Originally Posted by
Nerull
CO2 cartridges contain liquid CO2 and are filled by weight, not pressure. Liquid CO2 will always stabilize around 900PSI (Depending greatly on temperature) no matter how full it is, until it's nearly empty.
Same reason they fill propane tanks on a scale instead of using a regulator - some gasses don't work that way.
Any gas that liquifies within the working pressure range of the container has to be sold by weight, because once you reach the liquification pressure any added gas liquifies, and while the percentage of liquid to gas changes, the pressure is constant.
As you draw off gas the liquid boils to maintain the gas pressure until there's no more liquid, then the pressure drops steadily.