What Does the Air Weigh?

[grand-galop]

KJ!!  Due to the enlightment of the new discovery by Bob, I might say that the better efficiency will probably comes with bigger caliber and valve opening in those calibers.... The theory behind the AIR WEIGHT is meaning at the same pressure, it will move the at the same speed down the barel independent of the caliber used..

[rsterne]

Sorry, kj, a vacuum in the barrel will only improve the performance of a PCP by a very small amount.... A perfect vacuum would only increase the force across the pellet by 1% if you are using 100 bar HPA.... and only by 1/2% if you are using 200 bar air.... etc.etc.... Likewise, the air in front of the pellet does not weigh very much, because it is only at 1 bar pressure.... In the first example I gave (25 cal, 20 barrel), the air in front of the bullet only weighs about 0.3 grains.... adding just over 1% to the weight to be accelerated.... Applying a vacuum to the barrel would at most add 1% to the velocity and 2% to the FPE....

You are likely thinking of the impressive performance of ping-pong ball vacuum powered cannons.... They get that performance from very large calibers with long barrels, and a very light projectile.... where the weight of the air in the barrel is many times more than the ping-pong ball, and the driving pressures are quite low....

Bob

[K.O.]

well hot air molecules do matter... say the rifles reservoir is 150 cc of 3000 psi in a -10*f Montana winter Snow Shoe hunt...duh molecules is much

more densely packed than the same rifle on a  100*f  summer Nevada  Jackalope hunt... has a charge that weighs less...

Energy state/velocity of course matters..

[lloyd-ss]

well hot air molecules do matter... say the rifles reservoir is 150 cc of 3000 psi in a -10*f Montana winter Snow Shoe hunt...duh molecules is much

more densely packed than the same rifle on a  100*f  summer Nevada  Jackalope hunt... has a charge that weighs less...

Energy state/velocity of course matters..

Well, -10F and +100F are both outside my comfort zone, LOL, but you do make a good point about how temperature affects the energy available in a compressed gas.  For CO2, we know that would make a big difference.  But for a PCP running on air, I am not positive, but I think it relates to degrees Kelvin.
-10 F is 250K and 100F is 311K, or 24 percent higher. Does that mean 24% more energy is available?  Maybe a little off topic, but it seems relevant.
Lloyd

[lloyd-ss]

...............................
From what I can tell, the difference in weight can varry according to both the pressure of the compressed air in the cylinder, as well as the amount of moisture in the air going into the tank when it's compressed.  Pressure becomes a little less of an influence the drier the air is, but only to a point. Then the weight starts to go up the higher the pressure goes for the given size of vessel. Could be because there's no way to be absolutely certain that 100%of the moisture in the compressed air is removed while filling. I don't know this for sure, but that's my train of thought. ................................

Ricky, moisture in the air can certainly be a nuisance for people who pump-their-own in places with high temps and high humidity. (Is that you?  ) This sounds like a good topic for another thread. Just for yucks, air pumped at 95 degrees F and 95 percent relative humidity will have 10 times as much water in it    as air pumped at 60 degrees F and  30 percent RH.  Think of that ice tea glass in the humid summer and how the water just pours off of it.

[ackuric]

I am curious on the subject matter of temperature effecting available energy.

I understand that pressure rises/lowers as temperature rises/lowers...with that aside

Does temperature have another effect on compressed air and available energy? IE:

3000 psi @ -10f vs 3000 psi @ 100f, or 250k vs 311k?

I was under the impression that temperature only effects pressure by either raising or lowering it. I am too uneducated on thermodynamics to state the above concretely though D:

[lloyd-ss]

If you fill two identical tanks at standard temp and pressure (STP) and then chill one to -10F and heat the other to 100F, the pressures will be different, but the density and weight of the gas will remain the same.
Pv=nrT   {pressure x volume = number of gas molecules(kinda, ha ha) x constant x temperature}
So, if the container volume and the number of gas molecules stay the same, when the temp goes up, the pressure has to go up too.  If you have a digital gauge on a tank and watch it throughout the day, you might think the gauge is goofy with the readings always changing, but really, the pressure in the tank is changing with the temperature.

[grand-galop]

After reading all the post from the beginning, is it right to say that the weight of air is costing air gunners a price in performance??? In one post it says humidity in the air causes the tank to weight more..  But to push that air charged with humidity cost in the ACCELERATION DEPARTEMENT.
If the laws of physics are valid, the more an object weight, more energy is required to move it.. That being said,an object in motion weighting a certain amount retain it`s momentum more that a lighter object..  I think in both cases, the weight factor cancel out because the lighter DRY AIR accelerate faster but contains less momentum..

[lloyd-ss]

Per Bob's original post, the lighter the gas (generally) the higher the achievable velocity.  Helium guns are a lot faster than air or nitrogen guns.

For the internal ballistics of an airgun, momentum is rather irrelevant.  You are accelerating a specific mass, along with the mass of the air (a=F/m) and the heavier the mass, the slower the acceleration.   But after the pellet leaves the barrel, momentum is a good thing.  Heavier pellets carry more momentum (weight x velocity) and more energy (weight x velocity squared), there fore, they can generally hold their velocity better, resist cross winds better, and hit harder.

