co2 and self regulation

[Ribbonstone]

Basic valves do self regulate to a point (basic being the striker/valve stem set up used on most guns).

Can get a PCP valves to self regulate over a 1000 psi change. (Lets say from 3000psi to 2000spi).  Lets call that 30%.

So what’s wrong with co2 guns?  Same valve system, so why can’t they self regulate over the same pressure range?

Well…they can and do.

Co2 pressure is not all that simple and not all that easy to predict.  It is not a direct function, has other variables at play.  But for the sake of this discussion, will act like it’s easy.
http://www.privatedata.com/doomlabs/science/co2pv.gif

If you consider the PCP valve as being able to handle a 30% change in pressure, then so can a co2 valve.  If set up to run well at 78F (call it 900psi), they do pretty well up from 15% below that to 15% over that.  That works out to 1035psi (86F) to 765psi (64F).

Most owners tune their co2 guns for high power at normal temps.  By smacking the valve that hard, they are moving the self-regulation towards the upper end.  When I tune a co2 gun to work well in the summer (lets call it 90F) it probably still has a 30% range where it self regulates.  But because pressure changes more drastically with small changes in temperature, that gun has a narrower temperature range (same 30% pressure range, but that might be only 15 degrees of temperature..call it 97F to 83F ).

Co2 guns tend to self-cool.  It’s not the ambient temp, but the temp. of the gas that makes for the pressure.  Shoot enough times, moving enough gas, converting enough liquid co2 to gas, and the co2 in the gun (an sitting in the valve ready to go) will think it’s 50F even if it is 90F outside.

[rangerman]

Given enough time CO2 temperature will equalize with the ambient temperature outside the tank.
CO2's self regulating characteristic relies on the fact that at a certain temperature (say 21°C) the vapor pressure stays at approximately 855 psi as long as there is still liquid CO2 available in the tank to turn into a gas. Regardless of the gas volume in the tank the vapor pressure would be the same as long as the temperature is maintained and liquid CO2 is still available. Higher temperature would of course result in higher vapor pressure.
Now, as far as providing power to a pellet, you would want to have enough gas volume at a certain high pressure to continue pushing the pellet all the way to give it the highest acceleration. A high pressure/low volume CO2 that does not maintain high pressure long enough ( due to rapidly increasing working volume behind the pellet inside the barrel) may not be any powerful than a lower pressure/larger volume CO2 that continues to provide adequate pushing pressure longer to a pellet all the way to the end of the barrel.
That is where the amount of valve opening time and proper CO2 volume come into play to provide that optimal pressurized CO2 that is needed to impart the best energy to a pellet.

[rsterne]

It's not particularly difficult to tune a CO2 gun to self-regulate over a rather wide range of temperatures.... The benefit is EXTREME shot count.... Here is a 2250 fitted with an 18" .177 barrel tuned so that it self-regulates between about 35*F and 75*F.... That is nearly a 40% range of pressures (low = 60% of high).... If you want a different temperature range, it woud just be a matter of tweaking the RVA (power adjuster)....



The three shot strings on "High Power" show the natural tendency for the velocity and shot count to decrease as the temperature drops.... The three strings on "Low Power", at the same three temperatures, show anywhere from 2-3 times as many shots all within about a 8% ES between 46*F and 75*F.... Note that the lowest velocities (but highest shot count) were at the highest temperature.... I did try the gun straight out of a fridge (about 35*F) and the velocity was still over 500 fps....

The two power settings are the result of a "Dual Power Hammer" made some years ago by a tuner.... It simply had two cocking positions and a built in RVA....



I have since removed the DPH and gone back to a non-adjustable rear plug and achieved the same self-regulating results by just clipping the hammer spring and using a lighter hammer....

Bob

[Motorhead]

That hammer looks very much like the SSP-250 unit that also has dual power positions.
* I have noted on my own SSP that on low power velocity with 8 grain pellets in in around 500 fps and can shoot near 70 shots !!
Bump to high setting velocity jumps up to @ 650 and shot count dives to @ 25 shots and shots very much sound like dump shots having a LOT of Co2 fog leaving muzzle each shot.  VERY efficient on a lower setting, Lousy efficiency when set for power. 

[Ribbonstone]

Pretty much what I was thinking. Its not that the valves of co2 guns are some how different, they do self regulate,  its that the pressure range needs to be thought of as a percentage AND that rise  by temperaturew/percentage of fill volume is an almost non-euclidian change.