Everything you wanted to know about CA tanks, but were afraid to ask...


CA stands for Constant Air.  CA tanks are designed to hold CO2 (carbon
dioxide) under pressure and provide paintguns with a supply of
CO2 at a Constant pressure, usually around 800psi.  This is required
by the paintgun, because a change in pressure will cause a change
in the velocity of the paintballs, and we try to keep that right
around 290fps.

First, some physics.  Every substance can be a solid, liquid, or gas,
so what I am saying about CO2 applies to most anything else, but some
of the "constants" such as specific temperatures, pressures, and
volumes will be different.  CO2 can be in three states: solid,
liquid, and gas.  The state it is currently in depends on the
pressure, volume, and temperature of the CO2.  If you change one of
these three things, one or both of the others must change as well,
to maintain a sort of equilibrium.

If you increase temperature, increase volume, or decrease pressure,
CO2 will move toward a gasious state.  Decrease temperature, decrease
volume, or increase pressure and it will move towards a solid state.

Lets start with a CA tank, and magically drop some liquid CO2 into
it. (not so much that it fills the tank though)  At room temperature
CO2 will change between liquid and gas at about 800 psi.  The pressure
in the tank currently is 0psi. (no pressure) Since liquid CO2 cannot
exist as a liquid at 0psi and at room temperature, it begins to change
state to a gas.  It's actually boiling.  As liquid is changed to gas,
pressure increases in the tank. (gas takes up a lot more space than
liquid)  When enough liquid has boiled, 800psi is reached in the tank,
and boiling stops.

If this tank were to be heated, temperature would be increased, and
therefore either volume must increase, (but it is confined in the tank)
or pressure must increase. (which it would)  If the tank is chilled,
volume would decrease, (again, no go) so pressure would drop.

OK you put the tank on your gun and fire a shot.  You've actually
increased the volume available to the CO2 by opening your gun's
valve and allowing some CO2 to move into it.  Pressure must drop
or temerature increase.  Since you're not torching the tank, the
pressure will drop, from 800psi to maybe 790psi.  Oops, now we can't
have liquid CO2 at room temp and at 790psi.... so it boils again,
until 800psi is reached.  Each time you shoot, CO2 boils to return the
tank to 800psi.

After a lot of shooting, all the CO2 will have boiled away.  The tank
may still be at or near 800psi, but it is actually considered 'empty'.
If you shoot again, pressure drops to 790psi, but cannot return to
800 because there is no liquid to boil.  Each shot drops the pressure
in the tank further, and your gun's velocity drops rapidly.  If you
have a semi, the drop in pressure causes your gun not to recock properly
after a certain point.

Due to another interesting point in physics, a substance cannot just
simply change state from solid to liquid, or from liquid to gas.  Energy
is required, in the form of heat.  It's not much, but it does take some.
If you fire your gun fast enough, you'll notice the tank get cold, or
perhaps even frost over on the bottom.  Prolonged rapid firing may even
temporarily FREEZE the liquid CO2 in the tank.

Now look at fill stations.  Here you have a 'bulk tank' that has a lot
of liquid CO2 in it.  These tanks have special tubes running from the
valve on top, down into the tank, almost to the bottom.  They're called
'siphon tanks', and the purpose of the siphon is to deliver liquid
CO2 from the bottom of the tank, rather than gas CO2 from the top.
(it's not safe to use a bulk tank upside-down)  Bulk tanks can work
without the siphon, but they are much more of a hassel, and fill
slower.

