A sure way to tell how much gas is left in a cylinder is to weigh the thing! This might not always possible. However, there are other ways to work out how much gas is left in a cylinder.
As far as I know, for most of the methods to be effective, the gas cylinder must be “in use” – ie providing gas to an appliance. This needs to be the case for half hour or so. Also, you need to be using a fair amount of gas, so the oven or boiler for example.
A closer look at the left hand cylinder in the photograph above clearly shows ice has formed on the cylinder. LPG is present in the cylinder at the “ice level” and below. This cylinder had been in use for a while, providing gas to the central heating system.
You may also have seen magnetic devices that stick onto the side of cylinders. These work by showing where the temperature differential is on the cylinder – ie the difference between where the level of LPG is and where there is nothing! Again, the cylinder has to be in use for these to be effective. If you just stick one of these magnetic devices on the cylinder and wait for a reading, you could get a very inaccurate result if you have not been using the gas before testing for the levels.
Personally, I do not like the devices. If the sticker is on the cylinder too far above the LPG level or two far below it, you will not get a reading at all. The magnetic sticker needs to be in line with the levels within the cylinder. Consequently, to use one, you need a rough idea how much gas remains anyway.
How much gas is left in a cylinder – the wet flannel trick!
Another trick, again when the cylinder is in use, is to place a warm, damp cloth at the side of the cylinder and note where the condensation line is – the condensation level reflects the level of LPG within the cylinder, just like the example with the ice as shown above.