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Post by snyper on May 14, 2008 1:55:34 GMT 8
Working with bulk-fill CO2 guns - PART Iby B.B. Pelletier information from Pyramyd Air Blog We have discussed bulk-fill CO2 guns several times in this blog. It's time to talk about how they are filled. Last week, we got a question about this from Jim. Since the answer is not straightforward, I thought it was time to talk about it in some detail. Meet my 10-meter pistolMy 10-meter pistol is powered by CO2 and can use either 12-gram powerlets of bulk CO2. I have been running it on bulk gas from the beginning - about seven years. I find the bulk method gives me more control over the fill, so I know when it's time to top off. The importance of knowing the status of a CO2 fill is crucial in a match. Since few CO2 target pistols have a means of displaying the fill status (how many shots remain), controlling the fill is very important for a competitor. The one time my gun failed me was during an important regional match, when my pistol's bulk tank was running low and did not give me the fill I had anticipated. I shot a perfectly held 10 that dropped to a 6 (just below the bottom of the bull at 6 o'clock) because of decreasing gas pressure. The four lost points dropped me from the standings in my class (top three places) to fourth place. My type of gun does not have a removable bulk tank like the top-quality target pistols. Instead, my gun is filled in a smaller fixed reservoir by a separate small bulk tank. Bulk fill as a cost-saver!Even though my story sounds negative, it illustrates that bulk filling is a method of precisely controlling your CO2 gun. But it's more than that. It's also much cheaper. I can bulk-fill a gun for a nickel and get the same number of powerful shots that someone else gets from a 50-cent powerlet. If you shoot gas guns a lot, bulk-filling is the best way to go. Gas-guzzlers, such as the Farco air shotgun (which drinks 2.5 oz. of gas for 20 shots), have to use bulk gas. A CO2 powerlet would only last for one powerful shot with a gun like that. How do you get set up to bulk-fill?You need a bulk gas tank. I own three 20-lb. CO2 tanks and one 5-lb. tank that is more portable. The twenty-pounders are similar in size to an 80 cubic-foot scuba tank. They start out life as fire extinguishers and soda fountain gas tanks. They are easy to acquire, though you won't find them at Wal-Mart. Consult your yellow pages for the nearest industrial gas supplier or restaurant supply house. The industrial gas place will probably also fill the tank for you and do any maintenance you need. Gas tank requirementsLike scuba tanks, CO2 tanks have to be hydrostatically tested every five years. As a huge user of CO2 10 years ago, I used to consume the contents of a 20-lb. tank in about two years. Calculating the shots I got for a Crosman 111 pistol, one 20-lb. tank provided almost 30,000 shots for $14. That was eight years ago, and the cost of gas has no doubt risen since then. I have purchased two additional tanks in the meantime, both filled, so it's been that long since I went back to the gas supplier for a refill. A scuba tank holding air may give 2,500 to 3,000 shots per $3 fill in an equivalent air pistol, so both gasses are relatively inexpensive. Bulk tank needs a siphon tube.Since you want liquid CO2 to come out of the big bulk tank, there has to be a siphon tube inside. It reaches down from the outlet valve to the bottom of the tank. The CO2 gas in the tank pushes down on the liquid, forcing it up the tube and into whatever you connect to the tank. Without the siphon tube, you would have to hold the CO2 bulk tank upside down to force the liquid out first. As heavy as the tank is, you don't want to do that! Are large CO2 tanks safe around the house?Since the other name for CO2 tanks is fire extinguishers, they're not only safe, they ought to be in every home. Once when some kids abandoned a stolen car in front of my house because the engine was on fire, I put the fire out with one of my bulk tanks. So, yes, they are safe. Just store them where they cannot fall and damage the valves.
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Post by snyper on May 14, 2008 1:56:31 GMT 8
Working with bulk-fill CO2 guns - PART II
Today, we'll learn how to move the CO2 from the large bulk tank to a smaller tank or to the gun, itself.
It takes an adapter To connect anything to a 20-lb. CO2 tank requires an adapter to connect the big tank to whatever it fills. The threads on the big tank are always the same 1/2" National Pipe Taper threads (NPT). The threads on the tank or gun you are filling can be anything, including metric, as many of these guns are now coming from outside the U.S. Twenty-five years ago, adapters weren't much of a problem. The Crosman model 190 gas tank was the most common small tank that needed to be filled, and it has 1/8" NPT threads. The guns it filled had female threads to match the tank, so bulk-filling was a pretty straightforward process.
