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Education / Training How air pressure relates to filling a precharged airgun

How air pressure relates to filling a precharged airgun

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.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

37 thoughts on “How air pressure relates to filling a precharged airgun”

  1. B.B.

    Thanks for answering this. Everything is clear now.

    The key sentence that I needed was this: “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.”

    and I now understand that in real life, you can never fill a 3000 psi airgun up to 3000 psi exactly, when using a 3000 psi scuba tank! It will always be slightly less, even the very first time.

    Thanks a lot for your help B.B.


  2. BB,
    Off topic, but have you got any information on a Crosman Powermaster 66? I acquired one of those old cannons last night in need of pump repairs. I’d like to know if there’s a blog on one. JP

  3. Andreas,

    And 3000 psi guns don’t operate at 3000 psi, either. Most operate at between 2000 and 2200 psi. They have valves that self-regulate (i.e., they can work with) 3,000 psi, so they operate at peak performmance until the reservoir pressure drops below 2000 psi (or whatever their true operating pressure happens to be.


  4. BB,

    do You know some results from the FT World Championship in Pulaski last weekend? Which guns and scopes were used?

    There are no informations on the official GOBFT site and their registration form is still online. Why are ther no polish or other continental european shooters enlisted?


  5. B.B.

    I don’t know your policy about where you want an unrelated posting to go—attached to the most recent article or to the relevant one even if it’s some time ago? I don’t know if you have some way of flagging new comments wherever they appear in your blog. As long as I’m here, I want to say thanks for your great work. I’m systematically reading all of your blog and I’m learning a ton, and I want to thank you particularly for your work on the IZH 61 which led me to purchase one. I read your reports very carefully along with everything that I could find about the rifle and decided to take the risk that it has not deteriorated too much with the plastic conversions. I’m glad I did; the gun is all that you ever said it was. From a sitting position at 7 yards with iron sights, a 10 shot group (minus one flyer) of Crosman Premier lights (7.9 gr) went into .4 inches CTC. Standing groups are very close to sitting. But I couldn’t have done any of this without your description of the artillery hold that makes all the difference. Thanks for making all this possible. Now, I’m going to try all the different pellets and the Leapers Bug Buster scope. (Pyramidair should thank you for the ton of money I’m dropping on them.)

    I do get some blue smoke out of the barrel every so often after a shot. Is this what’s called “dieseling”? Will it cause damage? Is there something I can do about it?


  6. BB and All,

    I have an operational alert for owners of the RWS 850 AirMagnum. I’ve posted problems with a loss of velocity and CO2 in the past, and I beleive I have found the reason.

    I have used Crosman 88 gram AirSource cartridges in my rifle since day one. They are readily available at the local Big-Mart. With each new AirSource, I’ve always placed one drop of Pellgun oil on the tip of the cartridge before installing it. After a half dozen cartridge changes, I began to lose performance and CO2 rapidly. So I took the rifle action out of the stock and took a good look at the valve.

    What I found was that the cartridges dont seal against a flat seal, but rather the tip of the cartridge is inserted through a round O-ring and the O-ring settles into the concave depression near the tip of the cartridge, or so I thought. In order to pierce the cartridge, the threads on the tip of the cartridge must come in contact with that O-ring. If you don’t back the cartridge out after piercing it (about 1/2 to 1 full turn as suggested in the owners manual) the threads will cut and distort the seal, and you’ll begin leaking CO2. It’s been my experience that when a small leak like that begins, the humidity in Florida can turn it into ice and begin to freeze the valve and it’s internals, lowering performance, or in my case, grinding to a halt.

    As if that’s not bad enough, when you remove the spent Airsource cartridge, the sharp edge of the crimp (or ridge from the machining) in the very tip (you can feel this on a new Airsource cartridge by running your finger lightly from the threads towards the tip) will shave off a few thousandths of the seal on it’s way out. Each time you remove a cartridge, you shave off a few more thousandths, until the seal no longer makes adequate contact with the cartridge.

    So after a few seal replacements, I’ve begun to prepare the Airsource cartridges by taking the burrs off the tip with a small diamond file, and polishing the tip with steel wool. You can also take the sharp edge off the first few threads to reduce the chance of cutting the O-ring seal while piercing the cartridge. Clean the tip carefully and apply a drop of Pellgun oil to the tip where it will be pierced, and around the concave depression where the O-ring seal will make contact, and you should have no further problems.

    Umarex recommends the use of Walther 88 gram cartridges, and when you compare the tips of the Crosman and the Walther cartridges, you can easily see the differences. I’m not recommending one brand over another, and free advice is worth what you pay for it, but some simple preparation of the Crosman brand will save you from some problems with this particular rifle.

