this isn't mine, but i ran across it on another forum:
Link to Original Thread
Why YOU should consider using CO2 instead of HPA
These days, every where you go you’ll hear the chant “Get HPA”, “Never use CO2 on a high end gun”, “If you can afford HPA there is no reason to use CO2”, etc. HPA does have certain advantages, but most of the ideas that a majority of people have concerning HPA and CO2 is based on one thing – BS
BS#1 – HPA is in everyway better than CO2
This is nonsense. Without even thinking, I can name one advantage of CO2 over HPA – Shot capacity.
We know that CO2 maintains a constant vapor pressure of about 850 psi @ 70 degrees from 100% fill down to about 18%. Below 18% fill there is no longer ANY liquid CO2 in the tank. That is why the pressure drops linearly just like compressed air or any gas for that matter. In otherwords, from @ 18% capacity a CO2 tank behaves completely like a HPA tank of the same volume with 850 psi of air in it – the fact that the gas is CO2 and not N2 or Air (78% N2 20% O2 and 2% other stuff) has no bearing on its pressure/volume relationship as long as everything stays gaseous -- remember PV=nRT; the Ideal Gas Law? Therefore, there should be no difference whatsoever between two tanks of equal volume @ 850 psi, whether it is filled with HPA or CO2, in terms of the number of shots you can get out of it.
We also know that at 100% capacity, a CO2 tank has 5.5 times as much propellant gas as it does 18%. Therefore we can expect 5.5 times as much working gas from a fully filled Co2 tank as one @ 18% capacity. If the gas doesn’t liqueify, then you’ll have to pressurize it to 5.5 times the pressure to squeeze the same amount of working gas into the given volume. That is for a HPA vessel, you’ll get 5.5 times as much working gas compared to the same vessel @ 850 psi if it is filled to 4675 psi.
What we can deduce from the above is that a CO2 tank is roughly equivalent to a HPA tank of the same internal volume filled to 4675 psi. The difference is that for CO2, from 18% capacity onwards pressure is constant and part of the gas liquefies. For HPA, pressure just keeps going up. Because the CO2 vessel does not need to be rated for 4500~5000 psi, it can be made lighter and cheaper.
BS#2 – HPA is more consistent than CO2
OK, this is generally true. HPA setups are generally more consistent than CO2 setups. But this is not because it is HPA. It is because CO2 is sometimes used unregulated (as is frequently the case with Tippmann users) or with a single regulator, whereas HPA is almost ALWAYS double regulated. A single regulator will have an effective ratio of between 50:1 to 100:1. That is on the average, the output pressure increases 1 psi for every ~75 psi drop in input pressure*.
In a double regulated setup, the output pressure from the high pressure regulator on the tank (when you go from 4500 down to 900 psi) will increase by about 48 psi. This is why there has to be a second regulator inline between the first and your marker’s gas input. The second regulator reduces the output pressure fluctuation to only a 0.64 psi decrement. This is where HPA’s consistency comes from.
In a CO2 tank, the pressure changes are actually MUCH LESS than in a HPA bottle. At 70 degrees F, CO2 is practically constant pressure between 15% fill and 100% fill – about 850 psi. The major change in input pressure from a CO2 tank comes from the chilling of the tank due to vaporization of CO2 within the tank which is an endothermic process -- which can be substantial sustained high rates of fire. If you empty a whole hopper in five or six rapid bursts, you stand to chill the tank by about 20 degrees F. This reduces the tank temperature to roughly 50 degrees F. At this temperature, the vapor pressure is roughly 660 psi. This is a 190 psi change, but still significantly less than what you will experience in a HPA tank. Even at a freezing 30 degree F CO2 maintains a constant vapor pressure of ~500 psi from 100% down to almost 10% of its tank capacity. Remember it takes a lot of shots to effect the kind of temperature change we are talking about, and even then CO2 has less pressure change than HPA. Also, at any particular temperature, CO2 holds constant pressure for almost 90% of the tank’s capacity, HPA on the other hand loses pressure constantly from 100% down to 0%. Hence, CO2 setups only need ONE regulator to maintain reasonable output consistency – about 4.7 psi across 40 degrees of temperature flux. This is MUCH less than the operational output flux of 48 psi were you to use one step regulation on a HPA setup!
