CO2 and HPA/Nitro – F.A.Q.

In this article, I will discuss the pro’s and con’s about these two gases and common terminology used. I do hope that this article will help you choose which gas is best for you.

CO2 (carbon dioxide)


  • Stored as a liquid in cylinders and expands into a gas,
  • Cannot fill compressed air or nitro into CO2 tanks,
  • Cylinders typically can screw right into an ASA and has a pin valve,
  • Output pressure depends on environment temperature…. 80*f is about 950psi…. 40*F is about 550psi,
  • Capacity of tanks is measured in ounces,
  • This gas causes velocity jumps due to liquid expansion,
  • Most are made of either steel or aluminum,
  • Aluminum cylinders over 2″ in width needs to be tested every 5 years by hydrostatic testing,
  • You can get CO2 in disposable 12g cartridges to a refillable 20oz tank,
  • You can get refills pretty much at any field or store,
  • When shooting with CO2, the tank will get cold. The colder the tank, less air pressure.

Nitro/Compressed Air (N2/HPA)


  • Both N2 and HPA can be used interchangeably in N2/HPA specific cylinders in a gas state,
  • When speaking about these tanks, both the terms N2 and HPA mean the same thing,
  • Depending on size, the small units are all steel construction. More common is an aluminum cylinder wrapped in a composite or fiberglass material,
  • All cylinders needs to be tested every 3 years with a maximum life of 15 years,
  • All Nitro systems can typically screw into an ASA and has a built in pressure regulator (can be purchased as a fixed or adjustable output) to reduce the internal pressure of 3,000 or 4,500 psi inside the tank to around 850 psi output on a non-adjustable tank or a wide range on an adjustable tank,
  • Using a HPA tank, you are not severely effected by temperature,
  • HPA/Nitro tanks come in 48cu to 114cu refillable tanks in either 3,000 or 4,500 psi,
  • PSI means “pound per square inch”,
  • not all fields can supply you with refills,
  • When shooting tanks filled with N2, the tank will gradually warm.


Typical questions that I have gotten is the following…

Q: I hear the terms CU, CI, OZ, HPA, compressed air, Nitro, and N2 when talking about tanks for paintball. What does all this mean?

A: CU and CI mean the same thing. Just different units of measurement used. CU means Cubic Units. CI stands for Cubic Inches. This refers to the area inside of the tank that is filled. So, 68CI means there is 68 cubic inches inside of this tank. A 68CU tank has 68 cubic units inside of the tank… basically, the same size area. This term describes the size of nitro tanks.

OZ stands for ounces. This is how much liquid CO2 the tank is filled. From 4oz to 20oz.

HPA stands for High Pressure Air. Same thing as compressed air. This is the type of gas used when filling these types of tanks. Basically, it is the same air we breathe in everyday. A compressor will suck in air and jam in into the tank your filling.

A Nitro system is just another term used for HPA tanks. It is a type of gas used to fill a tank. Instead of the atmospheric air that is used to fill these tanks, it is almost pure nitrogen, or N2, that is used to fill the tank. HPA and Nitro can be use to fill the same tank specified as such. You cannot fill CO2 into a Nitro tank. Same is true about not able to fill HPA into a CO2 tank.

The difference between Nitro and HPA is when you fire a tank filled with Nitro; the tank will get a little warm as you fire it. A tank filled with HPA will not. And as many of you already know, CO2 slowly chills a CO2 tank.

So, CU and CI mean the same thing. It is the area inside of the tank to be filled. HPA and Nitro system means the same type of tank.

Q: About how many shots can I get out of a CO2/HPA tank?

A: It would depend on the operating pressure and general workings of your marker.

For the CO2 users, the number of shots depends on how cold the tank is and outside temperature. The warmer it is, the more the liquid CO2 will evaporate. On a typical 75* day with a marker shooting at about 850psi, you can get a rough estimate on the number of shots in the chart below.

CO2 Tanks

Tank Size Est. Shots Tank Size Est. Shots
7oz 350 14oz 700
9oz 425 16oz 900
12oz 600 20oz 1100

* Remember, the more you shoot, the colder the CO2 tank will get. The colder the tank gets, the less chance the CO2 will evaporate into a gas. So the PSI will decrease.