    Michael in Florida

  7. B.B.,
    I’m hoping Pyramyd will have enough of their staff on hand in Roanoke to help me (and others) choose what I need and help me avoid any of those regretable “impulse” purchases. The truth is I know what I want to do with a new gun or two, but am having serious trouble narrowing the choices. I know there will be other knowledgable folks there too. Possibly you with your unveiled identity???

    And a hypothetical question on air tanks. If you started with two fully charged scuba tanks, and always used the first tank to do the initial fill of your gun, and then used the 2nd tank only to do the final topping off, might you get more more usable fills? I don’t have a PCP (yet) so I am clueless.

  8. Michael, thanks for your detailed information on the RWS 850 AirMagnum leak problem! I feel fortunate that I haven’t had a problem with my 850.

    .22 multi-shot

  9. hi bb
    your killing me here. 5 tables of airguns! on sale! and parts guns! im so mad i cant make it. do you know if this will become an annual occurance for pyramyd or is this a once in a lifetime opportunity?

    Nate in Mass

  10. B.B.
    Thanks for the aditional info on filling PCPs. For me the quietness of airguns is one of the big attractions. Most of the PCPs are listed as “loud”, except the S410, and the Webley Spectre Mark 2 is listed as “Quiet”! Do you know if the Spectre really is “quiet”. Also, do you have a ball park on what the true additional costs of owning a PCP are: tank, hand pump, fittings, fill-ups, etc, etc.
    Thanks again,

  11. BB,

    i dont think a sporting pcp that uses the usft method of 1600psi fill is a good idea…

    the big tank that is on the usft is to big and is like 3lbs. It is great on the usft but you would not walk around the woods with it.

    it is getn cheaper and easyr by the day to get 3000psi.

    If you were thinkn about 2500 to 2700psi thats a good idea (i think)! Just to make it ok to just have a scuba tank. You can get a scuba tank for $50 so that is better for some people than the $600 it takes to get the cf tank

    i have a cf tank so 3000 psi looks small to me. I know you do too so you know how i feel.


  12. Pestbgone,

    The Spectre has a shrouded barrel and is also quiet.

    A pump costs about $225 – 250 today. An 80-cubic-foot dive tank sells for $150-175 new.

    I pay $3.00 to fill my scuba tank and $6.00 for my CF tank (that’s the way to go).

    Fittings should come with the gun.

    There is an annual test that costs $10 for each tank and every 5 years a scuba tank must be hudrostatically tested ($25).


  13. Anybody know anything about the daisy 15xt? I have been thinking of buying one for blasting cans in my backyard, being only $30 it must be great or suck. A review on it in the near future would be great.

  14. dylan,

    i have a daisy 15xt and it works well but its only accurate for cans (as you said) and trees lol. It is great for that! It has good power.

    Its a fun gun.


  15. B.B.

    I believe you may have misled Pestbgone. There is simply no trick to get around the properties of compressed gas. Starting with two tanks using one for the bulk of the fill and topping off with the second tank will yield exactly as many fills as using the two tanks sequentially, minus the air lost in the additional connects and dis-connects. Part of the confusion regarding fills and tanks is explained by your post where you draw the distinction between compressed air and a tank full of water. Unfortunately, confusion creeps back in when you describe charging a gun with terms like flow and decant. When charging a gun yes, there is airflow but it is not like pouring (decanting) a bottle of wine. What is happening is that a compressed gas is expanding to fill the enclosed volume. The operation of the valve on the tank moderates the rate of expansion and allows one to stop the process at a desired pressure transfer if the storage vessel has a greater pressure than the desired pressure in the gun. If you are charging a gun with a 3000psi tank and wish to charge to 3000psi you will never get there. You may get close since the volume of the storage tank is far greater than the gun’s but you simply can’t take a compressed gas, put it in a larger volume and have the same pressure (unless you heat the system). Gage error may lead some to think that they have taken 3000psi from one tank and put it in another but that’s just not the way it works. If you want to charge a gun to 3000psi, you have to start with a tank holding more than 3000psi so that the total enclosed volume would still contain at least 3000psi.
    People who want to use their guns starting at 3000psi need to get 4500psi tanks. Folks that are happy to use their guns at lesser pressures (often resulting in fewer shots but similar performance) should realize that physics can’t be tricked.

  16. The 15XT is a fun gun for the money – far more accurate than, say, the Walther PPK. It’s a little awkward to load and the trigger has a long and heavyish pull – but remember that this pistol is really a double-action gun.

  17. B.B.

    I recently recieved an old RWS spring pistol and I was hoping you could help me identity it. The only form of identification is : “MOD 5 g” and the serial number “627647”. The pistol looks very similar to the P5 magnum but without the fiber optic sites or the muzzle brake. Other than that this pistol and the P5 seem identical, mine has a globe post site. can you help me identify it?


