On the concentrated contact as a viscometer

Lubricant viscosities from published concentrated contact-based measurements are compared with measurements by conventional viscometry for overlapping pressures and viscosities. Contrary to published claims there are important differences. The greater than exponential pressure-viscosity behaviour, which is known to occur at high pressures, is absent from contact measurements. Deficiencies in contact-based techniques are probably due to assumed constitutive behaviour and are not related to time scale. In general, contact-based methods are poorly conditioned for the measurement of pressure-viscosity coefficients.