by B.B. Pelletier

Before we begin, I have two announcements.

First, the weekly podcast has been changed to a monthly podcast. It takes nearly a full 8 hours to produce one podcast when everyone’s time is factored in, and the time vs. listeners doesn’t justify doing it week.y. The next podcast will go up in early November.

Second, Pyramyd Air will have five tables at the upcoming Roanoke Airgun Expo. It’s the largest airgun show in the world, attracting people from all over the U.S. and Europe. This year, the show coincides with a large gun show (on Saturday), which should bring several thousand additional attendees, since the public gets into both shows for one price. Pyramyd Air is cleaning out their warehouse – overstocks, repaired guns, parts guns, hundreds of repair/spare parts and the usual bits that accumulate in a multi-million dollar business. They don’t want to go home with any of it, so bring cash and lots of boxes and gun sleeves. This is a once-in-a-lifetime opportunity for serious airgunners.

On to today’s blog, which is an answer to a question from a reader named Andreas.

“I’m trying to figure out why an 80 cubic-foot scuba tank can only fill two {Airforce} 490 cc tanks. Considering they are both at the same pressure, shouldn’t the 80 CF tank be able to fill MANY more times, since 1 cubic centimeter = 0.00004 cubic feet?”

Apples and oranges
Andreas, the problem you are having is a common one people have when discussing compressed air. You’re equating volume with pressure, and they are not the same thing. If compressed air were water, you would have every right to complain, because you could fill many 490 cc vessels from one 80 cubic-foot vessel. But the 80 cubic-foot vessel you are talking about (a standard scuba tank) really doesn’t HOLD 80 cubic feet of anything! That name isn’t a description of the interior volume of a scuba tank. It describes how many cubic feet of air at standard pressure (sea level, or 14.7 psi) can be contained safely inside the tank. To get all that air into the scuba tank, it has to be pressurized to 3,000 pounds per square inch (psi) or 206 bar.

The LARGER tank is even smaller!
Look at the 88 cubic-foot carbon fiber tank. It’s actually SMALLER in size than an 80 cubic-foot scuba tank, yet it holds more air. The air has to be pressurized to 4,500 psi in order to get 88 cubic feet of it into such a small vessel, and a really neat thing happens because of that.

As the air flows out, the pressure drops
When you decant air from a scuba tank to another vessel, the air that remains in the scuba tank begins dropping in pressure immediately. If the vessel you’re filling is large enough and also holds 3,000 psi, it won’t be long before you will notice a drop in the pressure in the scuba tank. You will see this because it will no longer fill other vessels to 3,000 psi. Although 490cc does seem small compared to 80 cubic feet, how does 2 cubic feet seem? Twice 2 cubic feet is 4 cubic feet. That’s what it takes to get two full (3,000 psi) fills of the 490cc tank. Losing that much air drops the air in the 80 cubic-foot scuba tank down to 76 cubic feet. Does that make more sense?

I’m not saying that the AirForce tank we are using for this discussion holds exactly 2 cubic feet, but it does hold something in that neighborhood. It’s a good enough approximation to comprehend why it can lower the air pressure in the scuba tank so quickly.

A couple full fills – many more partials
But listen to this – once the air pressure in the scuba tank starts falling, it doesn’t go to zero immediately. Maybe fill No. 3 would put only 2,975 psi into the AirForce tank. Maybe the fourth fill would put in 2,925 psi, while fill No. 5 would put in 2,850, and so on….Again, these are not precise numbers gathered from testing – they’re simply representative of what happens to the scuba tank as you fill the AirForce tank. As the fill pressure you are able to achieve in the rifle’s tank drops, the total number of shots you can get also drops. The shots aren’t less powerful – there are just fewer of them. Instead of 35 powerful shots, maybe you’ll get only 30 on a 2,800 psi fill. At some point, the pressure left in the scuba tank will fall to around 2,200 psi.

Eventually, you’ll get so few shots from a fill that it’s clearly time to go back to the dive shop and get the scuba tank refilled. I tell people to expect 15 to 18 fills from an 80 cubic-foot scuba tanks, but only the first couple will be full fills. The rest are partials.

The benefit of carbon fiber tanks
Remember the carbon fiber tank I mentioned ealier? It holds 4,500 psi to begin with. You can get a whopping number of full (3,000 psi) fills from a tank like that, because even though it is dropping in pressure with each fill, it has a long way to go before it gets down below 3,000 psi. That’s the beauty of using a carbon fiber tank to fill precharged airguns. Besides weighing less than half what a scuba tank weighs, they also hold a lot more usable air.

Another way to make it happen…
There’s another way to get the same beneficial effect. Instead of having the air tank hold MORE air, make the gun use LESS! The USFT from Mac-1 uses 1,650 psi to get over 50 shots of a heavy 10.6-grain Beeman Kodiak pellet moving at 915 f.p.s. If you were filling that gun from a 3,000 psi scuba tank, you could go a LOT lower before you had to return to the dive shop for a refill. The air volume of the USFT reservoir is MUCH larger than the volume of the 490 cc AirForce tank, so you won’t get as many fill as you might think, but you will definitely get more.

The USFT is a specialized competition rifle, and priced to reflect the hand work that goes into it, but imagine how great it would be to have an affordable sporting precharged rifle that used less than 3,000 psi! The first company to bring out a gun like that is going to break the market wide open, as long as it’s affordable.

I hope this explains why you don’t get that many full fills from a scuba tank.