The Magnetic Gas Level Indicator.
How it works:
On the label, each vertical bar is a different temperature liquid crystal (LC) mixture. There are 12 LC bars in total which ensures that whatever the ambient air temperature (where the bottle is stored), ther will always be at least one LC bar showing colour. When gas is discharged from the cylinder (ie being used) the liquid changing to gas inside the bottle causes cooling which creates a noticeable temperature difference on the cylinder's surface, above and below the liquid level. The cooling is caused by gas absorbing heat energy from the cylinder and it's surroundings as it changes from a liquid (under pressure) to a gas. The LC bar is highly temperature sensitive and is able to indicate the change in temperature by changing colour vertically (blue to green to red) and show where the gas level is. If the LC bar shows a constant colour the gas liquid is above (or below) where the indicator has been placed.