Perhaps one of the most difficult concepts for beginning divers to grasp is Surface Air Consumption Rate or SAC. Don’t feel bad, many seasoned divers also have difficulty with this. So, what is it and why do I need to know it?

Your SAC is the amount of air you breathe per minute, at the surface, performing moderate work. If you know your SAC, you’ll be able to determine how long a cylinder of gas will last at any depth. You can also determine if the cylinder size you’ve chosen will allow you to make the dive you’ve planned.

Finding your SAC

Finding your SAC is relatively easy. I’ll give both imperial and metric examples. These examples will assume you’re diving with the standard “does everything – not so well” aluminum 80 but any size tank for this exercise will do. Do this on a dive site where you can be sure you’ll be able to maintain a relatively constant depth.

Start by noting the pressure in your tank and depth. Check your time and continue with your dive. After ten minuted, note your pressure on your SPG. Now you know the amount of air in PSI or bar you used at a given depth in ten minutes. You’re done for now. We’ll figure out the actual SAC on the surface. Enjoy the rest of your dive. See you back on the surface.

The Math

The first thing we have to do is convert the depth of the dive to atmospheres absolute (ATA) of pressure. You should remember from your open water certification class that you add 1 atmosphere (atm) of pressure every 33 feet or 10 meters of depth in the ocean. Fresh water is a little less but that’s another class.

To find the ATA of your dive simply divide your depth by 33 if using imperial or 10 if using metric and add 1. Thus a dive to 70 feet/21 meters would look like this:

(70/33)+1=3.12 ATA

(21/10)+1=3.1 ATA

As you may have noticed, the metric system is a little easier to use. If math isn’t your forte, don’t change from imperial to metric just to make this process easier. Stick with what you know. You won’t have to do this calculation very often.

Next, find out how many PSI you used in the 10 minute test period. Subtract your ending PSI reading from your beginning PSI reading. If your beginning reading was 2800PSI/193 bar and your ending pressure was 2000 PSI/138 bar, then:

2800-2000=800

193-138=55

Now we know that we used 800 PSI/55 bar in 10 minutes at 70 feet/21 meters sea water. divide your result by 10 to get the amount of gas you used per minute.

800/10=80

55/10=5.5

So you used 80 PSI/5.5 bar per minute at 70 feet/21 meters sea water. Now we have to find out what that equates to on the surface. This is the Surface Air Consumption Rate after all.

The first part of this exercise we figured out how many ATA we had at depth. Now all we have to do is divide the pressure we used per minute by the ATA we were at and we’ll know the SAC.

80/3.12=25.64 PSI per minute

5.5/3.1=1.77 bar per minute

Voila!

What is it good for

Let’s say you were planning a dive to 100 FSW/30 MSW using the Aluminum 80 we used to find our SAC. (note: once you find your SAC with a particular cylinder, it only works with that size cylinder. You can convert your SAC to respitory movement volume or RMV but that’s another article. stick with whatever cylinder you used for now.)

You want to end the dive with at least 300 PSI/21 bar in your tank so subtract it from your starting pressure:

3000-300=2700 PSI

210-21=189 bar

Next, you have to subtract the air you’ll use at the safety stop. Find the ATA of 15 feet/5 meters then multiply your SAC. Multiply the product by 3 for the 3 minute safety stop. Finally, subtract the product from the difference above. Thus:

(15/33)+1=1.45 ATA
1.45×25.64=37.29 PSI per minute at 15 FSW
37.29×3=112 PSI for the entire safety stop
2800-112=2688 PSI remaining for your dive.

or

(5/10)+1=1.5 ATA
1.5×1.77=2.7 bar per minute at 5 MSW
2.7×3=8.1 bar for the entire safety stop
189-8.1=180.9 bar remaining for your dive

Now the final part. Our dive was for 100 FSW/30 MSW. Again, find the ATA for this depth, multiply by your SAC and divide it into the PSI od bar you have remaining from the calculation above. This will give you minutes of bottom time.

(100/33)+1=4
4×25.64=102.56 PSI per minute at 100FSW
2688/102.56= 26 minutes of bottom time at 100FSW

or

(30/10)+1=4
4×1.77=7.08 bar per minute at 30 MSW
180.9/7.08=26 minutes of bottom time at 30 MSW

Now, a quick check of the dive table shows that at 100 FSW/30 MSW you only have 20 minutes of bottom time. So, you’ll end the dive with more that 300 PSI/189 bar we had planned on.

Things to Remember

The actual bottom time we calculated above is still just an accurate guess. Other factors can affect your SAC and cause it to vary from dive to dive such as currents, cold, visibility and task loading. Always plan your dive well within the limits of your dive table. Use a computer to back up your table. As the old saying goes, “plan your dive and dive your plan.”

Next time, I’ll show you how to convert the SAC you found using an aluminum 80 to a different size cylinder. Until then, be safe and have fun.