Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/147079
LESSONS IN LUBRICATION electronically as signals to a computer or additional monitoring device. Factors that Affect Viscosity Measurements Although the viscosity of a fluid may appear simple to analyze, performing precise measurements on such fluids can be a difficult task. Several factors must be known and kept constant in order for a measurement to be effective, including the following: Temperature Common U-tube Viscometer Variations In terms of importance, temperature is to viscosity as viscosity is to the base oil. The temperature-controlled bath's sole responsibility is to ensure the entire viscosity measurement experiment remains at a precise temperature (usually at 40 or 100 degrees C) and is controlled within 0.02 degrees C. Managing the bath to this degree can be challenging, which is why more sophisticated viscometers include the temperature-controlled bath system as part of the unit. Viscometer Glass Capillary viscometers not only depend on temperature for accuracy but also on the precise inside diameter within the tube. For this reason, these glass tubes are formed with a fully annealed, low-expansion borosilicate glass and often come with the tube's associated viscometer constant as the correction factor. Recalibrating the viscometer's constant on a yearly basis and in accordance to the temperature at which it will be used is common practice to ensure as little error as possible. Calibration methods are specified in ASTM D446. Viscometer Size Viscometers vary in size due to a wide variety of viscosities that may be tested. It is recommended that a viscometer take at least 200 seconds (for manual measurements using a stopwatch) for the test fluid to pass from one marked point to the other in order to avoid human error. Rinsing Between Measurements Another crucial viscometer test factor is the quality of the rinse within the tube between 52 | July - August 2013 | www.machinerylubrication.com measurements. Typically, cleaning, rinsing and drying stages are performed using various residue-free solvents. In laboratories where samples are run continuously on high-volume viscometers, built-in cleaning systems ensure cleanliness efficiency. Nevertheless, a visual check is still required to determine if a second rinse cycle is necessary, especially for used oils. Other important factors that can affect viscosity measurements include the timing device's method and accuracy, proper sample handling, and in-depth and continuous quality control. Taking viscosity measurements has evolved over the years with various styles and methodologies. To the credit of ASTM and the International Organization for Standardization (ISO), viscometer testing methods, procedures, calibration, cleaning and other factors have evolved as well to ensure accuracy and consistency among the different viscometers. After all, with viscosity being the most important physical property of a base oil, the efforts to perform viscosity measurements through careful methods and stringent standards can equate to more reliable machinery lubrication and ultimately to longer lasting machinery. About the Author Bennett Fitch is a technical consultant with Noria Corporation. He is a mechanical engineer who holds a Machine Lubricant Analyst (MLA) Level I certification and a Machine Lubrication Technician (MLT) Level I certification through the International Council for Machinery Lubrication (ICML). Contact Bennett at bfitch@noria.com.