How Long Does a Tank of Air Last? (Part 1)
Have you ever been suiting up on the shore and talking to a non-diver? They are full of questions. Usually we can answer them pretty easily but then they hit us with the bomb: “So how long can you stay under with that thing?” If you’re like me, you give them the standard answer: “Ah about an hour.” That usually makes me put my hood on and run down to the water before they start asking me to explain how I figured that out.
In this week’s tip, we are going to learn how to figure out how long a tank of air will last us at a given depth and also how much air we still have in the tank.
Volume
Before we start talking about how much time we have, let’s talk a little about tank volume and pressure. You have most likely seen tanks that range in all different sizes from 13-cubic-foot pony bottles to 120-cubic-foot full scuba cylinders. What’s the difference? Well obviously the size matters. The volume of the tank is how much air it will hold when it is completely full. A 13-cubic-foot tank will hold 13 cubic feet of air when full. Well duh, teach us some thing we don't know...I’m getting there—bear with me.
Pressure
You have no doubt seen tanks with all different kinds of pressure ratings: 1800, 2250, 2400, 3000, 3500. Is an 1800 less then a 3500? Not necessarily. Let’s do a little math and see if we can figure who has more air in their tank at the end of a dive. We have two divers. One is using an Aluminum 80 that is service pressure-rated to 3000 psi. This diver comes out of the water with 1800 psi in his tank. The second diver has a Steel 108 that has a working pressure of 2640 (2400+10%). He comes out of the water with only 1400 psi in his tank. Who has more air? Let’s work it out. Remember the tank chart from last week? Well here it is again.
Dive Tank Comparison
First lets look down and find the aluminum 80 tank and see what its actual capacity is. According to the chart, it is 77.4 cubic feet. So if we know that if we have 3000 pounds per square inch of pressure in an aluminum 80 tank, then we will have 77.4 cubic feet of air in the tank. So how many cubic feet of air do we have left if we only have 1800 psi in the tank? We can figure that out by doing a little cross multiplication:
Now let’s look at diver number 2. According to the chart, a steel 108 tank actually holds 108 cubic feet of air when it is filled to capacity at 2640 psi.
Wow! So why use a high-pressure (3000 and above) tank? Well, what a high-pressure tank will do for us is allow us a greater volume in a smaller package. For instance, a steel 72 has a length of 20.5 inches and a diameter 6.75 inches but filling it to its capacity, 3300 psi, the tank will hold 72 cubic feet of air. On the other hand, an aluminum 72 has a length of 26 inches and a diameter of 6.9 inches. Filling it to its capacity of 3000 psi will only put 69.6 cu.ft. of air into it. So the steel tank will be smaller and easier to handle and will also carry more air! Well, that is part one of our tip. Next week we’ll take what we know about tank pressure and volume and figure out how long a tank of air will last us at a certain depth. I’ll also show you how to figure out how to tell how different cylinder sizes will effect us.
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