What is the principle behind the green and yellow Fuel Check indiactors on Blue Rhino propane tanks?

Answer 1

When you pour a small amount of hot water over the temperature indicator, the fuel remaining in the tank will act as a heat sink and cause heat in that portion of the tank to more rapidly dissipate than in the empty portion of the tank above the fuel level. The resulting temperature differential of the tank, i.e. the lower portion being cooler than the upper portion, is immediately sensed by the temperature sensitive ink of the fuel indicator, providing an indication of the remaining fuel level in the tank.