Beginners guide to compressed air and air rifles.
After browsing the forum it will become evident that two types of air rifle exist namely ‘springers’ and ‘PCP’s”.
Springers contain springs or gas rams that are manually compressed by the shooter to propel a piston forward when released by a trigger to compress air that accelerate the pellet down the barrel.
PCP’s or Pre Charged Pneumatic air rifles use air compressed by a compressor and stored in a cylinder to accelerate the pellet down the barrel.
It’s basically a lot of energy safely stored in a dive cylinder in the form of compressed air. Safely means all safety precautions are adhered to.
To quote an expert: , "The explosive potential in a fully charged 80cf aluminum SCUBA cylinder is approximately 1,300,000 foot pounds -- enough to lift a typical fire department hook-and-ladder truck over 60 feet in the air!", stated by A. Dale Fox on his web page.
Another guy relates it to 300g of TNT high explosive, about two hand grenade’s worth.
Let’s try putting it in perspective by means of visualisation. One bar = one atm, or atmosphere. So if we have a 300bar filled cylinder it contains 300 cylinders of air compressed into one:
Your car tyres contain around 2bar each, so we are talking MAJOR pressure compared to garage compressors and aerosol spray cans.
The idea here is for us to safely use all this harnessed energy pellet by pellet and not in one big bang.
What we’ll need to fill a PCP rifle:
-Tested and filled dive cylinder. 232bar or 300bar. Most cylinders are 232bar cylinders with 300bar cylinders becoming more commonplace.
There are two types of rifle cylinders used on PCP’s, non quick fill and quick fill. The non quick fill cylinders are removed from the rifle to be filled. These include the older CZ200’s, Air Arms S200 and MPR, older Weihrauch HW100’s, Anschutz, Steyr, Feinwerkbau and Walthers.
-Fill station for quick fill rifles or fill adapter for non quick fill rifle cylinders.
-Quick fill probe or adapter to fit fill station hose to rifle’s quick fill valve.
Let’s start by looking at the dive cylinder:
We have a dive cylinder as bought from a dive shop or someone on the forum. It will be fitted with a DIN pillar valve. It is filled with clean dry air pressurized to 232bar or 300bar.
The integrity of the cylinder containing the massive pressure must be unquestionable. To ensure this cylinders are tested once a year by certified people and include visual inspection and pressure testing. Pressure testing or “hydro” is done by putting the cylinder under pressure while in a water bath. Rise in volume of water bath indicates expansion of cylinder under pressure. When this test is passed the test date is stamped into the cylinder metal.
Most accidents happen while filling cylinders so respect the guy at the dive-shop’s right to refuse filling of an out of date cylinder. These accidents normally result in loss of a limb and often in loss of life and always in serious damage to property.
When in doubt, ask the dive shop guy to check the stamps for you and rather get it tested even if the guy tells you it is ‘slightly out of date’ but he’s prepared to fill it this ‘one last time’. Looking at photos of what a cylinder explosion looks like you might agree with me on this one, your cylinder rides in the car with you and most likely shares your house with you.
Ok, so we have a tested and filled cylinder, here’s how to treat it:
ALWAYS be aware of the potential energy in it and handle it with respect like you would a rifle with rounds in the magazine.
Do not drop it, let it roll around in a loading space, expose it to abnormal heat, store it near sources of heat (campfire, stove etc.)
The weakest point of the cylinder is the valve, when this break off the cylinder becomes a projectile that will penetrate a brick wall. The cylinder can safely be carried and lifted by holding onto the valve but do not drop it or knock the valve.
Store the cylinder sideways in your car and be sure to secure it so it does not become an airborne liability in case of an accident, it weighs in excess of 15kg. Should the valve get damaged in an accident the direction of propulsion will be sideways away from driver and passenger.
Never disassemble or fiddle with the valve or its threads. Leave o-ring replacement and valve servicing to the guys at the dive-shop, they have the know-how and tools to do this.
Using the cylinder for air rifle filling.
