Does anybody know what tool(s) and process I would use to measure the volume of gas contained in something? For example - imagine I have a 3 liter bottle filled all the top with jellybeans, and the inside of the bottle is at regular atmospheric pressure. What tool would I use to measure how many millimeters of gas the bottle contains? Footnote: try not to get caught up in this exact example. The container can be anything, the solid matter inside the container can be anything, and the gas can be anything. I just want to know of the tool(s) and process part for returning the measurement.