When activated, this propane tank gauge automatically senses the propane in your tank and indicates the amount remaining.  How does it work?  This Removable Acc-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.  Attach this propane tank gauge on the side of your 20lb. tank.  Magnetic and movable from tank to tank.  Fits all R.V. and B.B.Q. tanks.  Visually shows exact level of gas.  Easy to use and will last for years.  Will not fade in the sunlight.  Made by Hammerhead Products.  Made in USA.