Review on GrillPro 00030 Magnetic Gas Level Indicator: Effortlessly Monitor Your Propane Tank Levels by Philip Wagner

I see there is a manufacturer comment that they are rated for use with 20 lb tanks. Shame on them they didn't talk about it directly. But it works well for 20lb tanks if you're using fairly high BTUs. If you put this thing on a tank that isn't being used or is burning low fuel like I use my smoker, you might not see anything useful at all. However, if you pour hot water over the area, you'll see (within an inch or so) a temperature gradient between the liquid and gaseous phases, even if you're not burning fuel. Better than fumbling around with your hand, so to me it's worth the few bucks it's worth.