For Nitro, a general rule of thumb is to take the size of the tank and multiply it by 10 for a 3,000psi, 15 for a 4,500psi, or 17 for a 5,000psi tank. This will give you an approximate number of shots per fill on a marker shooting 850 psi. So a 68cu Java tank can give you about 680 shots on a 3,000-psi tank and 1,020 shots on a 4,500-psi tank (68 multiplied by 10 = 680; 68 multiplied by 15 = 1,020) on a fill. Weather doesn’t affect HPA tanks that much. Good N2 tank companies are Nitro Duck, Crossfire, PMI, and ACI. If you have the money, Smart Parts has a high recharge to them.

Nitro Tanks

Tank Size Est. Shots Tank Size Est. Shots
48cu 3000psi 480 48cu 4500psi 720
68cu 3000psi 680 68cu 4500psi 1020
88cu 3000psi 880 88cu 4500psi 1320
96cu 3000psi 960 96cu 4500psi 1440
110cu 3000psi 1100 110cu 4500psi 1650
114cu 3000psi 1140 114cu 4500psi 1710

The above tables are estimated shots per fill to expect from a tank. The actual shots will vary with weather, operating PSI, and overall mechanics of the marker. The above estimates is based on a marker operating at 850psi at a constant temperature of 75 degrees.

Q: Where and about how much can I fill my tanks?

A: For the CO2 users, most all of the paintball shops and fields can fill CO2 tanks. If they don’t, I would probably stay away from shops like that for anything paintball related. I have seen prices range from $2 to $10 for a fill of a 20oz tank.

For Nitro, this may be a little harder to fill. The larger fields and well-established paintball shops may have a compressor or have access to scuba tanks to fill the tanks. The ones that do, some may be limited to only 3,000 psi fills. The price for fills varies from $1 to $3 per 1,000 psi.

Another option that you can try is to fill these yourself. I offer guide lines on filling these tanks in Filling Tanks Article. For CO2 fills, you would need a scale, a valve to connect the cylinder to a bulk tank, and a bulk CO2 tank. For Nitro, a filled scuba tank and a valve to connect your Nitro tank and the scuba tank is needed. Some of the compressors that homebuilders use have an output of only 300psi or less. Ten times less than what is needed for a fill for a 3,000-psi tank.

Q: Are there any additions or modifications I need to do to my gun to use Nitro? I am currently using CO2.

A: No. Most of the time you can interchange a CO2 and a Nitro tank with no problems by screwing the tanks into the ASA. No need of add-on’s or modifications.

If going from CO2 to Nitro exclusively and have an x-chamber mounted on your marker, get rid of the chamber. It may hinder the HPA from fluently flowing into your marker. Better yet, change from an x-chamber to a regulator of your choice. This is an optional addition that you can add to your marker. Your shots will be more consistent and accurate. Some regulators will not work well with liquid CO2 getting into the reg.. I suggest an anti-siphon tube be installed into the CO2 tank. This is a bent tube that will keep the opening above the liquid CO2 and suck only the gas.

There may be a requirement by the manufacturer when using a regulator to only use nitro (i.e.: the Air America regulators are not CO2 tolerant and made only for HPA/Nitro use). Even if you do have an anti-siphon tube installed.

Another optional addition is a drop forward for an “on gun” system (CO2 or Nitro) to balance the marker. A drop forward will move the weight of the tank more forward and drop it down a little. If the marker is well balanced, you won’t get as tired holding the maker as fast because the tank is back heavy. It will also make the marker feel more compact. For a Spyder, an adapter is needed to “change” the off-center holes on the grip into in-line holes that most all drop forwards use. G3 Paintball has them for about $10. And drop forwards come in all shapes and sizes and with many features. They all do the same thing. Get one you like. There are at least two drop forwards that already have off center holes pre-drilled for the Kingman markers. Psycho Ballistic and Lapco are the ones.

