Abstract: A flash gas filter system detects a low level of refrigerant flowing in a conduit of a refrigeration system by measuring a temperature drop across a sub-cooler connected to the conduit. The sub-cooler has a liquid process circuit for passing the refrigerant and a refrigerating circuit connected by a manual expansion valve to an outlet of the liquid process circuit for producing a predetermined temperature drop across the sub-cooler when the refrigeration system is fully charged. The temperature drop in the refrigerant is reduced by the presence of bubbles caused by a low refrigerant level. The reduced temperature drop is detected and an alarm signal is generated by a computerized filter system control. The control also can monitor liquid line temperature for protection against malfunctioning condensers generate maintenance information and warn of potential malfunction conditions.