Now, comes the best part – who says you cannot use double regulation with CO2? Put two regulators inline and you’ll get a level of consistency which can exceed that of HPA setups! I do it on my Ion, and there a quite a number of Automag users in the old days which run a pre-reg between their CO2 tank and the A.I.R in the Automag.
* This holds true until the input pressure drops below the set output pressure at which point the regulator becomes effectively a gas-through foregrip (or a gas-through cylinder), and the output pressure is the same as the input pressure.
BS#3 – CO2 cannot be used with high end guns
Yes, a lot of high end marker manufacturers tell you that using CO2 in their guns voids the warranty. And there is a good reason for that – IDIOT USERS. Whereas doing something really stupid can cause a HPA tank to blow up and maim you, that is not really the marker manufacturer’s problem. However, running liquid through a electro-pneumatic marker can trash your solenoids which is the manufacturer’s problem!
The fact is that as long as you use an anti-siphon tank (and orient it correctly), use a regulator and do not do moronic things like fire 30 shots with the marker upside down. You are not going to get liquid in your marker. Even if a little liquid gets into the anti-siphon tube it will boil off before getting to the reg for the most parts. If not, it’ll boil off within the reg or at the regulator output where pressure is 140~400 psi on most higher end guns. The chances of you getting an incompressible liquid slug into the solenoids is practically zero. Even if you shoot 5 or 6 shots sideways or upside down, it is not going to happen. You have to literally dump two or three dozen shots while drawing liquid to screw up.
Another concern is whether the marker – or rather its lubricants – will function as CO2 chills down. Well, if you marker uses Dow 33 grease or a proper marker oil like KC Oil, it will. It will because these lubricants will work way below 0 degree F and at that point you’ll run out of working pressure before you run the lubes solid!
BS#4 – CO2 is a dirty gas
We are not exactly running medical grade CO2 in our markers. So I wouldn’t venture to say the CO2 we use is squeaky clean. But all you have to do is look at a mesh filter placed between a tank output and an ASA adapter after a dozen paint balling sessions. Let me tell you want you’ll find… probably nothing. I rest my case.
BS#5 – CO2 is less safe than HPA
Actually I think the reverse is true even though neither is really dangerous unless you do really moronic stuff to it. CO2 cylinders will withstand about 1500 to 2500 psi before bursting the pressure relief disc. HPA tanks also have safety releases to prevent them from blowing up due to over filling. But fiber wrapped HPA tanks are actually much more delicate than any CO2 tank. From a safety stand point, a 850 psi pressure vessel is inherently less dangerous than a 3000, 4500 or 5000 psi one.
You SHOULD consider HPA and CO2 based on their respective merits.
Choose HPA if you do not need shot capacity but somehow feel the need to shoot 3 hoppers full at 20 bps in 2 minutes. Choose HPA if you enjoy free air at your local field or you like to do the DIY Scuba fill stuff. Choose HPA if you somehow feel the need to shoot the gun sideways or upside down all the time – I don’t know why you will want to do so and you’ll need to address your feed problems first by getting a Q-loader or the like.
For most other purposes, you should consider CO2. You’ll get a lot more shots for the same bottle size. You’ll hence be able to avoid having to trek back to the fill station between games. You can actually get as good or better consistency with CO2 if you invest in a second regulator – a convenient route is to use a Palmer Stabilizer Female in place of your current ASA adapter and retain your current reg. Even if you don't a good single regulator will give you immaterial consistency differences between CO2 and HPA. CO2 tanks are also a lot more affordable than HPA tanks, and generally a lot more hardy. Just remember… ALWAYS use an anti-siphon tank (unless you use a remote or vertical bottle) and never do stupid things like shoot two dozen rounds upside down for dry firing fun at the chrono.