I installed two of these in-line with my two 20lb tanks... It always reads in the green with the same reading with the tanks turned on. Then one day it read empty. I switched to the other tank and now it reads in the green all the time.
I don't think I have much faith in this gauge... I was hoping they would start showing lower pressure as they were being used.
I can usually tell in the mornings how much propane is in the bottle by just looking at the sweat line on the tanks...

I try to use the same tank all the time until it goes dry then switch to the full one. Then I can get the empty one filled back up. I use one of those D-RINGS with the spring lever on them hanging on the tank I am always using.

Roy Ken