If we were to put a line between a bulk tank and an empty CO2 tank,
here's what happens: liquid CO2 rushes down the line, and even as it
is running through the line, it begins to boil.  By the time it gets
down the tube, enough liquid has boiled that the pressure in the CA
tank is rapidly rising.  Very quickly, the tank reaches 800psi, which
is what the bulk tank is at.  When pressures equalize, no more liquid
will flow.  There is a small amount of liquid in your CA tank, but
not much. (maybe 2oz in a 7oz tank, or 5oz in a 20oz tank)

At this point, the operator will close the valve on the bulk tank, and
then open a special "dump valve" in the line.  This causes the CO2 in
the CA tank to rapidly expand. (volume has been increased)  The CO2
in the tank changes state from liquid to gas very rapidly, and requires
heat, which it draws from the walls of the CA tank. (causing the tank
to frost over)  Once the pressure in the CA tank has been released, the
dump valve will be closed, and the bulk tank valve opened again.  The
liquid begins to flow into the CA tank again, but this time the
temperature of the two tanks is not the same, so the pressure at which
gas changes to liquid is different in the two tanks.  When the CA tank
reaches about 770psi, the pressure is great enough (at the lower
temperature) to actually change the gas into a liquid.  As more gas
arrives, (and pushes the pressure above 770psi) the gas changes to
a liquid, which takes less space, so pressure drops back to 770psi.
This continues until the temperature of the CA tank rises back to
room temperature, the CA tank becomes completely filled with liquid
CO2, or the operator closes the bulk tank's valve. (which is what
he should be doing)

Note that this "constant air" effect is working in the reverse way it
worked when you fired your gun - you dropped the pressure and it raised
it by boiling.  Now you RAISE the pressure and it responds by
*condensing*. (into liquid CO2)  The preferred pressure at the given
temperature is maintained.

If you manually chill the CA tank, or heat the bulk tank (NOT advised),
you won't have to waste CO2 to chill the tank first.  I have a 20lb
bulk tank at home, and throw my tanks in the freezer for 20 minutes
before filling them.  Storing your tanks in the freezer helps keep them
up to pressure when filled.  All tanks leak to some degree, and chilling
drops the pressure in them, thereby slowing the leak.  So if you are
not going to play for a few weeks, toss your tanks in the freezer so
they'll still be ready to go when you play next time.  Just be sure to
grab them before you leave - I forgot mine once.  That was fun to
explain to the group... "You forgot your tanks WHERE?" ;-)

Since most tanks are rated to hold about 40% of their volume in liquid,
the rest of the space is reserved for gas.  Above that, they may
burst, and that could be fatal if it is near your head at the time.
For this reason, there is a "burst disk" on your CA tank's valve.
When the temperature increases, and it cannot increase its volume,
pressure rises.  The pressure rises faster than the special pressure
at which gas changes to liquid, so the gas starts changing to a liquid.
This reduces volume, (liquid takes less space) and helps to slow the
rising pressure.  If not enough "head space" is left, (if you overfilled
it) all the gas will change to liquid, and after that, a small
increase in temperature causes a dramatic increase in pressure.
The burst disk will go quite soon after all gas has condensed.

It's under the bolt with the holes in it on the side of the valve.
If the pressure gets much above 1200psi, this thin wafer of brass
cannot hold the pressure, and a hole is punched in the middle.
Through the hole, the tank is allowed to rapidly vent all its CO2.
Similar to when filling the tank, this rapid drop in pressure causes
the tank to chill.  Most tanks, if they have much CO2 in them, will
outright FREEZE.  Some of the liquid in the tank will change to a
solid, and because the temperature is so low, the pressure in the
tank is very close to 0psi, so it'll take a bit for it to thaw.
Also, this is VERY cold, and can easily cause frostbite, and if
you are stupid enough to hold onto the tank while it is venting,
your skin may become frozen to the tank.  *ouch*    You will almost
certainly be able to shake such a tank and hear a large hunk of
crozen CO2 (dry ice!) rattling in the tank.

Burst disks should not cost more than about $1 each, (and even that
is really a rip-off, but they need to make their money) so it's
not a terribly bad thing to have happen.  It will happen if you
overfill the tank beyond its specified weight in CO2.  Use a digetal
fish scale to weigh it before and after filling.  It may also burst
simply because it got too hot.  Fill tanks 'short' a few ounces
on very hot days.


Some physics for thought...
The formula we use to calculate things is: pv=nrt

pressure * volume = number of moles of substance * substance's constant * temp
I don't have any of the units information to make this formula usable,
but you can see how changes in one thing cause a change somewhere else,
and in what direction.