Scuba tanks for precharged airguns
by B.B. Pelletier
If you get into precharged airguns, sooner or later you're going to get a scuba tank. You can get by with a hand pump for a long time. They do the job as long as you are willing to do the work. One day you'll see somebody fill his Big Bore 909S from a tank in a handful of seconds after you just pumped yours for five minutes to get the next four shots ready, and you'll decide that a scuba tank is not such a bad idea.
Two types of valves: DIN & K
There are two types of scuba tank valves for airgunners - the K valve and the DIN valve.
DIN valve: The DIN valve comes in two pressure ratings. Ten-meter target guns are mostly filled from DIN tanks because most 10-meter guns come from Germany where DIN is king. A DIN (Deutsches Institut für Normung) valve is characterized by female threads into which all attachments are screwed. Ten-meter guns come with brass adapters that fit the DIN valve and attach to the removable air tanks on the other side.
A DIN valve has a hole with threads to accept any devices. This is the deeper 300-bar DIN hole, though both have the same diameter and threads.
DIN valves come in both 200-bar and 300-bar ratings. One bar is about 14.5 psi, so you can do the math. The 300-bar DIN valve is just a deeper hole with more threads to effect a stronger grip on any attachment. Other than the length, the threads are the same for both DIN ratings. An O-ring on the face of every attachment seals the DIN-type valve interface when the attachment is screwed down flush against the flat bottom of the interior of the DIN valve.
K-valve: The K-valve uses some kind of clamp that surrounds the valve. A screw in the back of the clamp adjusts the tension between the clamp and valve. The valve has the O-ring, not the attachments, so this is something you should check frequently. If the O-ring falls out, you'll never get a good seal.
The K-valve is flat with an O-ring to seal it. This type needs a clamp that fits around the back of the valve to hold it.
A refill clamp fits around the K-valve.
All sizes and pressure ratings!
Scuba tanks come in different sizes and pressure ratings. If your gun needs to be filled to 3,000 psi, your tank has to be able to be filled to that pressure, and this is where people sometimes get confused. A tiny tank can hold 3,000 psi air, but not very much of it. And a large tank may only be rated to 2,200 psi. It is the pressure rating that determines how much pressure can be put into a gun, and the volume of the tank determines how many fills you'll get from it. A very common standard scuba tank that works for most precharged airguns is an aluminum 80 cu. ft., 3,000 psi tank. They cost $130 to $150 brand new and can be purchased used for under $100. Last week a friend of mine bought a used one for $35.
Tanks must be visually inspected every year
A dive shop will empty the tank and visually inspect it for signs of corrosion and work-hardening (from flexing due to pressure changes). Either problem can condem the tank - meaning that no dive shop will fill it for you. This is a risk you run when buying used. A visual usually costs around $10.
Hydrostatic testing every five years
A hydrostatic test determines whether the metal in the tank walls has work-hardened beyond specifications. For the test, the tank is pressurized well beyond its working pressure and the flexing of the outer circumference is measured. If it is still flexible, it passes its hydro and can be returned to service. A failed hydro means the tank can no longer be filled. Fortunately, the tanks that PCP shooters use seldom go below 2,000 psi before they are refilled, so the amount of work-hardening or fatigue they suffer is much less than what a scuba diver would subject it to by taking it all the way down to empty.
Carbon fiber tanks are the next thing
Carbon fiber tanks are lighter than scuba tanks and hold a greater volume of air. They're what the rescue workers and firemen use when they can't breath the air. One of these tanks is half the weight when filled and holds several times as much air as an aluminum 80 cu. ft. scuba tank. The big drawback is the cost, but some shooters need this kind of performance at any price. The other consideration is will you be able to get the tank filled to its 4,500 psi rating? Most dive shops won't be able to fill that high, but a fire station can usually do it - so, do you have any friends there?
The BIG question!
You don't have to be a diver or have a dive card to get a scuba tank filled. There is no law governing who can purchase high-pressure air, but dive shops need to be approached respectfully the first time you go, because they don't HAVE to sell you anything! Take your refill clamp in and explain that you're an airgunner and most dive shops will take it from there. Many will want you to sign a release promising you won't breathe the air they sell you. That way, if you abuse their courtesy and wind up face-down in your swimming pool, they can show the six o'clock news that it was YOU who screwed up - not them.
If you want to get into PCP airguns, consider a scuba tank to be the same as the transformer for your electric trains. It's not sexy and you don't play with it, but it makes everything else work.