Little standardization! Outside the U.S., all sorts of other standards prevail, and guns made in other countries use many thread patterns that are not typically found in this country. Your first duty when purchasing a bulk-fill airgun is to ascertain where the adapter will come from to fit that gun or its tank to an American bulk CO2 tank. You have to use an American-approved bulk tank because they are the only tanks for which our DOT has technical data. As a result, they're the only tanks a filling station may legally fill. This is not a problem, as there are an abundance of bulk tanks, but you always need to think about the adapter situation.
Adapter seals The adapter that attaches to the bulk tank has some kind of gas seal on the tank side and perhaps a second seal on the small tank or gun side. These may be o-rings or flat seals, and they have to work because CO2 is under 850 to 900 psi at room temperature. Treat these seals with the same care as your gun seals. They need a light coating of diver's silicone to maintain their supple sealing ability, and they need to be kept very clean.
Make the connection Although I used to fill a number of different tanks and guns, I usually shot the same type of gun at any one time, so the adapter for that gun was left on the bulk tank. When making the connection of the tank or gun to be filled, it is important to remember that the adapter seals do the sealing job - NOT the tightness of the connection! Remembering this will preserve the longevity of your adapter seals as long as possible. In twelve years of bulk-filling with nine different adapters, I have never had a single seal fail.
Open the valve! When you fill from a scuba tank, you open the valve very slowly to keep from overheating the seals from the sudden compression of the air. With bulk CO2, it's just the opposite. The seals and everything else will cool down during the fill. Therefore, you can make the fill very quickly. I can fill an empty 10-oz. bulk tank in about 20-25 seconds. During this period, the first liquid that enters the empty tank flashes to gas, because the pressure in the empty tank is relatively low. As the liquid changes to gas inside the tank being filled, it cools that tank, lowering the pressure of the CO2 gas inside (remember, CO2 pressure is determined by temperature). Because the bulk tank you are filling from has been sitting in a room for a long time, it has assumed something close to the ambient temperature of that room, so its internal pressure will be higher than the pressure of the gas that's now inside the tank being filled. As a result, liquid CO2 will continue to flow into the smaller tank. You'll notice that the outside of the tank being filled will frost up and then melt into a heavy coating of dew. This happens in just a few seconds, and the relative humidity determines the extent of this phenomenon.
Weighing the small tank to be filled Because there is really no simple way to control how much CO2 enters the small tank during filling, the safe procedure is to weigh the small tank after the fill. Therefore, you MUST know the empty weight of every small CO2 tank you own! By law, a new gun comes to you with an empty gas tank, so that is the time to weigh that tank. Don't take anyone's word for how much it weighs until you weigh it on YOUR scale!
What if you overfill? An overfill of a small gas tank can be very dangerous, though there is time to deal with it safely if you do not delay. After the fill, the small tank will be very cold, which means the CO2 inside will be at a low pressure. The tank was designed to safely contain a certain amount of CO2 within a certain temperature range. If you discover that the small tank has been overfilled, simply fill your gun from it immediately. (The tank shown in the picture is rated to hold up to 4 oz. of CO2, so it's not overfilled. Max fill info comes with each new bulk tank.) That drains off the excess gas. You can verify the tank is now safe by weighing it again. If it is still too full, shoot the gun and refill it from the tank. If you don't have time to do this, simply store the small tank in a freezer until you can get to it again. The object is the keep the tank's temperature as low as possible until you can get the fill weight down into the recommended range.
Sound too technical? Bulk-filling CO2 guns is not rocket science. However, you are working with highly compressed gas and there are safety procedures that must be followed. The same is true for filling pneumatic guns from a scuba tank. I have tried to give you as much detail as possible, which I don't believe you'll find anywhere else. Even the owner's manuals of modern bulk CO2 guns don't go into the detail I have here, but these are things you need to know.
I need one more post to cover all the details I want to pass along, then I'll wait for your questions to tell me if there is anything else we need to look at. You may be a little confused about how "full" to fill the small tank you're transferring to. Don't worry, it's almost no-brainer. However, I want to spend a little time talking about it, because there are safety considerations.
