Film Thickness by Electrical Resistance

Let us consider the lubricant film between two metallic surfaces of length I and width w a distance h apart. The resistance of the lubricant film is given by 

In the same investigation [1] Crook demonstrated another effect in a dynamically lubricated system which has an important influence on the validity of film thickness determinations by electrical resistance. Direct measurement showed that the resistivity of a commercial turbine oil decreased from 10 ohm-cm at 273 K (32 F to 10 ohm-cm at 549 R (530 F). It was also demonstrated that the bulk of the metal in the rotating disks acted as such a large heat sink during a run that it took considerable time for the temperature of the system to stabilize, and even then the temperature of the oil film was not exactly known. Both of these influences cast doubt on the reliability of the electrical resistance technique for evaluating fluid film thickness. The usefulness of the resistance method is restricted to special circumstances such as light loads, very slow speeds and simple linear geometry. 

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