TargetTalk

I don't claim to be an expert on this, but it seems to me there have been a lot of mistaken assumptions expressed in this thread. The general rule is that, in a closed container, the vapor pressure over a liquid is a constant at any specific temperature. Therefore, overfilling a cylinder with liquid - which does not compress - is not possible, and filling the cylinder completely with liquid will not cause it to explode. However, allowing a cylinder with ANY amount of CO2 in it to get overheated, may cause it to explode.

In the case of CO2 cylinders for air guns, there needs to be enough gas-filled space left in the cylinder so that first, the cylinder valve can be opened by the striker or the pin fitting on the gun, and second, when the valve is opened, a large amount of liquid CO2 does not come through along with some gas. If that happens, the variable amount of liquid CO2 will vaporize into a variable amount of gas, thereby giving a variable velocity to the pellet.

This was a problem with the first generation of CO2 target pistols, e.g. FWB Model 2 and the corresponding Walther (don't remember the model number), in which the striker contacted the cylinder valve directly. So the pellet gun designers started putting an expansion chamber in between the cylinder and the main valve, instead of metering directly from the cylinder. This improved consistency quite a bit. Those first generation CO2 pistols are far more prone to CO2 related velocity problems than are the later ones.

The bottom line is, if you have a first generation CO2 pistol, weigh your cylinders. And if you have any CO2 pistol, do not expose the cylinders to either extreme heat or extreme cold. In extreme cold, there may not be enough vapor pressure to reclose the cylinder or gun valve, and the cylinder may vent. And in extreme heat, there may be too much vapor pressure against the valve for the striker to overcome, or there could be an explosion.

HTH,
FredB

Ed Hall wrote:The one time I overfilled, as referenced above, was prior to a State Match. I filled a frozen cylinder on a room temperature tank for a Walther CP2. It shot poorly through the entire match. It did not readily display a malfunction, but it might have been dispelling liquid with the gas. I couldn't explain the wide pattern which was also 15 clicks off from my normal zero. I zeroed during the sighting targets, wondering what had gone on - had the gun taken a hard knock during the three-hour trip to the match? It looked fine. Additionally the group was much larger - was I a little unsettled from my three-hour drive? I felt alright and didn't see any excessive movement. I fired the entire match and then asked the director if he could weigh my cylinder. After 70 plus pellets, it was still overcharged!

Since then I've gone back to room temperature filling prior to any outings and I've never run out and never overcharged again. I may not have enough to fire all week, but I have enough for a couple training sessions or matches without worry.

Take Care,
Ed Hall
http://www.airforceshooting.org/
http://www.starreloaders.com/edhall/

And you didn't weight it!

Matt wrote:And you didn't weight it!

Quite correct, Matt! I grabbed it out of the freezer, pumped it overfull and headed out the door. In fact, I don't weigh my cylinders now when I'm filling them at room temperature. I don't remember what the weight was the times I checked, but it was below max and there's always enough for more than a match.

I guess perhaps what I'm saying is that by filling at room temperature I don't personally worry about overcharging, weighing or having to vent anything off. I just fill and go...

Hey FredB,

Good post! I think perhaps the full weight for the CP2 cylinder provides for a liquid level below the valve, which, when in the shooting position, is at the halfway mark of the cylinder. For my described trouble the gas pressure for the cylinder had not risen to some dangerous level like a PCP tank might if overcharged, but I think the valve being able to let liquid CO2 into the system caused the trouble. If I remember right the shots were actually extrememly low - possibly due to a low volume of gas.

Further query: The CP2 seems to be designed such that the cylinder and chamber become the same vessel. The vapor pressure holds throughout (above the liquid) and the flow is controlled during a shot by the structure of the porting and release valve. Do later models have some sort of secondary valve system that isolates the chamber from the reservoir during the shot?

Take Care,
Ed Hall
http://www.airforceshooting.org/
http://www.starreloaders.com/edhall/

pgfaini wrote:

Elmos said:

An overfull cylinder won't shoot properly. The resistance of the cylinder valve will be too high. But, if you use the bleed screw and let some CO2 out ... you'll find that solves the problem

The only rwo things I can imagine an overfilled cylinder would do, is not have enough volume of gaseous CO2 to: 1- propell the pellet, or, 2- allow the valve to open, which would require it to compress the gas a bit as it opens Am I understanding this correctly?

yes, to my mind your explanation is the more logical and the more correct !

Elmas

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I shot a Pardini K2 in co2 for several years, and I'll share my experience for what its worth.

I didn't want to sit around waiting for the cylinders to chill, but found with some experimentation that a "timed" 10 minutes in the freezer compartment of my fridge was optimum for a near perfect fill every time. Handling the cylinders with gloves or a cloth to minimize heat transfer from your hands was part of the regimen.
My interpretation of "overfilling" was that the cylinder was full of liquid with no room for gas to expand, which resulted in the first few shots showing the frosty expulsion. If mine weighed more than the "tara" plus the specified maximum amount of co2 I always bled off the extra. The Pardini did have a well designed fill adapter and bleeder valve. You might say that thats all well and good for a Pardini but I did fill a friends FWB model 2 cylinders several times with excellent results.