We need to transfer a small amount of the compressed air to the air rifle. Even though the air rifle cylinder is much smaller than the dive cylinder it is by no means less dangerous when abused.
Do not confuse pressure with volume. If we have a cylinder the size of a petrol tanker pressurised to 232bar and we fill a cylinder the size of a BIC lighter from it, the small cylinder cannot be filled to more than 232bar from the large cylinder. It will only contain a smaller volume of air at 232bar.
Air rifle cylinders are “safe pressure” rated. Most are tested to withstand an accidental “full fill” from a 232bar dive cylinder. The danger now lies in the more often used 300bar cylinders;
When filling from a 232bar dive cylinder the pressure in the rifle cylinder cannot go higher than 232bar, it will equalize at 232bar. BUT, with a 300bar dive cylinder a 232bar rated rifle cylinder can accidentally be filled to 300bar. Also, it would be wise never to assume that your dive cylinder filled by the shop is only pressurized to 232bar. Shit happens. Your cylinder might have been exchanged or accidentally filled to 300bar. The valve threads on 232bar and 300bar cylinders are of different depth so confusion is unlikely but rather be safe than sorry.
Manufacturers stamps on cylinder:
I know of two cases in less than a year in my local air rifle community where 232bar rifle cylinders were filled to 300bar, one case because of a broken manometer that was ‘home fixed’ and the other because of gross neglect. Luckily no bodily harm occurred, an EV blew a regulator and the other rifle was shot to release the extra pressure.
To know what we’re doing a manometer or ‘pressure gauge’ is added for safety. This brings us to the next component needed to fill the rifle, the “filling station”.
The cut open hosepipe section stiffens the rather thin fill hose to prevent it from kinking with a heavy fill adapter attached to the end.
The fill station is bought separate from the diving cylinder and is specific to air rifle and paintball shooting. Some dive shops sell them and some PCP suppliers stock them.
It’s basically a fitting that screws finger tight into the DIN pillar valve of the dive cylinder. It has a manometer mounted on it and a short high pressure hose attached that terminates in a female brass fitting.
The specific rifle’s quick fill adapter is screwed into this brass fitting using plumber’s tape to ensure a good seal. Make sure the quick fill adapter comes with the rifle when you buy it. This stays on the fill station unless you have invested in a “Best Fittings Quick release adapter” which enables the use of different rifle fill adapters by quick releasing them from the fitting on the hose.
Also on the fill station is a bleed screw, holding a small ball bearing in place over a hole. This screw is used to release the pressure in the hose between the rifle cylinder and dive cylinder after the dive cylinder valve was closed upon completion of a fill.
Should you find that your fill station’s thread won’t match with the thread in the pillar valve on the dive cylinder it might be that there’s a diver’s adapter screwed into the thread. This can be removed using an allen key which will fit into the center hole of the adapter or ‘plug’.
The rifle fill procedure:
Screw fill station into dive cylinder pillar valve after inspecting the o-ring on the face of the protrusion that goes into the pillar valve thread and ensuring that the threads and recesses are clean and dust free.
Fasten finger tight, the o-ring does the sealing so force is not deeded. Position the station so the manometer is visible without you facing it directly. (There is a very, very slim chance of manometers exploding, cut this possibility of a mishap out the procedure.)
NOW CHECK THE RIFLE’S SAFE FILL PRESSURE. This differs from make to make. Most will be 200bar and some as high as 300bar. If you don’t have the rifle’s manual check for marking on the rifle’s cylinder. There should be a Max safe pressure and a Max Operating pressure. We are aiming for the Max operating pressure. Next, CHECK THE MANOMETER SCALE to make sure you know how to read the needle and where to stop filling.
We’ll presume you have already fitted the rifle’s specific fill adapter or fill probe to the end of the fill station hose. Position the rifle so that the adapter can be inserted on or into the rifle cylinder quick fill port. The port is usually covered by a dust cap. Now close the bleed screw.