  18. Michael, thanks for the heads up on the 850 Air Magnum. I just got mine today and put the first Airsource canister in it. Next time I’m going to smooth the edges off if it needs it.

    This is a great rifle. I only got to shoot 2 clips with open sights before it got dark. I’m going to put a scope on it Wednesday and really test it out. I start my week long vacation then so this is going to be awsome. Thanks for your article on the 850 B.B.

    This is my 6th air rifle in the last year and a half. People are starting to think I’m crazy. Does anyone else spend a lot of time trying to explain why they drop $200 or more on an air rifle?


  19. dylan, vince,

    i have the ppk and yea i agree, the daisy is more accurate than the ppk. I was thinking about my cp88 and it blows them away! As far as co2 goes!

    I say the gamo p23 is a joke! It jams with some ammo and has a 1000 pound trigger. The bbs go all over. The pellets are no better! DO NOT BUY ONE OF THEM!!! I did not get a lemon… I have a freind with one and his is worse, and it leaks! I have used 4 of them!

    Out of all those guns the cp88 is the best but the daisy is 2nd and its cheaper!

  20. shawn,

    yea, but thats only to ppl that dont appreciate airguns…the same goes for ppl who dont like art, going up to an art collector…its ignorance of the subject, and lack of caring…lots of ppl out there think youre probably spending $200 on the red ryder(or any bb gun), when these are precision air rifles, which takes a lot of engineering to produce(especially the expensive guns…im going to order a quigley V when it becomes available, which retails for about $3000)


  21. bb,

    i was thinking about the gamo hunter extreme, and even though im never going to buy one, i was thinking about its accuracy and usability as a hunting gun…in your review, you said even the kodiaks wobbled…so i was thinking, why not use round lead balls, but the round lead balls are apparently not reccomended for high powered airguns…i thought i remembered you saying you can use them…is there a problem with using them in powerful guns, or not?

    also, i got my cfx .22 tuned by cdt, and he chronied it with gamo hunters at 700 fps…i tried sighting it in with a number of pellets in the 14.3-14.6 grain region, and all of them had oblong holes in the paper…theres no way the gun in shooting them over 900 fps(i cant tell you the exact #, but it takes longer for the pellet to hit with it then when i shoot my g-1 extreme, which i know shoots 900 fps with jsb predator pellets, which is what i was shooting(btw, under 1/4″ groups at 30 yrds after a cdt turbo tune and grt-III trigger…amazing gun now), so its def not going above 900 fps…when i shoot crow magnums in the cfx, is has perfect holes(same with everything heavier)…im very confused…got any advice? im trying to stay away from heavy pellets, so i ordered some gamo hunters, and i might get some jsb exacts.


  22. B.B.

    Thanks for the weight on the USFT. It will be nice if someone can take that concept and turn it into a rifle for those of us who haven’t gotten into FT.

    .22 multi-shot

  23. DED,

    You ask a complex series of questions. While the Predator pellet may leave the muzzle at 900, it will slow down faster than a domed pellet, so the distance at which you shoot has to be factored in, as well.

    I don’t like to think about all these things, so I just focus on group size and let the other stuff rest.

    Remember – best accuracy is ALL that counts. Everything else is a waste of time. You must hit what you aim at first. Nothing matters until you do that.

    I think round balls have too narrow a driving band and no skirt to flare out, so I don’t use them in powerful airguns, as a rult. If we were talking black powder and I was using a patch, things would be different.


  24. I used the Ideal gas law (pV=nRT) to calculate the volume in my Career Infinity, 300cc. 61 liters of air at 3000psi 😉

    Talking about quiet pcp’s, the Infinity should be on that list!

    Take care n shoot safe!

  25. B.B.

    These comments and questions have prob came up before but just in case they haven’t:

    I can’t help but wonder how safe all these pre-charged guns in relation to their 3000+ PSI working pressures. I read where there have been several aluminum scuba tanks exploding and causing serious damage to property and certain death of anybody close enough to the blast. I guess the thought of packing around and shooting a potential bomb makes me nervous.

    In your opinion just how safe are these pre-charged guns for everyday use?

    Bill S.

  26. Bill S.,

    PCPs are as safe as any properly handled technology. The explosive capacity of the gasoline in your car’s gas tank is far greater than a scuba tank’s, yet millions of cars drive the roads daily. That hazard is familiar to us, while the hazard of high-pressure gasses not nearly as familiar.

    Another example – I investigated an accident in which a propane grill blew up three apartments, yet we still have gas grills. Far greater power in a five-pound propane tank than a 3000 psi scuba tank, but more people are familiar with the technology and are willing to look beyond the danger.


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