1.34 cfm at 70 feet.

a 72 cubic foot tank will last you 53 minutes at 70 feet.

now, i'll stop here with the SAC concept, before moving on to gas planning, which
is a whole new subject.