Merry Christmas,
SP

I had a FWB Model 2 Junior. I always chilled the cylinders and weighed them. I do not remember having any overcharged. I now have an LP1 in CO2 which tends to take on too much CO2 and does require bleeding.

My first really good AP was a FWB Model 2. I bought a triple beam scale to weigh the cylinders being extremely cautious.

I had trouble getting the cylinders full at room temp, so I did the fill-bleed-fill-freeze-your-hand thing, too. Someone suggested that I start cooling them. I started chilling them for 10 minutes in my freezer first. I could them fill them easily to full or almost so by counting x number of seconds on the bottle. I then verified their weight with a cheap postal scale. Worked great.

Stan

Ed Hall wrote: Further query: The CP2 seems to be designed such that the cylinder and chamber become the same vessel. The vapor pressure holds throughout (above the liquid) and the flow is controlled during a shot by the structure of the porting and release valve. Do later models have some sort of secondary valve system that isolates the chamber from the reservoir during the shot?

Ed,

It's been a while since I borrowed a friend's FWB 2, and I've never examined the Walther CP2. However, here is what I THINK is the case. IIRC, the FWB cylinder valve is normally closed when the cylinder is fully mounted on the gun. The gun's spring-driven striker hits the end of the cylinder valve pin, depressing it to release an amount of CO2.

There are two potential problems with this arrangement. If the cylinder has too much liquid CO2, then a variable amount of liquid will be released along with gas, and, depending on the temperature, will vaporize only in part, giving erratic velocities. Also, each cylinder valve is slightly different, potentially leading to different cylinders providing different velocities. This, in fact, was dramatically the case with the gun I borrowed.

I'm pretty sure the first generation Walther operated the same way, since I believe the Walther and FWB were based on the same patent. I don't know if the CP2 is one of those early guns. You could probably tell by removing the cylinder and firing the gun in live fire mode. Presumably you would see the striker move forward.

On the later guns, the cylinder valve is normally OPEN when the cylinder is fully mounted on the gun, and CO2 immediately fills a staging chamber on the cylinder side of the metering valve. The gun's spring-driven striker operates on this internal valve. Since the cylinder's valve is open, it would seem that liquid CO2 could enter the staging chamber, but that seems to be less of a problem with this design. And, since it's the gun's valve that does the metering and that valve does not change when the cylinder is changed, velocities should be more consistent. I am not aware of any secondary valve that isolates the cylinder, as you asked above. Perhaps Scott would know this for sure.

I hope this is the answer you were looking for, because it's about all I know on this topic ;-)

Regards,
FredB

Thanks Fred,

The CP2 has the second valve that is struck by the striker. When you mount the cylinder, it opens the cylinder valve and the gas flows into a small chamber area within the gun itself. Since the pressure is the same throughout the gas area, you would expect a consistent velocity, unless some liquid is allowed into the gas area. The only other things (I can think of) that might affect the system to a small degree (possibly not even measureable), would be the flow of gas through the system, and possibly the change in surface area that can allow the liquid to turn to gas. But, that's some pretty deep fluid mechanics, and it is probably not going to exhibit enough inconsistency to create a noticeable velocity change.

However the way the CP2 cylinders mount would allow liquid to be at the valve level when the cylinder is just under half full of liquid. So you could still have a pretty good gas area and have it contaminated by liquid.

Take Care,
Ed Hall
http://www.airforceshooting.org/
http://www.starreloaders.com/edhall/

Hi Ed,

Just looked at the exploded diagram for my Steyr LP1 (CO2), and noticed that the internal metering valve lies considerably above the mid-point of the CO2 cylinder. Of course the cylinder valve must be at the mid-point because the cylinder screws on the gun. Thus, there could be overflow of liquid CO2 through the cylinder valve and into the staging chamber, but, unless the chamber was nearly full of liquid, the metering valve would only admit gas. Maybe the same is true for your CP2? Probably a good argument for not shooting the gun upside down :-)

