Hammerhead Products Accu-Level Removable Magnetic Propane Gauge Removable Accu-Level Propane Tank Gauge utilizes a very special yellow ink. When slightly heated, this ink will change color from yellow to orange. Upon cooling, it will change back to yellow. There is nothing that gets used up or wears out. Accu-Level Propane Tank Gauge will last for years. When activated, this Propane Tank Gauge automatically senses the propane in your tank and indicates the amount remaining. Fits all R.V. and B.B.Q. tanks and will not fade in the sunlight.