Ok, so I tried my Craftsman digital temp gun on Thanksgiving, Had to get an early start because of a birthday/Thanksgiving day feast/open house. My job was to fry two fifteen pound birds. I had two propane cylinders and I knew one was close to empty and the other was close to full,,(by weight) I had the close to empty hooked up to the fryer already but didnt know exactly how much propane I had,, SO after reading this thread I wanted to try your idea of using the digital temp gun.
Worked great!! first tank I started my reading from the top of tank to the bottom,, top of tank read 34.6F, close to the -1/4 mark it read 35.2F. As I used the propane and the liquid turned to gas it boils,, after a few minutes of
burn time I could see a definitive liquid line on the tank,, top was 34+F while just below the 1/4 tank it read 36+F,, As time went by the differential got higher and I could really tell the tank level.. Also, as I burned more gas I could see a condensation line forming, until the tank went empty,, Swapped out tanks and again the test worked perfectly!!!
So,, next week when I pick up my trailer form storage I will measure and draw a line so as I know my current level!!!
Thanks for the tip!! That was fun!!!( gave me something to putz around with while the birds cooked)!!