They don't always empty it before filling. Somebody might make the mistake of trying to add 20oz on top of what you already have left in there and burst your burst disk. You can empty it yourself by opening your valve all the way or you can ask the store to do it for you. I'd ask them to do it for me.
Why are you wasting your co2 like this?
With 20oz (or 24, 16, 12, 09 cylinders), if anyone tries to fill your tank without draining it completely, I'd take the tank back and say no thank you. Aluminum cylinders, or vessels, designed for CO2 can have a weight variance of up to 3oz. This being the case, the only way to accurately fill one is to completely empty the vessel and place it on the scale, resetting the scale to zero. Then the CO2 is introduced to the vessel and filled according to the more accurate weight on the scale. Some places have pumps where they can force the amount of CO2 into the tank. Those that don't, use the cascade method of fills. Chilling a tank causes it to draw CO2 in. To start a fill, several oz of gas is sent to the tank. Depending on the size of the tank, the fill is stopped and the fill valve is closed. Then the CO2 in the tank is vented.. This chills the tank and the fill valve can be reopened. The chilled tank will then suck in more CO2 until the tank is filled.
When a filler does not drain the tank and start at zero, your fill volume could be anything other than what you bargain for.
If a paintball shop has the right adapter, 5lb cylinders can be filled in a similar manner. I fill mine at our shop.
If anyone is in Northshore New Orleans area and needs fills, pm me. Special treatment for aquarium people. ;-)
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