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Post by snyper on May 14, 2008 1:58:14 GMT 8
Working with bulk-fill CO2 guns - PART III
Today is the last post on this topic unless there are questions.
Small bulk tanks Small bulk tanks have been part of bulk-filling airguns from the beginning. Perhaps the best-known is Crosman's 10-oz. tank that accompanied their rifles and pistols back in the late '40s and early '50s. It was also marketed as a tire inflator to be carried in the car, because those were the days when tires went flat for reasons of their own.
Crosman's original idea was that shooters would send their empty tanks to a refilling station - BY MAIL! - and wait patiently until their return. Sounds good in the conference room - doesn't work that way in the real world. That's where the 20-lb. tanks came into the picture. People were unwilling to wait for their tanks to be returned. Because they're so simple to fill at home with the right equipment, many of them began doing just that! Pyramyd Air sells bulk CO2 tanks as well as CO2 adapters to connect to certain airguns.
Safety burst disks The Department of Transportation (DOT) requires some kind of positive pressure relief device that can vent certain tanks when the pressure climbs too high. The tanks on which this is required must contain gas at a certain pressure (and higher) and have an outside diameter of two inches or more. The pressure relief device is a small sheet of metal that ruptures at a given pressure. When it bursts, the sound is very alarming, but people nearby are relatively safe. Without it, a tank would explode like a hand grenade. The problem is that not all small tanks have them!
If the tank is less than two inches in outside diameter, it is not required to have a burst disk. Many airguns, such as the Drulov DU-10 and the Tau rifles and pistols, come with these smaller tanks and usually hold 125 grams or 4.4 oz. That's why I emphasized (in yesterday's blog) weighing the tank after filling it. Because there is no safety burst disk, you have to make certain there's enough room remaining in the tank to absorb a great amount of gas expansion. This room has been designed into the tank by the manufacturer. If a small tank is subjected to excessive heat, such as from a fire, it could become very dangerous.
Working pressure This is the most abused area of bulk-fill operations, because some people will not use their heads! A CO2 bulk tank is rated to a working pressure of 1,800 psi. But, we know that CO2 NEVER gets anywhere near 1,800 psi unless it becomes very hot! So this "working pressure" is really your safety net pressure for when the CO2 tank is heated to 130 degrees F (54.45 degrees C). CO2 at 70 degrees is at 853 psi, give or take. But some people read the number 1,800 and think, "I know everything is really over-engineered. So, if I fill this tank with air to 3,000 psi, that's not even double its working pressure. It should be safe." No, you ARE NOT SAFE! You are almost halfway to destruction. If your tank has no burst disk, you will be the last person to know when it blows.
Filling a gun Every bulk CO2 gun has its own special procedure for filling, so I chose my target pistol - the one I showed you two days ago - as the demonstration gun. To fill this gun, I first remove the protective cap from the threads at the bottom of the pistol grip, then flip the gun upside-down so the grip bottom is pointed straight up. I always put three drops of Pellgunoil into the filling port before attaching the tank. A 125-gram bulk tank is then screwed down tight on the grip. I can hear a brief hiss as the liquid in the tank rushes into the grip reservoir - but I'm not finished. With the tank attached, I thingy and dry-fire the gun twice in the upside-down position. CO2 gas puffs out the muzzle heavily when I do this. Then, I wait for one minute before disconnecting the tank. If I follow this procedure, the pistol will have more than 45 shots at good velocity - providing there was sufficient liquid CO2 in the bulk tank to start with. I will shoot only 40 and then refill the gun. In a 60-shot match, I arrange the pellets in my holder so I can see when it's time to refill.
Connecting the tank, firing two shots and waiting a minute is the entire procedure for filling this gun. Other guns will have different procedures. You have to get to know each gun's characteristics to fill it properly.
Is this bothersome? Yes, but I have the procedure down so well that nothing can derail me except when my bulk tank runs too low. I miss the positive feedback that manometers give on precharged pistols, but training has taken their place. When I shoot for pleasure instead of competition, I always find a way of counting my shots. Bulk-fill operations require more participation in the process than some shooters may like. However, a good bulk-fill gun can be a wonder to behold. I guess it comes down to making a choice.
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