We have had much debate about the following:
Where the quantity of agent in the container is determined by special measuring devices, these devices shall be listed.
the substance of the debate is this: Does a listed ultrasonic measuring device meet the standard? If all you are doing is marking a spot on the outside of the cylinder, what are you comparing to? Even if the measurement is always taken at the same temperature 7 the cylinder pressure remains the same, how can you be sure this was the correct level to begin with?
With a listed Liquid Level Indicator as part of the cylinder assembly, you are able to accurately determine the weight of the agent in the cylinder based on the LLi reading, temperature and pressure. My stance is that in the absence of a listed LLi as part of the cylinder assembly the only accuarte method for determining agent quantity is to weigh the cylinder.