How much air?

[arnie2b]

It's not hard to work out.

I fill my FAC rapid to 210 bar, 400cc bottle, so 210 x 0.4 = 84ltrs of air
I get 55(ish) full power shots before they drop off at 120 bar, 120 x 0.4 = 48ltr
84 - 48 = 36ltrs used
55/36 = .654
so 654cc's per shot .25 at 42ftlb
or there abouts.

.177 is least economic with air, .25 most economic.

Is your gun regulated or not? I'm guessing it is.
I've read a rule before of about 1cc to 0.5cc of air consumption per ft/lbs in a regulated gun so that must be wrong?
Edit:that is plenum volume not air.

[RobF]

When we did the calculations using digital gauges on the Steyr .177 running at about 11.5 ft-lb it was using about 200cc at atmospheric pressure. (From memory)

[angrybear]

Is your gun regulated or not? I'm guessing it is.
I've read a rule before of about 1cc to 0.5cc of air consumption per ft/lbs in a regulated gun so that must be wrong?

No it's fast flow .

[trajectory]

Reading the replies & comments with interest. I like the idea of catching the air in a baloon to measure the resultant volume. Might try a modification to that with a plastic bag as being less constrictive it wont exert much pressure on the air inside thus reducing its volume a fraction.

Rich mentioned that a PCP might use a bit more air than a springer.....is this because the pressure increase with a springer is more efficient at getting the pellet moving? I suppose the pressure build up with a springer is much more gradual than with a PCP. Then if a PCP is a bit louder is this 'wasted air' expanding / venting after the pellets left the barrel, if so why isn't the dose of air reduced a bit, or is this what a regulated PCP is?

[angrybear]

Reading the replies & comments with interest. I like the idea of catching the air in a baloon to measure the resultant volume. Might try a modification to that with a plastic bag as being less constrictive it wont exert much pressure on the air inside thus reducing its volume a fraction.

Rich mentioned that a PCP might use a bit more air than a springer.....is this because the pressure increase with a springer is more efficient at getting the pellet moving? I suppose the pressure build up with a springer is much more gradual than with a PCP. Then if a PCP is a bit louder is this 'wasted air' expanding / venting after the pellets left the barrel, if so why isn't the dose of air reduced a bit, or is this what a regulated PCP is?

A pcp will use more volume because the air is the only energy source, a break barrel has a chuffing great spring

Remember school physics ? - the amount of energy that exists in the universe is finite all we can do is change the form of that energy,
So you transfer energy (which you got from food) from your body to compress the spring, which stores that energy in the spring tension, when released the spring transfers that stored energy in to the air as both pressure, heat & recoil, the air in turn transfers the energy in to propelling the pellet which in turn transfers it to the target.

The pcp only has the energy stored within the air pressure, so no effort from you & no recoil but the pay off is more air is used per shot.

A regulator (very basically) is a valve between the main store of air in the bottle & the single shot store in the firing chamber,it uses spring & air pressure to only allow a pre-set pressure in to the firing chamber rather than the actual pressure in the bottle so the pellet always has the same pressure pushing it across the whole bottle charge.

Air is "wasted" because the pellet has to be accelerated up to the required velocity within a very short time & barrel length and the transfer system is not all that efficient due to the need for the pellet to be free to move & exit the barrel.

[Rich]

With a PCP you start with air at high pressure and that pressure falls from the moment the firing valve opens.

A springer starts with air at NTP and initially compresses this air when the piston is propelled forward. Now, the work done by the spring heats the air, and anyone who has pumped up a bicycle tyre knows that the pump gets warm, and the harder you pump the warmer it gets.

The compression is done so quickly in a springer that this heat is not taken up by the metalwork, and it results in the air temperature rising. This increase in temperature leads to an increase in pressure, substantially more than the simple compression from swept volume to head volume would predict. Thus the springer effectively gets more work per cc of air than a PCP does.

It's called adiabatic compression.

[xbow]

I bought a 10m target rifle and modified the reg to use it for FT. The original reg had a very small air space within it. I made new parts for the reg to increase the volume to about one cubic centimetre and easily upped the power enough to use it for FT. Not sure of reg pressure but I think most are normally in the 50-60 bar range?