Various methods of measuring the percentage of diluent in used crankcase-oils are summarized in this paper but the broader questions of deterioration of the oil due to other factors are not considered. The characteristics of viscometric methods and of steam, atmospheric and vacuum-distillation methods are discussed. It is pointed out that as dilution is not the only change the oil undergoes in service, methods based upon the assumption that oil is unchanged except by the presence of diluent may yield misleading results.
Distillation methods seem best suited for this determination and those which are rational, in that the evaluation of the diluent is based on the change in the properties of the distillate as the distillation proceeds from diluent to oil, seem to promise the greatest accuracy over a wide range of diluents and oils.
Since the accuracy that is necessary for research work is not attainable with the simple apparatus and procedure desirable for general routine work, two rational methods are suggested for the determination of percentage of crankcase-oil dilution. The first is a vacuum-distillation method carried on at a pressure low enough, about 4 cm. (1.575 in.) of mercury, to avoid cracking of the oil. In this method distillation data, volume of distillate and vapor temperature are taken and the percentage of diluent deduced from these data or from a curve plotted from the data. The results are accurate to within about ½ of 1 per cent.
The second method is a rapid atmospheric distillation in which no thermometer is used. The distillate is fed from the condenser through a funnel having a capillary section in the stem. The transition from diluent to oil is accompanied by an abrupt increase in viscosity of the distillate and also in the rate of distillation with constant heat-input. These two effects combine to produce a rapid rise in the head of distillate in the capillary section. The volume of distillate corresponding to this rapid increase is taken as the diluent. Results may be expected to fall within 2 per cent of the true value.