"A best film thickness model in using interferometry in finding the pressure-viscosity coefficient alpha of a fluid"

With ever increasing power densities, the so-called pressure-viscosity coefficient alpha of fluids is becoming more and more important. An example is the high pressure viscosity of diesel fuel with common rail systems, where maximum pressures will soon reach 0.3 GPa (3,000 bars). At the occasion of the 3rd Arnold Tross Colloquium (2007) a new method was introduced to employ very accurate film thickness measurement results in determining the pressure-viscosity coefficient of an arbitrary fluid. In this method, the most recent film thickness model, and the believed most accurate one to date, was used: the Moes (2000) central film thickness formula for elliptical EHL (elastohydro-dynamically lubricated) contacts. The measurements are carried out on commonly used interferometric devices and is a cheap alternative to high pressure viscometers. Back in 2007, it was assumed that the Moes model would yield the best results. But many models have been developed, and which one really is the best? In this presentation 11 models are compared with calibration measurements. The unknown pressure-viscosity value is matched in such a way that its value minimizes the least squared error sum for the calculated film thickness with the measured values. The range of investigations was limited to 1.5 < L < 7 for the reference lubricant. The Moes model is the best one at very small film thickness values. The Moes model is the most versatile and general one available, but in the range of the measurements it lacks the accuracy of the Chittenden approximation. The Chittenden et al. model turned out to be superior in the full range of the measurements. The validity of the Chittenden formula transcends the area where it was originally designed for. It is recommended to perform film thickness measurements for fluids with high L values, as traction fluids, and check whether the conclusions still hold.