The correct amount of refrigerant is critical for the performance and reliability of the heat pump.
Why weighing is used
Charging with a scale allows the precise amount specified by the manufacturer to be introduced.
Base quantity and additional length
In split systems, an additional amount of refrigerant may be required when the piping length exceeds the manufacturer's base limit.
Why pressure is not enough
Pressure changes with outside temperature and load, therefore it is not a reliable way on its own to determine the refrigerant quantity.