Yes they do.. Here is how.
First the strip is an LCD (Liquid Crystal Display) that is sensitive to heat. It changes color as it changes temperature.
So you start by using the tank to power your stove and put on a pan of water to boil.. As the propane (liquid) in the tank evaporates it gets COLD.
THen with the tape on the tank you dump the boiling water over the tape on the tank,, where it changes color. that's the level of liquid in the tank