PSI vs. Cuft

Making Sense of SAC Rates

by TDC Instructor, Jim Vafeas

Whatís your SAC rate? Pose this question to your average diver and the answers may surprise you. The facts are, most recreational divers donít calculate their Surface Air Consumption (SAC) rates once beyond their Open Water training (if then!). Knowing your individual SAC rate is a vital piece of information useful in dive planning, selecting what type of cylinder size works best for you, and helps you determine which dive buddies are most compatible with your breathing rate. How many times have you dived with someone new, and when they signaled, "low on air - letís surface," you looked at your gauge and discovered more than half a tank left!? Scenarios like this can easily be prevented by comparing SAC rates. As well trained divers, we should be recalculating our SAC rates frequently in cubic foot per minute (cuft/min) and the reasons and benefits, as you will see, are numerous.

Many divers fail to see the value of air consumption calculations but analogize it this way. If you were planning a road trip in which you needed enough fuel to get to where you were going and come back, wouldnít you want to know how far you could go? We know what our vehicleís MPG rating is and if we know our fuel tank capacity, we know how far we can go. Knowing our SAC rates as scuba divers is as, if not more, important as knowing a vehicles MPG rating as travelers. Now that I have made my case, how can we go about making sense of these SAC rate numbers?

Letís first analyze the way most of us learned to determine SAC rates. We dived at a certain depth for a certain amount of time, monitored how much air we used, then plugged it into a magical formula, or a SAC Rate Calculator, and got our SAC rate in psi per minute (psi/min.).

SAC Rate=

Total psi used
------------------
   Total time

ų

Depth in ATAís

So for instance, if we use 500 psi in 10 minutes, thatís 50 psi per minute at 33 feet or 2 ATAís. This translates into 25psi/min. at the surface. Now we know that our SAC rate is 25psi/min. The only problem is that psi is not a measure of volume, but a measure of pressure that is dependent on cylinder capacity and working pressure. For this reason, our 25psi/min SAC rate works only for the specific tank we used for the calculation. If this was an Aluminum 80 and we recalculated using a LP Steel 95, our SAC rate will be completely different. So when ever we use a different type of tank we have to recalculate our SAC rates. To get around this dilemma we have two choices: 1) Buy and always dive with your own tanks, or 2) Learn how to convert psi/min into cuft/min.

Since option 1 only works if you always carry your tanks with you, even on vacation, letís look at option number 2. In option 2, we are determining our SAC rate in cuft/min. Cubic feet is a measure of volume. We breathe cubic feet of air, not psi of air. The benefit of determining how many cubic feet of air we breathe per minute, is that we can carry this number over to any type of tank we use, regardless of size or capacity.

When we know how many psi/min we breathe, all we need to convert this information into cuft/min. is something called a tank factor. A tank factor is a measure of cubic feet per psi, or how many cubic feet equal each psi of air. This number is simply the tankís capacity, divided by itís working pressure.

Tank Factor =

  Tank capacity in cubic feet
--------------------------------------
Working pressure of tank in psi

The working pressure is the pressure that a tank is filled to in order to reach its rated capacity. For instance, using our trusty Aluminum 80 when filled to its working pressure of 3,000 psi, contains 80 cuft of air, while a Steel 72 contains 71.2 cuft. of air at its 2,400 psi working pressure. Now if you don't already know, you can easily determine your tanks working pressure.

Every cylinder approved by the DOT or CTC for scuba is required to have certain information stamped into the crown of the tank. Here you will find the tank's size and fill pressure either in a coded form on older tanks, or actual psi and cubic feet numbers on newer tanks. If you cannot determine these numbers yourself, ask the facility where you rented or purchased the tank from.

To determine the tank factor for the Aluminum 80, we divide 80 cuft by the 3,000 psi working pressure and get 0.0266 cuft/psi. This tells us that each psi in that Aluminum 80 represents 0.0266 cubic feet of air. If we use 25 psi/min., and multiply that by 0.0266 cuft/psi, we are left with a breathing rate of 0.665 cuft/min. (The psiís cancel out)

 25 psi
--------

  min

x

 0.0266 cuft
---------------
      psi

= 0.665 cuft / min

If we were using an Steel 72 instead, we divide 71.2 cuft by itís working pressure of 2,400 psi and get 0.0296 cuft/psi. So each psi in that Steel 72 represents 0.0296 cubic feet of air. If we use 25 psi/min., and multiply that by 0.0296 cuft/psi, we are left with a breathing rate of 0.740 cuft/min.

 25 psi
--------

  min

x

 0.0296 cuft
---------------
      psi

= 0.740 cuft / min

As you now can see, a SAC rate calculated in psi/min can yield very different results depending on the type of tank used. This should also make it obvious that an Aluminum 80 at 3,000 psi is not the same as a HP Steel 80 at 3,500 psi.

You may be wondering if there is a way to determine our SAC rates in cuft/min. without doing all these conversions. Unfortunately, since pressure gauges read psi, we must use that information first, then proceed with all the necessary conversions.

You now have all the knowledge and formulas to properly determine your SAC rate in cuft/min. monitor you SAC rates and notice how it varies during different dives and conditions. Feel proud to answer the "Whatís your SAC rate?" question with "half a cubic foot per minute during a relaxing dive and up to three-quarters of a cubic foot during a working dive." Sharing this information with your fellow divers and dive buddies will bring you pre-dive planning to another level. Good Luck and Safe Diving!

By the way, I have an average .46 cuft/min. SAC rate diving singles in warm climates, and a .83 cuft/min. SAC rate wreck diving off Long Island in a drysuit using doublesÖin case you were wondering.

Jim Vafeas has been diving since 1986 and is currently working in Long Island, New York as an Instructor for Tiedemann's Diving Center.   Jim welcomes all comments, questions and criticisms at his e-mail addresses jimv@tdconline.com.