The amount of refrigerant to charge in the system can depend even from the season when the charge is done (winter or summer). In winter, for example a lot of CO2 could stay inside the Gas Cooler.
Theoretically the receiver should be sized to manage any CO2 fluctuations (between winter and summer) and to collect all the CO2 in the circuit in case of a pump down and guarantee any volume/pressure variation due to an increment of temperature.
Typically the system must be pre-charged in order to cover the minimum sight glass present in the vessel (typically there are 2 or 3 sigh glasses: Min and Max or min, medium and max level).
If the level of CO2 decreases below the minimum level during the operation, it’is suggested to add some extra refrigerant in order to have the level above the min one.
The intermediate level should be reached only in a few cases (i.e. after a pump down of the system).