And, don’t forget, if you add a new regulator and/or a drop forward, you may need new air lines and fittings.

Q: I understand that there is a regulator on a Nitro tank, should I bother with replacing the x-chamber with another regulator?

A: All of the HPA tanks comes with a regulator built into the tank to converts the 3,000, 4,500, or 5,000 psi down to about 850psi that is used by most paintball markers. Some are preset to a certain psi output (typically 850psi). You can get an adjustable regulator on the tank that you can adjust the air from 900 to 0psi.

It is not a bad idea to do this. This setup is known as a “dual-regulating” or “double regging”. It is not needed, but would not hurt either.

For most beginners, having two regulators is not needed if s/he has a Nitro tank. CO2 tanks do not have a built in regulator, so an “on gun” regulator is a good idea. Just make sure it is CO2 tolerant. Some good regulators for CO2 applications is the Bob Long Reg, the Palmers Pursuit Shop Stabilizer, Gen-X, Messiah, and I have also heard that the AKA Sidewinder does well. The Stabilizer in particular will not only regulates the pressure, but also does a great job at keeping liquid CO2 out of the marker. Not 100%. But is pretty good. CO2 has a bad habit of jumping up and down in psi because of the liquid CO2 expanding into a gas. Having a CO2 tolerant regulator will keep your velocity stable. Also, having the regulator just before the valve is the best. If you have an x-chamber after the regulator, you will make the regulator work extra hard.

For some heavily modified markers that use Nitro, it may be important to have certain psi input. For instance, my setup is a 110cu ACI tank (with a non-adjustable reg.) and a Vigilante on the marker. The output pressure of the tank is about 850psi. But my marker is modified to run at a much lower pressure. So the vigilante will convert that 850-psi down to my operating pressure of 300psi.

As a side note for the adjustable regulator on your Nitro tank, be careful not to turn down the tank regulator too much or you may starve your second regulator. Also, if the regulator is turned down to far and shoot fast, this can also starve the marker. This is called a ‘drop-off’.

Those that want to use both CO2 and nitro off and on, pretty much any regulator will work. So long that the regulator is CO2 tolerant.

Q: Why and where should I get testing done to my tank?

A: Strictly for safety to you and others off and on the field. It would put a damper on a day of play if all of a sudden, your tank explodes during a game or while it was being filled. CO2 tanks should be tested every 5 years. Nitro tanks, every 3. The reason there is a difference in years is because the CO2 tanks is not subjected to the stresses that Nitro tanks are. If I remember, CO2 tanks are rated to 1600 to 1800 psi. With a properly filled tank, the internal psi is around 1000 max. A Nitro tank is subjected to either 3,000 or 4,500 psi (depending on the tank) at a constant pressure. Considerably a lot more stress than CO2.

Now, do not concern yourself about tanks exploding. This happens on very rare occasions. All tanks have a safety mechanism called a burst disk. If the psi inside the tank gets to a certain point, the burst disk will break. All of your air will exit out of the tank in two different directions. Much better than if in one direction like a rocket. These burst disks can be replaced very easily and I have instructions in my burst disc article.

Where to get it tested is kind of easy. Some paintball shops may offer that or may know where to send it for testing. I have heard of a place that sells scuba gear can test (or know where to send). Keyword to ask for is “hydrostatic testing”.

These shops may also provide a service to fix tanks if they are damaged in some way. To help prevent some damage to the outer shell, a neoprene cover can be placed over the tank. Also a fill nipple cover can be used to keep unwanted particles from entering the tank and eventually clogging the internal regulator for nitro tanks. Thread protectors are also good to keep the threads in good condition. These can be used for both types of tanks.

I am sure I missed a few other items, but this is a start.

A small tip for those that use a regulator is to let the regulator do all the work for adjusting velocity. Doesn’t matter if you use CO2 or HPA. What I do is I screw in the velocity screw 3/4 of the way in. This will increase you velocity. Now, turn down the regulator and use it to adjust velocity. When you get within your target velocity, lock the regulator in place. Use the velocity adjuster to fine-tune the velocity. Most likely, you will need to decrease the velocity.

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