PDA

View Full Version : Playing with your SAC



MSilvia
12-16-2007, 23:29
After getting a pm about the SAC calculations I made in another thread, I got to thinking that it's a very handy thing to be able to calculate, and I thought I'd list some of the ways I've used it, and invite others to do the same in the hope that we might all pick up some new ideas.
Tracking my workout's impact on my gas consumption
Planning my dive so that I know when I have to end it in order to ensure I have enough gas to surface while sharing air with a buddy under stress
Planning how much gas I need to bring to ensure a dive can last for a given amount of time
Figuring out how much gas I need to carry to meet a deco obligation
Determining the adequacy of "Spare Air" for bailout from a given depth

MSilvia
12-16-2007, 23:29
It also occurred to me that SAC calculations are not something that everyone knows how to do. In the pm, I was asked if I had a formula for that, and I thought I'd share my response here for the edification of those who are interested, and the scrutiny of those who might be able to correct the mistakes to which I know I'm sometimes prone.


Yes, the formula is "air consumption in cubic feet of gas per second divided by absolute atmospheres of pressure exerted, or to put it differently, "the product of (tank volume/service pressure) multiplied by (change in psi/minutes) divided by the sum of (depth/33 or 34)+1". I think it's much more useful to understand the calculation than to memorize the formula, so I'll walk you through it quickly.

SAC, or "Surface Air Consumption", is a measure of how much gas you breathe per minute at sea level, expressed in cubic feet. It gives us a standard by which to compare breathing rates, and you can use that information in all sorts of interesting ways.

Since you know how many psi are consumed per minute, at what depth, and out of what kind of tank, we can figure it out. I'll walk you through it using Shaggy's fresh water fun dive: "At 40ft used 31.1 psi per minute out of AL80."

First, to figure out how many cubic feet of gas is represented by a given number of PSI, we need to look at the tank and determine it's capacity and service pressure. Most tanks are true to their names, so you can expect (for example) a steel 120 to hold 120 cubic feet of gas at it's rated service pressure, which is stamped on the neck of the tank. A typical "aluminium 80" is an exception to the rule in that it is not actually rated for 80 cubic feet, but rather 77.4 at 3000 psi. If we divide 77.4 by 3000, we can find out how much gas each pound per square inch represents. I get .0258 cubic feet per psi. Shaggy used 31.1 psi per minute. Multiply .0258*31.1 and we'll discover that he used .802 cubic feet of gas per minute... at 40 feet.

The depth is important because of the properties described by Boyle's Law, which says that for a fixed volume of gas kept at a fixed temperature, pressure and density are proportional. In other words, the more pressure is exerted on you from the combined depths of the atmosphere and water above you, the greater the volume of gas you suck out of the tank with each lungfull.

Not surprisingly, the Earth's atmosphere exerts 1 ATM (ATM stands for "ATMosphere") of pressure. That's the pressure at the surface, with no water pressure. Water exerts roughly 1 ATM per 34 feet of depth, or 33 for salt water, and Shaggy was at 40 feet. If we divide 40 by 34, we'll see that the water was exerting 1.18 ATM of pressure on him. Add one for the atmosphere, and the ATmosphere Absolute (ATA) is 2.18.

He used .802 cubic feet of gas per minute under 2.18 ATM of pressure. All you need to do is divide .802 by 2.18, and you'll see that he consumed 0.37 cubic feet of gas per minute at sea level. In other words, he has a SAC rate of .37.

texdiveguy
12-16-2007, 23:39
And for those of you which might not like doing math and so shying away from messing around in your spare time calculating your SAC rates and applying the info. to your dive planning, here is a link that might make it a bit fun:
Gas Consumption (http://www.spearfishing.org/bruces_tips/java/gc.html)

Daved
12-17-2007, 22:17
I'm sure glad you guys cleared that up for me--I'll sleep alot easier tonight.