As I've written in another comment: > That's because the weight of the gas cylinder is relative and you got the point of reference wrong: The base weight is wh

As I've written in another comment: > That's because the weight of the gas cylinder is relative and you got the point of reference wrong: The base weight is when it's a vacuum inside the cylinder, because otherwise it doesn't even displace the air which you don't want to measure for your reference weight, making it appear heavier. Whatever you add inside the gas cylinder now will make it heavier and the difference you get is the weight of the gas.