Normal filling practice:
1. By law, every cylinder under 100 lbs. must have a OPD (overflow preveition device) valve, which prevents any it from being filled more than 80%.
2. Add tare weight of cylinder to number of lbs. of propane it will take to fill it to 80% , and set the scales at a reading approx. 2 lbs. more than that combined weight.
3. The cylinder is placed on the scales, propane introduced into the cylinder, and the OPD valve should shut it off at 80% of tank capacity.
4. If scale tips, indicating the OPD valve didn't shut it off, dispenser refill valve is turned off and tank considered 'full' (80%).