Harold Import 019pro Accu-level Propane Magnetic Bbq Tank Gauge Durable; 2-pack HAROLD IMPORTACCU LEVEL REMOVABLE PROPANE MAGNETIC TANK GAUGE DISPLAY OF 32 This 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. Easy To Use: Attach This Propane Tank Gauge On The Side of Your 20lb Tank. Pour A Cup of Hot Water Down The Propane Tank Gauge.