Dynamic Viscosity Measurement Made Easy

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.

• The fundamental principle behind the design of the RHEOTEK™ AV-1 Automatic Viscometer is the use of a constant force for driving the used oil sample under test. This eliminates the effects of fluid density and gravity. The resulting measurement is a dynamic viscosity result, which may be converted to a kinematic viscosity result and recorded as such by the user. A density-measuring cell is available for this purpose, to eliminate uncertainties introduced by assumptions made about fluid density.
• Temperature control is the key to precise viscosity measurement and the RHEOTEK™ AV-1 Automatic Viscometer deploys two heat stoves with independent temperature control. Each heat stove comprises of a solid aluminium block with miniature heaters and temperature sensors. Each stove can be set to a different temperature, which enables the user to measure the same sample at two different temperatures. Alternatively the RHEOTEK™ AV-1 Automatic Viscometer can be set so that it measures two samples, at the same temperature, simultaneously. This enables a throughput of up to 45 used oil samples at 100oC per hour.
• Two capillaries are embedded into the heated blocks, with opto-electronic sensors to monitor sample flow rates and flow times. A precision-regulated positive pressure is used for driving the sample through the capillary. Additionally, pressurized air, solvent and vacuum systems are controlled by a series of electro-mechanical valves. The complete instrument is controlled by a PC.
• The viscometer is calibrated using viscosity oil standards calibrated in accordance with ASTM D445.
• Samples are automatically loaded using an auto-sampler. They are then heated to the required temperature and measured. The fluid is then purged and the system is thoroughly cleaned. Optionally, a bar code reader may be added for sample identification, monitoring and tracking. Serial LIMS data can then be transmitted to the laboratory's central monitoring system.
• The viscometer has a throughput of up to 45 samples per hour.