Buying a high-pressure air tank - Part 1
by B.B. Pelletier
This report is for Abe, but with the large number of folks coming into precharged airguns, I suspect many people need to read it.
When I started in precharged pneumatic guns (PCP) in 1996, the need for air was not as critical as it is today. The problems back then were overcoming the personal fears of high-pressure air, getting dive shops to fill the tanks for non-certified persons and, of course, adapters.
The adapter problem continues to be bad, though there are movements to standardize at Crosman, Daystate and in the aftermarket. But the tanks themselves are more critical now than ever before. That's because the guns of today need more air than they used to.
Size doesn't matter
Here is the problem. The physical SIZE of the scuba tank means next to nothing. What MATTERS is how much air it holds. You say, "Of course!" But until you become a PCP user, you don't really understand what that means.
The most common scuba tank
The standard scuba tank today is an aluminum 80 cubic-foot tank that's pressurized to 3,000 pounds per square inch (psi). The physical tank measures just under 30 in. high by about 7 in. in diameter. It weighs just under 40 lbs. when filled (mine weighs 38.5). All these specifications will vary slightly from manufacturer to manufacturer.
And it will completely fill a PCP rifle that needs 3,000 psi one to three times, depending on the volume of the gun's reservoir. AirForce rifles that have a huge 490cc air tank get about two complete fills from such a tank. Remember, I'm now talking about complete fills to 3,000 psi.
After a full fill come partial fills
After that, the rest of the fills will end at less than 3,000 psi, because the scuba tank's pressure has dropped. Taking an AirForce Condor as an example, each successive fill could look something like this:
And so on for about 15-18 fills. When the pressure in the scuba tank drops below 2200 psi, there isn't enough air left to fill the AirForce tank high enough to get the gun on the power curve. That means you have to get the scuba tank refilled.
Actual results will vary
The actual number of gun fills you get varies, based on how low you allow the gun's tank to decline before refilling and the size reservoir you're filling. So the numbers I've just given are approximations. Don't try to create mathematical formulae based on them--they're just general observations.
Get a bigger tank
If you want more fills, the solution is to get a tank that holds more air. In the scuba tank world there are aluminum tanks rated to hold 100 cubic feet of air at 3300 psi and steel tanks rated for 120 cubic feet of air at 3500 psi. Obviously, these tanks will give you more complete fills of any PCP than the 80 cubic-foot aluminum tank. The 100 cubic-foot aluminum tank is larger than the 80 cubic-foot tank, but the 120 cubic-foot steel tank is about the same size as the 80 cubic-foot aluminum tank. The 3500 psi steel tank weighs about five pounds more than the 80 cubic-foot tank. Both of these larger tanks cost more than the 80 cubic-foot tank. You have to look for sales when buying tanks like this.
Smaller is more
Then there are the tanks made of carbon fiber. They're actually aluminum bladders wrapped with carbon fiber fabric. They seem to be contradictions, because they're smaller and lighter than 80 cubic-foot aluminum scuba tanks, yet they hold more air. My 88 cubic-foot tank is 24 in. by 7 in. and weighs about 20 pounds when filled to 4,500 psi. It will completely fill the AirForce reservoir perhaps 7 to 9 times and will get many more partial fills than an 80 cubic-foot scuba tank. It costs several times as much as an 80 cubic-foot tank. So, as with most things, you pay for performance.
Carbon fiber tanks are rated to hold breathing air, but for land operations. They are most often encountered in rescue service operation.
So far, all I've talked about are the largest portable air tanks. That makes what I'm about to say meaningful. If I were shooting a big bore rifle that gets two or three shots per fill and needs 3,000 psi to be completely filled, an 80 cubic-foot tank would be inadequate. The tank that most smallbore PCP owners use today could not supply the air needed to keep a big bore operating very long.
But if I had a Benjamin Discovery rifle that has a maximum fill of only 2000 psi, an 80 cubic-foot scuba tank would be great! I would get a great many full fills and lots more partials from that tank. That's why I pushed hard for the 2000 psi fill level when we developed the Discovery. Besides being easier to fill with a hand pump, it extends the use of scuba tanks many times.
Okay, Abe, that's the first part of the report. Next week I'll cover smaller scuba/carbon fiber tanks, tank valves and buying used. What I need YOU to do is ask me questions now so I can work the answers into the next report. The same goes for all readers who have questions.