This may or may not be of any interest but I thought I'd just keep a little record of it here for future reference for myself for that inevitable time when I want to know how much gas there is left in a part used Campingaz cylinder.
These figures relate to the 907 Campingaz butane cylinder only which I am planning to use as my "standard" fuel supply for car based camping.
A full cylinder weighs 6.5kg.
When full a cylinder contains 2.72kg of gas.
An empty cylinder will therefore weigh 3.78kg.
From the above by weighing a part used cylinder and subtracting 3.78kg the weight of gas remaining can be simply established with a reasonable degree of accuracy. This makes it easy to estimate cylinder life and whether a new one needs to be taken on a trip.
Alternatively, one can just shake it and guess :-)