Dynamic Viscosity Measurement Made Easy

Tags: viscosity, oil analysis

Poulten Selfe and Lee Ltd is a UK company specializing in the viscosity measurement of new and used oils. The company was incorporated in 1931 and manufactures an established range of glass capillary viscometers, viscosity oil standards and automatic laboratory viscometers. Stephen Gosling, director of the family owned business, actively participates in contributing to international rheology panels including IP and ASTM. He is currently drafting a code of practice for the determination of the viscosity of used crankcase lubricants from diesel engines.

Poulten Selfe & Lee Ltd has recently launched a new viscometer, the RHEOTEK™ AV-1 Automatic Viscometer, which supersedes the VMU 300. Like its predecessor the RHEOTEK™ AV-1 Automatic Viscometer is designed specifically for the viscosity measurement of used oils. In addition the AV-1 can be used to measure fuel oils and new oils. The most important feature of the instrument is that it measures dynamic (absolute) viscosity (reported in centiPoise) directly, and if required, density can also be measured, simultaneously, in order to present the results in kinematic viscosity units (e.g., centiStokes). Dynamic viscosity is measured in order to overcome the possible problems associated with kinematic viscosity from used oil samples. Contaminants commonly found in used oil can alter the density of the sample and, as such, might have an adverse effect on the trended viscosity result.

From fundamental physics theory, Poiseuille's equation states that the viscosity of a fluid may be determined from the flow rate through a given capillary, provided that a constant force is applied to the fluid.

For convenience, gravity is usually used as the driving force in capillary viscometers. However, this can potentially introduce an error into the calculation of viscosity. An assumption must be made about the density of the fluid if accurate measurements are required. In used oil analysis it is not possible to make assumptions about the fluid density over its service life. It has been shown that various fluid samples with similar viscosities may exhibit a wide range of densities, according to their chemical makeup and suspended contaminants. These fluid samples with similar viscosity would therefore be considered as having different viscosities if measured by a conventional kinematic viscometer deploying gravitational flow. However, with dynamic viscosity the measurement would reveal the true result, i.e., that their viscosities were similar.

The following is a summary of the RHEOTEK™ AV-1 Automatic Viscometer.