Last time we talked about surface air consumption rate and how to calculate it using an Aluminum 80 cylinder. But what if you change to a different sized cylinder? This time, I”ll show you how to convert your Surface Air Consumption Rate to Respiratory Minute Volume so you can calculate bottom time with different sized cylinders.

Let’s say a new buddy wants to go diving with you. You agree and he says, “don’t worry about getting tanks. I have some already filled.”

You like this guy already. Saves you a few bucks for a fill because he has extra tanks. You agree to meet him at the dive site for a beach dive the next day.

The next morning, you pack your equipment and drive to the coast for a couple beach dives. You pull up and your buddy is already unloading the equipment. “Help me with these tanks,” he says. You look lean over the trunk to grab one but something doesn’t look right. These tanks look a little smaller and they’re heavier than what you’re used to. Not wanting to look stupid, you state the obvious, “these aren’t Aluminum 80’s.”

“Nope, they’re not,” you buddy replies. “They’re steel 72’s.”

What do you do now? You have a tank that’s smaller than the Aluminum 80 you calculated your SAC with. No problem if you convert your SAC to Respiratory Minute Volume or RMV.

You may be wondering if that extra 8 cubic feet of air you’re missing is really worth worrying about. If you plan the dive using the same PSI rules you use with an 80 cf tank, your dive won’t be a problem, but the safety stop is where you may run in to trouble with a smaller tank. Each pound of pressure has less cf of gas in it.

One other thing, many scuba cylinders do not hold as much air as they are rated at. For example, an Aluminum 80 actually holds only 77 cubic feet. And that steel 72 you’re going to use only holds 71.4 cubic feet if it’s less than 5 years old. This is because you can overfill a steel cylinder by 5% of it’s rated pressure if it’s a new tank. When you get the second hydrostatic test done, that steel 72 that used to hold 71.4 cf now only has 63 cf in it.

So, get out your calculator, we’re going to do a little math.

The Math

Last time we figured out our SAC using an Aluminum 80 to be 25.64 PSI or 1.77 bar per minute. Since tanks are usually marked in cubic feet and not liters, we’ll be using imperial for these calculations. If you figured your SAC using metric, convert it to imperial by multiplying you SAC by 14.5.

1.77 X 14.5 = 25.665 PSI per minute.

Right now we have to figure out how many PSI are in each cubic foot of that tank. For your aluminum 80 rated to 3,000 psi:

77 cubic feet/3,000 psi = .0256 cubic feet in every psi in an aluminum 80.

Now multiply this number by your SCR:

25.64 x .0256 = .66 cubic feet per minute.

Your surface RMV is .66 cubic feet per minute.

Let’s say your beach dive will take you to a maximum depth of 40 fsw. Now you can use the RMV to calculate the duration of the dive using the steel 72 with 63 cubic feet of air. Again, plan backwards. Calculate the gas you will use during your safety stop like so:

(.66 RMV X 1.45 ATA) X 3minutes = 2.87 cubic feet of air consumed during your safety stop.

Before we go any further, I want to point out that this is a relatively shallow beach dive. Therefore, I’ll make an educated guess as to how much air to have as a reserve in the tank when you surface by adding the amount of gas we’ll consume during the safety stop rounding off to the nearest cubic foot; in this case 3cf. There is another way to use RMV which is much better. I’ll show you below.

Add the reserve gas to the gas for your safety stop and subtract it from the maximum capacity of the tank:

63 – (3cf + 2.87cf) = 57.13 cubic feet of gas remaining for your dive.

I said this dive would take you to a maximum 40 fsw. To find the total bottom time you could get from this dive divide the remaining gas in the tank by your RMV at 40 fsw:

57.13 / (.66 X 2.21 ATA) = 39.16 minutes of bottom time with a steel 72 containing 63 cf of gas.

Not bad for a little tank. To compare it to an aluminum 80:

(.66 RMV X 1.45 ATA) X 3 minutes = 2.87 cubic feet of air consumed during your safety stop.
77 – (3cf + 2.87cf) = 71.13 cubic feet of gas remaining for your dive.
71.13 / (.66 X 2.21 ATA) = 48.88 minutes of bottom time with an aluminum 80.

Advanced Dive Planning

As I said above, there is one place where RMV really out performs the SAC for dive planning. This is when you use the rule of thirds. Using this rule is a requirement anytime you go into an overhead environment such as wrecks or caves. You should also use it for dives deeper than 100fsw / 30 msw. Tec divers use it for every dive they do.

So what is this rule of thirds? In a nutshell, you divide the capacity of your tank by three. You use one third for travel (this is for both your ascent and descent as well as your safety stop), one third for exploration and one third in reserve for emergencies.

You may remember when I showed you how to calculate your SAC, our final dive was to 100 fsw with an aluminum 80. We planned that dive to surface with 300 PSI. Lets what size tank we would need to make the same dive for 20 minutes of bottom time using the rule of thirds:

(.66 RMV X 1.45 ATA) X 3 minutes = 2.87 cubic feet of air consumed during your safety stop.
(.66 RMV X 4.03 ATA @ 100fsw) X 20 minutes = 53.2 cubic feet consumed at depth

As you can see, you’ll consume way more gas on the bottom that you would traveling. So to comply with the rule of thirds, you would have to carry 159.6 cubic feet of gas to make this dive. A set of doubles using steel 98’s would do it, or you could have a single 120 cf tank on your back and a 40 cf stage bottle. Both of these configurations would require advanced diving skills taught in the SSI Advanced Nitrox Certification. They are presented here for demonstration purposes only. Contact us if you’re interested in this course.

Now you know all the ins and outs of dive planning with SAC and RMV. You can plan bottom times with any size cylinder. If you still need help, post your questions or comments below. I’ll respond in turn.