At the 1956 National Meeting of the American Society of Lubrication Engineers, a paper entitled “The Dynamic Demulsibility Characteristics of Oils” was presented by the authors. In that paper, it was stated that an apparatus had been developed that would permit the evaluation of the demulsibility characteristics of paper machine oils under the same conditions encountered in the plant. Several commercially available paper machine oils were evaluated using the steam emulsion number (ASTM Method D 157) and the Herschel method. By these tests, the demulsibility characteristics of the oils rated poor; but field experience indicated that their demulsibility characteristics ranged from “fair” to “good”. Using the dynamic demulsibility apparatus, the oils rated in approximately the same order as observed in the field. Data for depletion of additive were also obtained by ash analysis. These data revealed that oils compounded to the same performance levels with a variety of additives exhibited different degrees of resistance to leaching of the additive by water. It was natural to attempt to use the dynamic demulsibility apparatus as a tool in studying additive depletion in steam turbine oils. Normally, the resistance to water leaching of the rust and oxidation inhibitors used in steam turbine oils cannot readily be studied in the laboratory, and the evaluation is made while the oil is in service. The design of this apparatus permits contact of the oil with water in a manner not unlike that found in full-scale equipment. Other demulsibility tests, currently in use, rely upon batch contact with water and do not permit equilibrium to be established between the water and oil. In a turbine installation, whether it is of the central station or marine type, water contamination is almost of a continuous nature and is being withdrawn either by gravity separation or by centrifuging. Although the extent of water separation at the water draw-off valve in a turbine system is of prime importance, the extent to which water remains dispersed in the oil phase is also very important. Extensive dispersion would result in impaired lubrication. An increase in volume in the storage tank would also occur, thereby necessitating the draining of some of the “lubricant.” An added advantage of conducting the dynamic demulsibility test is that an ample amount of used oil is available for determining the extent of additive depletion by means of performance testing. A flow diagram of the apparatus is shown in Fig. 1. The dynamic demulsibility test is conducted for 6 hr by continuously contaminating the circulating oil with distilled water at the rate of 5 per cent of the circulating rate at the selected test temperature. The oil and water are brought into intimate contact in the mixing chamber by means of a T-blade stirrer as used in the ASTM turbine oil rust test apparatus. The contaminated oil is pumped to the reservoir where it is allowed to stand for a period of 37 1/2 min. As the volume of liquid increases as a result of the water addition, a sensing control, located in the mixing chamber, actuates a solenoid valve at the bottom of the reservoir and permits the periodic and automatic removal of water or emulsified oil. Once equilibrium is reached, the solenoid valve opens at intervals of about 1 min.