Does Size Matter? Tank Size, That Is.

scuba tanksIs an 80 cubic-foot aluminum tank good for local diving below 60 feet?

NO.

Don’t take my word for it. It’s a simple numbers game. Let me show you.

What’s Local Diving?

First of all, let’s define local diving. Primarily local diving here in Canada is cold water diving. If you ask what cold water has got to do with size of the tank you need, read on.

While diving in warm water, we tend to have thin or no exposure suit, both of which require a light weight belt and create little drag. In cold water, we have thick wetsuits (7 mm) or drysuits which require more weight and create more drag. This increased drag will increase your air consumption rate, also known as SAC (surface air consumption) rate.

What Is Surface Air Consumption Rate?

While diving in warm water, your SAC (Surface Air Consumption) might be around 3.5 cubic feet per minute. In cold water, it can almost double to 0.6 or more cubic feet per minute.

SAC rate is a personal thing. It varies from diver to diver. And even from one dive to another for the same diver. It's affected by a whole bunch of things:

  • your size
  • your fitness level
  • your comfort level as a diver
  • whether you're smoker or not
  • the environment you are diving in.

Incidentally, you would be surprised to learn that smokers have been reported to have lower SAC rates than non-smokers. By no means am I promoting smoking here! :)

Anyway, before we choose a tank size, we should figure out our SAC rate. There are two ways you can use to determine your SAC rate:

  • Use an air integrated computer which will calculate your average breathing rate on every dive. Your breathing might vary from dive to dive so knowing your averages is important.
  • Alternatively, you can calculate your SAC rate by performing an underwater test. You want to swim for a period of time at a constant depth and record how much air in psi or bars you use. Once you have the required data - depth, time, used psi and size of your tank - you can easily figure out your air consumption rate.

How To Calculate Surface Air Consumption (SAC) Rate

Let's do an example. Say you swim at 33 feet for 10 minutes, and you used 400 psi. And sorry about all this math: the problem is that our pressure gauge give us the pressure in psi but SAC rates are calculated in cubic feet, so we have to convert all that psi into cubic feet. And our depth gauge gives us depth in feet but SAC rates (and all things diving) are calculated in atmospheres or bars of pressure, so we'll need to convert the depth from feet or meters to atmospheres absolute (ATA) or bar.

Step 1

Work out how much volume of air is represented by each psi in your tank. In North America the volume of the tank is given at its maximum working pressure. For example if you have aluminium 80 cubic-foot tank with a working pressure of 3000 psi, it means that your tank will hold 80 cubic feet when it’s filled to 3000 psi. Divide 80 by 3000 to get the volume represented by 1 psi in this tank:

80 ÷ 3000 = o.o26 cubic feet.

The number of cubic feet per psi is also referred to as the baseline of your tank.

Step 2

Time to convert those 400 psi you used into cubic feet.

To do this, multiply 400 psi by  the baseline we just worked out:

400 x 0.26 =10.4 cubic feet of air.

Now you know you used 10.4 cubic feet of air swimming for 10 minutes at 33 feet.

Step 3

Now, to convert the depth we were swimming at from feet to ATA (atmospheres absolute). You should remember from your basic open water course that the absolute pressure at 33 feet is 2 ATA. No? Here's how to calculate it:

(Depth + 33) ÷ 33 or

(33 + 33) ÷ 33 = 2.

So, now you have the absolute pressure of the depth you were swimming at when you did the swim test.

Step 4

We now have all the numbers we need to calculate our SAC rate:

10.4 cubic feet per minute ÷ 10 min= 1.04 cubic feet per min

then

1.04 ÷2 ATA =0.52 cubic feet per min.

Now you figured out your SAC rate is 0.52 or round it up to 0.6 to be on the conservative side.

Easy, right? Fine, if you disagree, you can always get an air integrated computer :)

OK, now that we know how much of an air hog we are, we can actually plan our dive by taking that into consideration and choose a size of tank that we'll need.

What Size Tank Do I Need?

Let’s plan a dive on the Arabia, one of my favourite wrecks in Tobermory. Arabia sits in 100 feet of cold water. As a nitrox diver, I would choose EAN35 which would allow me to stay at a maximum depth of 100 feet for 36 minutes, acccording to my  Suunto D9 computer, set at 50%RGBM - the less conservative setting.

Now, I want to have a safety margin, so I won't plan to dive to the limit permitted by my computer. So, let's just plan for a dive of 33 minutes of bottom time.

My SAC rate is 0.6 cubic feet per minute. I want to stay 33 min at 100 feet (or 4 ATA).

How Much Gas Will I Need?

33 min x 4 ATA x 0.6 cubic feet = 79.2 cubic feet.

So, based on my SAC, the depth of the dive, and the time I want to spend, I will need close to 80 cubic feet of gas just for the bottom portion of the dive.

Since I plan on going back to the surface, back into the boat, and then on to the Crowsnest for a pint, I also have to calculate the gas I need for:

  1. my ascent to the surface
  2. the required safety stop for 3 minutes at 15 feet
  3. sufficient reserve for any emergencies.

At a safe ascent rate of 30 feet per minute, it will take me 3 minutes to get to the surface:

3 min x 0.6 SAC x 2.5 ATA (average pressure from the depth to the  surface) =4.5 cubic feet.

The air we need for our 3-minute safety st0p at 15 feet:

3 min x 0.6 x 1.45 ATA = 2.61 or about 3 cubic feet.

So far, we calculated:

  • Air for the dive = 80 cubic feet
  • Air for the ascent to the surface = 5 cubic feet
  • Air for the 3-minute safety stop at 15 feet = 3 cubic feet.

That's a total of 88 cubic feet. This is just the air I'll use at depth, during my ascent, and for my safety stop.

If life were perfect, we'd never need to plan for emergencies.

Until then, plan for the sh*t to hit the fan. Better safe than sorry, etc. An easy way is to use the rule of thirds. Simply multiply your gas requirements by 1.5. I need a total of 88 cubic feet for the dive, so using the rule of thirds, I would multiply 88 by 1.5:

88 x 1.5 = 132 cubic feet

So, to safely execute a dive on the Arabia at 100 feet for 33 minutes, I will need 132 cubic feet of gas.

And my SAC rate is much better than average.

Do you still think that an 80 cubic-foot tank is enough for local diving?

Even if your SAC rate is 0.4, you would still need 90 cubic feet of gas for this dive.

3 replies
  1. Sean
    Sean says:

    Hey guys, you have had some interesting blog posts lately. This one and the one on spare air (gas reserves) have been very interesting and not surprisingly are inter-related.

    You could come at this the other way and work backwards instead of forwards. The first thing you would need to do is determine the amount of gas you would need to hold in reserve to get yourself and your buddy to the surface in case of emergency. For shits and giggles let’s say 64 ft^3 is what is needed on a 100′ dive. The capacity of an 80, as discussed previously, is about 77 ft^3. That leaves you with 13 ft^3 to conduct the dive (assuming you started with a full 3000 psi fill). Now you need to decide what portion of that is available to you on the actual dive, is it 100%, 50%, 33.3%? Taking into account your SAC you would find this leaves you very little time at depth and I think it would be hard not to come to the same conclusion you have in your post, it is hardly worthwhile.

  2. Ed
    Ed says:

    How does the baseline go from 0.026 in step 1 to 0.26 in step 2? I find it easier to use percents. if I use 13% (400/3000) then I use 10.4 cf (80 cf X .13). Same answer with out mysteriously multiplying my baseline by ten.

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