Get a positive grip on the pillar valve handle and VERY SLOWLY open it under full control while watching the manometer. This valve is also supposed to be closed only finger tight. If over tightened slow opening is more difficult. NEVER take your hand off the valve handle during the fill process. Your first instinct in case of emergency should be to close it instantly, clockwise like a water tap. If the phone rings or the kid bites the dog’s tail off, CLOSE before dividing your attention.
The manometer reading should now slowly rise as you pressurize the fill station and hose. The rifle will still have pressure in it and the pressure will be sealing the inlet valve. As the pressure in the fill system reach the pressure inside the rifle cylinder you’ll hear a ‘click’ as the rifle inlet valve opens up and the pressure gets transferred from the dive cylinder to the rifle cylinder.
The fill process must be done as slow as possible. The reason being that the inlet valve on the rifle cylinder is made of Delrin or Nylon. Forcing such a massive volume of air through the little valve opening causes friction which generates heat. The valve can melt or get distorted because of the frictional heat. Ideally, the fill process on a standard rifle cylinder should take at least two minutes. Heating the air because of a hasty fill will also cause it to expand and give an elevated manometer reading. Once cooled down you will end up with less pressure in the rifle than anticipated.
Once the desired manometer reading is reached the pillar valve is shut down, finger tight. There’s still compressed air in the fill station and hose. Once this pressure drops below that in the rifle cylinder the rifle’s inlet valve will be shut by the higher pressure behind it. To achieve this we slowly open the release screw (bleed screw) on the fill station before removing the fill adapter from the rifle. This will cause a loud hissing sound as the pressure is released from the fill station.
Put the dust cap back on the rifle’s quick fill port and you’re done.
The same procedure is followed for non quick fill cylinders with the exception that where the fill station screws into the pillar valve the non quick fill adapter screws in. The cylinder after removed from the rifle then goes in here directly and the manometer on the rifle cylinder is used for monitoring instead of the fill station manometer.
General precaution summary:
-Check dive cylinder test dates.
-Regularly inspect all visible o-rings on rifle cylinder, quick fill probes and fill station.
-Inspect fill station manometer for damage before each use.
-Use rifle’s manometer as double check.
-Never face a manometer directly, observe at an angle facing away from body parts.
-Ensure fill equipment is grime free, dirt in rifle cylinders cause slow leaks of valves.
-Caution kids about dangers of compressed air or keep dive cylinder out of reach.
-Store cylinder upright, if valve pops in case of fire the propulsion will be downward and upwards and hopefully not sideways through the bedroom wall.
-When transporting in car store sideways and secure it in place.
-Only have your cylinder filled with clean dry compressed air by a reputable supplier.
-Do not use any oil or grease near compressed air. Oil and grease can combust when pressurised. Use only pure silicone grease sparingly on o-rings as supplied by dive shops.
-Never hold a body part close to a compressed air exit, air released under such great pressure can penetrate the skin and cause severe air embolism, that is air penetrating flesh and the cardio vascular system. Same principle goes for fire arm blanks, Jon-Erik Hexum killed himself on a movie set by jokingly pressing a handgun loaded with a blank against his temple and pulling the trigger. A cubic meter of compressed gunpowder gasses had to go somewhere...
-Should you wish to de-pressurise a dive cylinder for air transportation or some reason, do it very slowly, preferably over night by opening the pillar valve slightly. Decompressing it too fast will result in water condensation in the cylinder which could lead to premature rusting.
-Do not custom paint your cylinder, the standard colors are in compliance with regulations, dive shops can refuse to fill it if painted otherwise.
-Avoid 'home made' parts, components and fittings when dealing with the high pressure end of a rifle.
RESPECT the energy contained in a dive cylinder, doing so will bring about COMMON SENSE.
This hose end block-off plug from Best Fittings enables the fill station manometer to be used to check the pressure left in the dive cylinder:
The rifle manometer must always be used as a double-check:
Happy and safe shooting, when in doubt ASK.