meaning of SAC?

[Biodiversity_guy]

"They are nice tanks if you can get your SAC low enough "

In the context of aluminum vs steel, what does SAC mean?

thanks

 

[H2Andy]

sac = surface air consumption.

it measures the amount of air you use, normally in cubic feet per minute, at the surface. you can then multiply by the atmospheres of depth you'll be diving and
figure out how much gas you'll need for a certain depth.

the lower the SAC, the less gas you use

the less gas you need, the smaller and lighter tank you need to haul with you.

people with good SAC rates could, in theory, do a dive on a steel 72 that somone
with a horrible SAC would need a steel 104 to do, for example (extreme example, just to illustrate)

aluminum tanks generally peak at 80 cf... so if you have a good SAC, you won't
need anything bigger. if you have a poor SAC, you may want to consider
moving to a steel 95, for example (although luxfer and catalina make 92
and 100 cf aluminum tanks ... but then you have to juggle weight)

 

[RICoder]

What H2Andy said... :11ztongue

Now all you need is the T-Shirt that says "Stop asking me about my SAC"

 

[bradshsi]

Andy pretty much covered it, but since I recognize my own statement from the other thread I guess it merits a quick explanation.

The aforementioned quote was in relation to steel 72 tanks. I was contrasting them with AL 80s.

I like the buoyancy characterisitics of the 72s but you need a good SAC, otherwise your dive time is rather truncated relative to the larger capacity aluminium tank.

My own personal SAC isn't bad, but then again it isn't good either. I still hit rock bottom gas limits on my 72 before I hit NDL limits. Therefore I need to improve to make best use of the tank.

 

[aquaagua]

my tshirt would read Sad SAC

 

[Pyromaniac]

sac = surface air consumption.

it measures the amount of air you use, normally in cubic feet per minute, at the surface. you can then multiply by the atmospheres of depth you'll be diving and
figure out how much gas you'll need for a certain depth.

the lower the SAC, the less gas you use

So how do you get it / work it out?

Must be a very iffy figure - I'l bet my breathing rate varies tremendously from sitting here to getting up and going downstairs, so how and when is it measured?

 

[Daylonious]

So how do you get it / work it out?

Must be a very iffy figure - I'l bet my breathing rate varies tremendously from sitting here to getting up and going downstairs, so how and when is it measured?

There's probably someone who can describe this better, BUT:

You get your SAC rate by swimming a known distance, at say, 40 feet or so. You take your guage pressure, swim as you normally would something like 50 meters, and that tells you how much air you consumed under "normal" conditions.

Borrowed from: http://home.flash.net/~table/gasses/sac.htm

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

Let's look at an example. Suppose you did a 50 foot dive for 25 minutes and used 1700 pounds of air. This would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formula we get:

SAC Rate = (68 x 33) / (50+33)

or: SAC Rate = 2244/88 or 25.5 pounds per minute.

We can then turn the equation around to determine our DCR for any depth.

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

Let's assume our SAC Rate is 25 and we want to know how fast will we use 2000 pounds of air at a depth of 75 feet.

Dropping our numbers into the equation we get: DCR = 25 x (75 + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81

This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this into the 2000 pounds, we see this amount of air would last 24.4 minutes.

-

Basically if you're a huffer, .8cf per minute is pretty bad.. really good fish type people are in the .35-.55cf/min range.

Hope that helps!

D.

 

[The Kraken]

Pyro,

Three things are required to determine one's SAC rate:
1: Volume of gas consumed
2: AVERAGE depth of dive expressed in ATA
3: Time of dive

In simple terms it can be expressed as (V/avg depth)/ATA.

And like Andy said, SAC rate is expressed in cu. ft or liters, not PSI.

PSI is a measure of force. However, if the volume of the tank, it's nominal working pressure, the starting pressure and the ending pressure, the volume of usage can be determined.

One should use a SAC rate to plan. It's not an index that one actively tries to achieve during a dive.

One of the best uses of a SAC rate is to determine a size of tank one would want to use for a given dive.

Case in point, if my wife and I do a dive on EAN36 that averages 65' I will use a HP 80 cf tank and she'll use a HP 119 cf tank. These tanks will give each of us a 57 minute bottom time based upon our personal SAC rates.

Hope some of this helps.

the K

 

[Biodiversity_guy]

Great and rapid response.

I figured it was a term I should know but just could not recall.

Excellent discussion and useful equations.

Wow, scuba board is an excellent resource!

Thanks all

 

[The Kraken]

You bet, Bio !!!
We hope you stay around for a while and become a valued member of the community.

The Kraken