Oh, yes you could Jean. But with helium you would be measuring "lift" (negative weight); we all are familiar with that, right? Remember, I'm figuring to start with a collapsed bag with little air in it. Not a perfect vacuum, of course, so not a perfect flow test. But I think it would be within 5%, which is a whole lot better than what he has now... nothing. If I did my arithmetic right, that one- cubic-meter bag of propane vapor would weigh 1.5# in room-temperature air (plus, of course, the tare weight of the bag). A limp bag, by the way, not a rubber balloon that would require the propane be at some pressure to inflate it.