Compressed-air tank capacity
by B.B. Pelletier
Today is Friday, when I usually have some fun, but I already did that with the dime article on Tuesday. I’m going to remain serious and address a topic that causes a lot of confusion. I’m going to talk about compressed-air tank capacity and how it relates to airguns.
As this report unfolds, I think you’ll see why this subject is so confusing. Every time I instruct a new precharged pneumatic (PCP) airgun owner about compressed-air tanks, their eyes glaze over when we come to this part.
“How can this scuba tank hold 80 cubic-feet of air? It isn’t that big!” That’s not what 80-cubic feet means.
“Well, why don’t they just say what they mean?” Because scuba tanks were developed for divers, who want to know how many cubic feet of air they have available to breathe. They can then calculate how much diving time they have, with a safety reserve built in.
Analogies don’t always work
We often use analogies to explain things like the capacity of a scuba tank. One analogy is the gas tank in a car. We might say that just because a car has a larger gas tank doesn’t mean that the car will go any faster. And the same is true for an airgun. The size of the air reservoir doesn’t relate to the velocity the gun can develop. But after that, the gasoline/compressed air tank analogy breaks down. Because gasoline isn’t compressible and air is. By varying the pressure inside the air reservoir of a PCP, we can stuff more air in and get more shots out or get more power from the same number of shots — or some combination of those two. You can’t do that with the gas tank on a car. Try to put in more gas than the tank can hold and it just overflows and spills out on the ground.
What do compressed-air tank sizes really mean?
One common size of scuba tank used by airgunners in the 80 cubic-foot tank. What does the term 80 cubic-foot tank mean? It means that 80 cubic-feet of air at sea-level air pressure are contained in the tank. Air pressure at sea level is approximately 14.7 psi. Now, follow this.
If you compress 80 cubic-feet of air to 3,000 psi, that’s compressing it 204.08 TIMES. The standard air pressure at sea level (which is accepted as 29.92 inches of mercury and can also be stated as 1013.25 millibar) is not accepted as exactly the same around the world, but it is close enough everywhere for this explanation. Notice the term millibar? That’s one-thousandth of a bar, which is a standard measure of air pressure.
A cubic foot of air at sea level on a normal day (and a normal day has a specific definition) measures just over one bar of pressure. Here’s the interesting part. If you multiply 14.7 times 200 bar you get 2,940 (psi). But remember that 14.7 psi is just OVER one bar? Multiply 14.5 times 200 and see what you get. The answer is exactly 2,900 (psi). So — 200 bar equals 2,900 psi. And 206 bar is very close to 3,000 psi.
What does that tell you about the 80 cubic-foot scuba tank? It tells you that if it’s pressurized to 3,000 psi, it’s holding just over 200 bar (actually 206 bar) and you now know that number (206) does relate to how much air the tank is capable of holding if the actual internal volume is around ONE cubic foot!
There are plenty of compressed-air tanks that hold air at 206 bar but are not called 80 cubic-foot tanks. I own a couple of small scuba tanks that hold 6 cubic-feet, each. Guess what? They’re pressurized to 206 bar (3,000 psi.), but the internal volume is much smaller than that of the 80 cubic-foot scuba tank. They hold much less air, but it’s at the same pressure. What does that mean? It means they’ll start dropping in pressure from 3,000 psi much sooner than an 80-cubic-foot tank will — given that they’re both filling the same airgun.
Stay with me
I’ll make sense of all of this in a moment, but first I need to tell you about one more thing — the carbon fiber tank. Actually, this tank is just wrapped with carbon fiber for strength. It has an aluminum “bladder” inside that holds the air, and the carbon fiber wrapping just adds tremendous strength to the bladder.
A common size of carbon fiber tank is the 88 cubic-foot carbon fiber tank. Okay, so it’s 8 cubic-feet “larger” than an 8 cubic-foot aluminum scuba tank. It must hold a little more air, but not that much. Right?
Yes and no.
An 88 cubic-foot carbon fiber tank does hold just 8 cubic-feet more air than an aluminum 80 cubic-foot scuba tank; so for breathing purposes, it holds only a little more air. That’s because people who breathe that air do so right down to almost the last cubic-foot — at least from an airgunner’s perspective. They use regulators that drop the air pressure that they breathe down to an acceptable level, and that level changes with the depth they dive.
But an airgunner usually needs air that’s pressurized to at least 2,200 psi just to start filling a PCP (that’s really the pressure at which many of the PCPs finish), and 3,000 psi is a very common maximum fill pressure these days. A tank that’s pressurized to 3,000 psi will usually give only one to three complete fills of a gun before the tank’s pressure starts dropping. It will still provide many more fills, but each of them will finish at a declining pressure. This is where an 88 cubic-foot carbon fiber tank shines, because more of the air it holds is at higher pressure, so it will give MANY more full fills to a PCP than the 80 cubic-foot scuba tank! How many more depends on which gun you’re talking about, but there will be at least 20-40 times as many full fills in the 88 cubic-foot carbon fiber tank.
WHAT? How can something that is only a little bigger hold that much more air?
Actually, an 88 cubic-foot carbon fiber air tank is SMALLER internally than an 80 cubic-foot scuba tank! Remember — we’re not really talking about the volume when we quote the size of the tank. We’re talking about how many cubic feet of air AT SEA LEVEL PRESSURE the tank will hold. The difference is like the difference between a year and a light year — and it’s not just a third less calories!
The smaller 88 cubic-foot carbon fiber tank is squeezing its air like a miser squeezes a toothpaste tube — trying to get the last bit of use out of what’s inside. As a result, you get one complete fill after another from this smaller, lighter carbon fiber tank. It holds air at 300 bar, which we can now calculate to be 4,350 psi, but filling stations commonly fill these tanks to 4,500 psi (310 bar). And it takes a long time and many gun fills for the pressure inside to fall below 3,000 psi, where the tank can no longer give complete fills. At that point, it acts just like a scuba tank — and the top of each fill declines from the fill before.
Do you see why low-pressure PCPs are so great?
This is one of the reasons I pushed so hard for the Benjamin Discovery to use a 2,000 psi fill. I actually wanted 1,800 psi as the max. Can you imagine how many more fills a gun like that gets from any compressed-air tank?
What’s in a name?
This report was prompted by confusion over the latest carbon fiber tank from Crosman. They call it a 342 cubic-inch carbon fiber tank, which has no meaning in light of the explanation you have just read. It accepts a 4,500 psi fill like most carbon fiber tanks, but I think Crosman is stating the actual internal volume of the tank rather than it’s air capacity. I showed you a picture of me holding this tank at this year’s SHOT Show, and you can see that it isn’t as tiny as it appears in the Pyramyd Air description. I believe this tank probably holds 40 to 50 cubic-feet of air, according to the explanation given here, but we’ll just have to wait and see.
What you want for your PCP, Grasshopper, is a tank that holds many cubic-feet of air at very high pressure. Carbon fiber tanks fill the bill. Such carbon fiber tanks weigh only half of what the lesser scuba tanks weigh, but of course they do cost a lot more.