Even though I have an automatic change-over regulator with an empty indicator, I will still take both tanks off and weigh them before an extended trip or anticipated high propane use trip. If my primary tank is below about 30 lbs., I will switch the regulator over to the other tank, and get my "empty" tank topped off. As these tanks seem to last forever, that procedure is probably unnecessary, but I still do it as a precaution.
My 30 lb. tanks weigh 54 lbs. full, and 24 lbs. empty.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263