As for the additional weight of the water in the air, at worst case (filling a tank a 95 degrees F and 95% RH), the weight of the air in the tank will increase by about 3%, but only if the water stays in the compressed air and does not condense out inside tank.

[ackuric]

Doesn't the Available Energy increase as pressure increases? Meaning 1 psi @ 1 cc has around .905 FPE available when at 1,000 psi, where as 1 psi @ 1 cc has around 1.023 available FPE when pressure is increased to 4,000 PSI (with adiabatic expansion only for simplistic reasons)...which is only a 13% increase in energy over the 300% increase in pressure but, does the Mass of air increase that same 13% which would inevitably negate the increased energy available at the higher psi since it has to accelerate its own heavier mass?

Physics hurts my brain

[rsterne]

Ultimately, I think it is the pressure that is the critical factor in PCP performance, not the temperature.... As you raise the temperature, the pressure in the reservoir increases, but in an unregulated PCP that can actually reduce the velocity.... If you fill your tank cold, and then warm it up, the pressure will increase, and with it, the shot count, THAT is where the increase in the heat energy shows up.... as increased number of shots.... for most PCPs....

There are a couple of exceptions where hotter means faster.... One would be a gun tuned for a dump shot (maximum power on the first shot), where the increase in pressure caused by the higher temperature would produce more velocity.... The other is in something like Lloyd's "Hypergun", that hit 2162 fps.... The faster moving molecules in hotter air may help the performance of such a beast, as least in theory....

For the most part, changes in temperature or humidity will have little effect on PCP performance.... However, the density of the gas, which affects the amount of work that expanding gas must do in accelerating itself.... has a very real, and profound affect.... That is why CO2 or Argon are slower, and Helium is faster.... compared to air or Nitrogen.... Nitrogen is marginally more powerful than air, by (29/28) = 3.5% in its potential effect on FPE....

Bob

[rsterne]

Matt, to answer your question, the density of the air at 1000 psi is 83.2 kg/m^3, but at 4000 psi is 302.4 kg/m^3.... Although the pressure is 4x higher, the density has only increased by 3.63X.... That is the VanDerWaals effect in action....

Bob

[oldpro]

 Heres some good reading if anyone is interested and my air weighs less than your air nanana lol. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch4/deviation5.html

[ackuric]

Matt, to answer your question, the density of the air at 1000 psi is 83.2 kg/m^3, but at 4000 psi is 302.4 kg/m^3.... Although the pressure is 4x higher, the density has only increased by 3.63X.... That is the VanDerWaals effect in action....

Bob

Thanks for the answer Bob. Based on what I understand of that, higher pressure operates more efficiently due to having less mass thus providing you with slightly more FPE per psi/volume when compared to lower psi. So it does not negate itself, the #'s you helped provide me with for fpe available at 1 psi per cc at specific pressure is due to the above effect?

[kbstingwing]

Wow, Getting Technical, this sounds a lot like a Refrigeration Class, since air is ruffly 78% Nitrogen and Nitrogen is basically inert (within normal environmental temps), only 22% of the air's content which is other gases are affected by temperature, so it's these other gases which constitutes the difference in pressures when exposed to a heat source.

[ackuric]

Wow, Getting Technical, this sounds a lot like a Refrigeration Class, since air is ruffly 78% Nitrogen and Nitrogen is basically inert (within normal environmental temps), only 22% of the air's content which is other gases are affected by temperature, so it's these other gases which constitutes the difference in pressures when exposed to a heat source.

Nitrogen is a gas and is still affected by changes in ambient temperature (about one psi for every 10° Fahrenheit or 3%) in a 30-35~ psi tire...it certainly experiences wild swings when 10-20F changes occur at 3000-4500 psi? My guess is the same 3% and that # coincides with my experiences. If I take my bottle out from my house (75F) into the sun (90+F) I experience around ~6% gains...can't give a proper % and temperature fluctuation though as that is very rough estimation

I've gone hunting from a heated house/car to outside with my bottle and the temperature drop there can be 60-70 F..if not more and the psi drop is HUGE.

To touch on that, someone shooting a string in the sun versus someone who is not, the person shooting in the sun could experience upwards of 6% if not more in shot count/efficiency

[kbstingwing]

yes it does, not every thing can defy the laws of Physics.

[rsterne]

Pressure changes as a result of temperature in accordance with the ABSOLUTE temperature (Kelvin or Rankine).... Since 70*F is 530*R, a 50*F change in temperature either way results in less than a 10% change in pressure.... This applies to all gasses and over almost all the pressures we use in a PCP.... or a car tire.... It does NOT apply to a liquid boiling into a gas, like CO2, of course....

Bob

[grand-galop]

So!!!  To get back at the original question of the tread, does a bigger reservoir full of air penalize in weight compared to an empty one??? The biggest reservoir COMERCIALLY AVIABLE is 500 cc... In a Benjamin Discovery I dont think it is significant but in guns like the FX Royal 500 is it?

[rsterne]

A 500 cc reservoir, filled to 3000 psi, would weigh about 5 oz. more than it does empty....

Bob