Fill 'er up!

[Rover]

Interesting article in the new (March 2015)USA Shooting News magazine explaining why you can get more shots out of a PCP cylinder one time and not another.

It depends on how much air is in the cylinder when you refill, which in turn generates more or less heat. More heat, more pressure, at least until it cools. Empty at refill, less shots. Near full at refill, more shots.

The article should be a regular at the top of this page. It might end many of the "is my gun leaking?" questions.

There was a blurb regarding the new lead-free "Green" pellets from H&N. Apparently they're almost good for only twice the cost.

[shaky hands]

Fascinating.

According to the data in the table, the authors claim that an initially unpressurized (1 bar) cylinder will have air inside it at temperature of 278 degrees of Fahrenheit immediately after filling the cylinder to 200 bar. They explain that the heating occurs because "the air already in it is compressed by the new air causing the temperatures to rise." This mechanism certainly is taking place during charging, but the number 278 F seems to be rather high. Yet, stranger things can happen in physics. I contacted the authors and asked for the details of their calculations, as they kindly suggested at the end of the article.

[David M]

Yes a empty tank will get warm to touch after a
fill from empty.
Leave it connected to the tank for a minute to
cool then remove. Little or no difference to shot
numbers.
Total tank temperature rise is only 15-20 deg only.

Tank temperature plays much bigger role in filling
a CO2 pistol and has a greater effect on volume/shots .

[mctrucky]

There was a blurb regarding the new lead-free "Green" pellets from H&N. Apparently they're almost good for only twice the cost.

What's not 'green' about airgun target shooting? It's not like we feed the spent lead to baby hamsters or anything. We tend to sell it to the scrap man for recycling - doesn't get any 'greener' than that.

[Spencer]

Was the number of shots in ether case lass than a shooter would need for a 60-shot match plus sighters; or is it merely an 'interesting'(?) bit of research?

[Rover]

But my hamsters love nibbling the expended pellets (especially the babies).

Oh yes, there was plenty of air for a match, but it seems many here are "deeply concerned" that they may not get more.

[TomAmlie]

When I fill my cylinders I always fill them slowly (e.g., 8-10 seconds to fill), under the assumption that if I can feel heat in/on the cylinder that's a sign that I'm putting some additional stress onto it. If I just crank the valve open right away the cylinder "fills" in a second, and gets warm. If I just barely crack the valve it takes longer, but doesn't generate the heat. My interest is in saving wear/tear on my gear, rather than getting 120 versus 110 shots out of a fill. After all, the air tank is 3 steps away in my basement range so taking the time for the additional fill is pretty much irrelevant.

Did they examine the effect of the "speed of fill" on the effectiveness of the fill?

[TomAmlie]

As an aside, I learned the hard way that the Steyr cylinder on the used compact LP10 I bought doesn't have the usual yellow/green/red bands; just the green/red.

Going to an informal AP shoot the other morning I looked at the cylinder and it was in the middle of the green range (didn't have my glasses on). In my Morini that's enough to do 80-90 shots, so being in a hurry I just packed up and went. Unfortunately, on the short Steyr cylinder - without the yellow zone - that's apparently the same as being on the yellow/green border on other company's cylinders. After 60 shots you're pretty much at zero PSI.

The last 10 shots I had to aim at the top of the bull to get into the 9/10 ring, instead of my usual deep sub-six hold.

[shaky hands]

Fascinating.

According to the data in the table, the authors claim that an initially unpressurized (1 bar) cylinder will have air inside it at temperature of 278 degrees of Fahrenheit immediately after filling the cylinder to 200 bar. They explain that the heating occurs because "the air already in it is compressed by the new air causing the temperatures to rise." This mechanism certainly is taking place during charging, but the number 278 F seems to be rather high. Yet, stranger things can happen in physics. I contacted the authors and asked for the details of their calculations, as they kindly suggested at the end of the article.

After waiting for three weeks for the details of their calculations, I have to conclude that the response is not coming. So much for the warm welcome to ask for them. The rest of the article has probably the same validity.

[David M]

Huumm.........just a lot of hot air

[holmqer]

I actually suspect that the temperature rise described is conservative, see an interesting but mathematically complex article on the Thermodynamics of a Tire Pump.

Google Thermodynamics of a Tire Pump and pick an article from Princeton

In principle an instantaneous rise in pressure from 1 Bar to 200 Bar starting at 23C (300K) would be huge, roughly 300K * 200 which would exceed the vaporization temperature of metals.

Imagine a fire piston which is a hand held device where you smack a handle to quickly compress air in a cylinder via a piston. The quick pressure rise in the cylinder sets flammable material inside ablaze which you pour out onto tinder to start a camp fire.

In filling an AP cylinder it is not instant, so heat from the low mass air entering is absorbed by the high mass vessel as the pressure rises so this theoretical temperature spike will never be seen. The vessel then radiates heat to the ambient environment. The surface temperature of the vessel will be much lower than the temperature of the air inside. The rate at which the heat transfers from the air to the vessel is proportional to the temperature difference so as the air heats up it transfers energy to the vessel faster. So a warm vessel after filling is an indication of hot air inside.

This is part of why you would fill the cylinder slowly to avoid creating a fire piston like event from occurring.

This whole thing is a very complex mathematical model.

[Andre]

When I used to shoot a Tau 7 match we'd put the tank in the snow to chill then fill the tank. co2 is most always at a constant pressure, so topping it off is a bit of a challenge sometimes. Worked like a charm.

But with modern day PCP's you don't need the extra air, unless your a sighter machine.

[holmqer]

Of course keep in mind that there is a difference between filling via a pump which takes 1 Bar air and compresses it to 200 Bar and filling from a SCUBA tank which moves 200 Bar air from one place to another.

Filling from a SCUBA tank gets some heat increase from air friction and some heat loss since the equilibrium pressure of the two tank system is lower than the pressure in the SCUBA tank.

Filling from a hand pump gets the whole pressure ratio times degrees Kelvin calculations in play (ideal gas law) but takes long enough that the cylinder absorbs heat from the compressed air fast enough to avoid any real temperature spikes.

[shaky hands]

Thanks for the article, Holmqer. Much more informative than the USA Shooting piece. Pretty much what it shows is that in case when you fill an empty (1 bar) cylinder up to a much higher pressure (200 bars) very quickly, the temperature of air inside the cylinder will be very close to 7T/5, where T is the ambient temperature. Of course T must be measured in Kelvins. For room temperatures of 73 F (T = 296 K) this gives 7*296/5 = 414 K which is 286 F. This is in the ballpark of what the article states, but somewhat different. The discrepancy becomes more significant with lowering the charging pressures. Apparently, the USAS authors obtained a different formula than the Princeton article did. I would be interested in comparing the two solutions, but unfortunately the response is not forthcoming.