Regards,
FredB

To fill cylinders you can but them in the freezer for a half hour or in the Fridge compartment for all night. Freezing for more than 1/2hour will result in an overfilled condition. You can also chill a tank by putting what liquid it will take and then blowing most of that out with valve upward. Dumping liquid out valve down will not chill tank but it will chill whatever your blowing on.
Once a cylinder is showing signs of chill (frost or Hazy condensation around tank) it is cold enuf to receive a full charge from any room temperature master tank. Excess CO2 should always be removed prior to tank getting up to room temperature. Tanks are filled by weight and I've found as long as the tanks are not left in a hot car, they will not cause pressure relief unless they are overfilled by more than 10%. 11oz in a 10 oz tank will see pressure relief disc rupture @ 135F which translates to 3000 PSI.
It is best to error on the conservative side and avoid those that recommend all night freezing or double bleeding vessels. If you don't have a scale you are not going to know what is going on and overpressrizing is inevitable with these practices..
Overfilled cylinders will OVERPRESSURIZE the system as the pressure rises exponentially with added volume above the normal fill. Normal fills are about 65% of what the vessel can physcally contain so overfills are easily accomplished when freezing tanks for too long.
CO2 is self regulating between tank capacity and no liquid remaining but it is far from self regulating when filled beyond rated capacity.
CO2 is a very odd gas that can generate 10 thousand PSI at 135 F if the tank is severely overfilled(15 oz in a 10oz tank). Pressure reliefs built into the valves dump the pressure @ 3000 psi in most tanks when the system is overpressured and the relief device needs to be rebuilt and the tank refilled.
If the master tank is really warm it can also cause an overfill condition so if you don't have a scale you need to purposely underfill and avoid chilling the recieving tank altogether
If a CO2 pressure relief goes off inside a closed vehicle get your windows down because CO2 will chase all the air out of the vehicle and you will not be getting any Oxygen.

Reading this thread reminds me exactly of why I now shoot CA (having previously owned a CO2 LP5).

Before leaving home to go to the range I simply screw all of my gun cylinders on to the scuba tank and check they are full using the gauge on the tank. No messing about with freezers or scales, just fill and go.

Once or twice a year I take the scuba tank to my local dive shop where they happily fill shooters tanks and even make me a cup of coffee while I'm waiting.

In the extremely unlikely event that 2 cylinders per gun will not be enough then I will take a small scuba tank in the boot (trunk) of the car. Even if I don't, there will always be someone with a compressor, tank or hand pump.

With CO2 you need just one tank there sporto! So easy for the adult airgunner.

Jim E wrote:With CO2 you need just one tank there sporto! So easy for the adult airgunner.

I only need 1 scuba tank. I just have an additional small one if I am going away for a shooting intensive weekend (mainly because the velocity of my LP5 has been increased to assist with the 5 target event). A CO2 shooter would also have to take a tank along.

Jim E wrote:With CO2 you need just one tank there sporto! So easy for the adult airgunner.

For compressed air you can get away without one. Most clubs and ranges have compressed air if you run out. If your CO2 runs out then you are in real trouble. Like David I travel to comps and I take a pump for emergencies, in case they don't have CA.

Rob.

Need to upgrade the clubs you go to --- high quality ones carry CO2 and compressed air, Beisdes with CO2 you get twice the shots over PCP .. as many as 300 per fill or two full competitions. Besides the poster said he carries a bottle just in case so why not a untra small CO2 bottle since it gets twice the mileage.

I think both CO2 and CA are excellent for shooting .

Nice move up from spring piston guns a la FWB 65.

Each one on here seems to present a good argument for his choice of gas propellant.

People defend their choice perhaps because of a niggling feeling of insecurity. They want to be sure they are making the right choice.

Rest assured both choices are the right one .

In my city Alexandria, in Egypt , air is more plentiful , cheaper and more immediately available than CO2 .

An 80 cu ft scuba tank is filled to 200 Bar for L.E 5 ,(( that comes to about $0.85 in american currency or 45p Sterling !! )) Its done while-U-wait and takes approx fifteen mins.

CO2 is sold per kilogram and to fill a 5kg master tank costs L.E.100 , that comes to $18 or just under 10 pounds Sterling. Takes two days to collect.

So here because there are plenty of Diving shops and air is cheap , air seems a good choice.

Elmas

Jim E wrote:Beisdes with CO2 you get twice the shots over PCP .. as many as 300 per fill or two full competitions.

That's fine, but I wouldn't dream of going to a competition without full cylinders. It would just seem foolhardy to me. Of course, if you have to go through the extra fuss of filling a CO2 cylinder that may become a concern.

Jim E wrote:Besides the poster said he carries a bottle just in case so why not a untra small CO2 bottle since it gets twice the mileage.

What I actually said was "In the extremely unlikely event that 2 cylinders per gun will not be enough then I will take a small scuba tank...". As that only happens about once every 2 years I could actually do away with the small tank.

I am not saying there is anything wrong with CO2, it's just that CA is a lot more convenient.

No extra fuss with CO2 over comprssed air for the experienced person and with 300 shot that is more then enough to cover two full AP matches ... all with only one cylinder. With two CO@ cylinders that's close to 600 shots --- that should cover a match or two ... or three!

Jim E wrote:Need to upgrade the clubs you go to --- high quality ones carry CO2 and compressed air.

Not in the UK they don't. I don't even think our national shooting centre has CO2 but I could be mistaken as I've never enquired.

The bottom line is that we do whatever is more convenient. I can get my scuba tank filled up within a few miles of my house and I have a choice of 2 dive shops. I have 5 air cylinders for my two guns so that covers me for 90% of the comps I do. When I really shoot lots I take the pump and/or fill up at the venue.

Rob.