Abstract: A sensor (1) is provided for sensing the entrainment of a fluid of low specific gravity in a fluid of higher specific gravity, particularly the entrainment of refrigerant gas in refrigerant liquid. The sensor (1) is typically installed into a higher pressure liquid supply line (24) of a refrigeration system (26). The sensor (1) has a differential float (16) including a float member (54) pivoted about a pivot (56) in a chamber (14) coupled to the supply line (24). The float member (54) has a specific gravity greater than the specific gravity of both of the fluids in which it operates. However, the location of the pivot (56) intermediate of the length of the float member (54) gives a net specific gravity for the differential float (16) which is less than the specific gravity of one of the fluids.