Air Gun Home :: View topic - The Importance of Volume

radar · Silver Status Member

I'm confused by the opening statement. Do all PCPs have regulators then? I was under the impression that the basic mechanical PCPs such as the Discovery and Sam Yang were purely controlled by a more-or-less self compensation of the hammer-valve-spring design as it operates directly against the air pressure in the storage tank.

rsterne · Moderator

PCPs come in two flavours.... regulated and non-regulated.... A non-regulated PCP works (ideally) by operating in the balance zone between hammer strike and air pressure.... This creates a "bell-curve" of velocities starting a bit below the maximum (4-5% is nice), slowly climbing, plateauing, and then slowly decreasing and you stop shooting when the velocity drops somewhere (4-5%) below the maximum....

Some PCPs use a regulator placed inside the air reservoir right next to the valve.... especially if they are converted from a non-regulated gun.... That arrangement restricts the power because the regulator can't "top up" the pressure during the shot cycle, so you only have the air that is in the valve available.... The same thing applies if you have a PCP with a valve with a very restricted inlet, like a stock Disco (the gauge mounting bolck only has a 1/8" hole through it)....

Think about it this way.... Let's say the valve holds 4 cc and uses half of that to produce a shot.... It starts at 2000 psi and ends at 1000 psi, so the average pressure during the shot is 1500 psi.... If you open up the inlet side of the valve to the air reservoir, then (assuming a large reservoir) the pressure stays at 2000 psi during the shot because the air can get into the valve easier than it can get out.... You can get more power because the average pressure is higher.... While this is unimportant at low power levels, it becomes critical as you ask the gun to produce more and more power....

The importance of volume between the regulator and valve in a regulated PCP is what this thread is all about....

Bob

radar · Silver Status Member

Now I fully understand. Thank you for your patience. I'm still a bit of a PCP noob and trying to learn all I can. The physics and mechanics are fascinating.

garththomas · Veteran Member

I suppose this sort of mod wouldn't work in a qb without it being first m=changed to pcp

rsterne · Moderator

Opening up the valve can add a bit of velocity to a QB on CO2.... Archer Airguns sell a high performance valve.... What you have to understand is that the shot count will go WAY down just to get a bit more velocity on CO2....

Bob

Doc Sharptail · Member

This could conceivably get worse than the airguns themselves....

From Luke's Machinery in Winnipeg. There was a golden glow coming from the west end of the shop- where the lathes and mills are Tense Smoker

Good thing is I was on my way to work, and short on time...

Good things are happening. Finally on my way to a "done by me" brass flat top piston Mr. Green

Regards,

Doc Sharptail

spike589715 · New Member

Hi Guys
is it really ok to put these pressures in this chines tube ?
they were designed to hold together with co2 pressures - not 1800 psi
this sounds to me like a bomb waiting to go bang ??
my HPA 79 runs on a regulated 850 psi which is about as high as i would go ?

are you all using uprated tubing ?

cheers
simon

rsterne · Moderator

CO2 regularly reaches 1200-1400 psi (90-100*F), and can reach 1900 psi at 120*F....

Any CO2 gun SHOULD be engineered to stand at least 1900 psi with a decent safety margin.... The boxes of powerlets come marked "do not store over 120*F".... CO2 bulk tanks are rated at 1800 psi, and the burst disc is, in fact 3000 psi....

Am I recommending you use those pressures in a QB?.... no, I'm recommending that you protect the gun with a 1.8K burst disc so that the pressure won't exceed that produced by CO2.... The lowest pressure failure I have heard about is the tank block screws in a QB79 shearing off at 2600 psi because of a regulator failure and no output side burst disc....

Bob