Summarizing Discussion of Film Thickness Measurement

In the foregoing treatments of the methodology of film thickness determination, the experimental results have been examined with respect to lubrication theory rather than direct engineering application. Still, the ultimate utility of establishing the validity of a theoretical relation is practical to predict the engineering limits for the existence of a fluid film, which in turn can be confirmed or tested by laboratory experiments amenable to close control and measurement, e.g. rolling/ sliding disk experiments. To be useful from both a theoretical and a practical viewpoint, the validity and accuracy of the film thickness measurements must be established on a primary basis. 

Direct measurement of oil film thickness by electrical conductivity has two basic drawbacks from the point of view of primary calibration. One is the sensitive response of the resistivity of the oil to temperature. Unless the temperature of the oil under operating conditions is known accurately, it is uncertain that the calibration obtained under static conditions can be applied. Direct measurement of film temperatures by an infrared emission technique has shown temperatures within the oil film as high as 388 K (115 C) [20]. The other source of uncertainty to be considered is the increase in field intensity as the oil film be- 

Electrical breakdown of very thin oil films is a basic difficulty in the capacitance method also. The other difficulties associated with applying static calibration to a dynamically generated film are chiefly manipulative exclusion of or compensation for stray capacities, balancing the capacitance bridge, etc. 

The basic theory for the absorption of X-rays is the simple exponential relation 

The method which seems to be soundest and most free of empirical complications from the point of view of basic calibration is interferometry. The calibration is derived from fundamental optics. A technique for dealing with the effect of pressure on the refractive index of the fluid is available via the Lorenz equation. An implicit difficulty is the effect of temperature on the density of the fluid and thus on the evaluation of the Lorenz equation. The necessity of passing the light through a flat transparent plate to form the fringes imposes some limitations on the applicability of the method. 

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