To find out how much gas is left in the canister, the canister should be weighed and the result compared with the weight indicated on the canister.
Weight indicated: 5 kg canister
Newer canisters have the total weight (somewhere in the region of 12 kg) printed on them in ink. Older canisters have the total weight (somewhere in the region of 7 kg) stamped into the collar.
Weight indicated: 10.5 kg canister
Many canisters have the total weight printed on them in ink. Depending on how old the canister is, the total weight will be between 22kg and 26kg.
Some canisters have a metal chip in the outlet on which the total weight minus 20kg is indicated: so, for instance, “2.7” means 22.7kg.
If neither of the above is present, the empty weight is stamped into the base ring. Depending on how old the canister is, the total weight will be somewhere between 12kg and 16kg.
Weight indicated: composite canister
Composite canisters are not individually weighed at the end of the production process. This means that, unlike with steel canisters, the weight of the remaining gas content cannot be calculated. Gently shaking the canister makes the liquid gauge visible, allowing you to make a rough estimate of how much gas is left.
Calculating the remaining gas content
Where the total weight is indicated: total weight – result of weighing = weight of gas used.
Where the empty weight is indicated: result of weighing – empty weight = weight of gas left.
How long will the remaining gas last?
The answer will depend on your rate of consumption. If your gas appliance gives an indication of grams per hour, you can easily calculate the remaining duration of gas consumption. Usually, however, no such indication is given, in which case you will be forced to rely on experience: if you know roughly how long a full canister will last on average, you can work out the remaining gas as a percentage of the whole canister and base your estimate of the remaining duration on that.