Why are barbecue gas cylinders weighed while they being filled?

Propane and butane are sold by weight, not by volume; the weight also determines how full the container truly is. Gas station gauges can be easily tricked, but not a scale. For safety reasons, refillable propane cylinders are designed to hold 80% of their water capacity by volume; some cylinders have an 80% fill line as a secondary way to indicate when to stop filling them. Butane cylinders are filled by weight according to the cylinder’s tare weight (TW) marked on the cylinder collar and the density (SG) of butane which varies according to ambient temperature.