Machinery Lubrication magazine published by Noria Corporation
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LESSONS IN LUBRICATION An enhanced variation of the rotational viscometer is the Stabinger viscometer. It uses a modified standalone floating spindle that is controlled by electromagnetic forces to generate rotation within the fluid. This is advantageous because it eliminates the difficult task of factoring in the bearing friction of an attached motor to the spindle. Falling Ball and Falling Piston Viscometer Tests Less common alternatives for measuring viscosity include the falling ball and falling piston viscometers. In these tests, a ball or piston is allowed to fall into the liquid, and the time is measured between the passing from one Rotary (Brookfield) Viscometer ASTM D2983 marked point to a second marked point. In order to calculate the viscosity in accordance with Stokes' law, the known terminal velocity, size and density of the ball or piston must be known. Other Test Methods In some cases uncommon in oil viscosity testing, a bubble method may be used. This test generally involves measuring the time it takes for a bubble to rise a specific distance. This measured time can then be proportionally associated to the fluid's viscosity. Another variation includes measuring the resistance to vibration from a probe. Types of Viscometers Although there are several methods by which viscosity can be determined, most viscometers are commercially sold to best fit their intended use. Below is a list of these viscometer categories. Portable Viscometers As the name implies, portable viscometers are intended for use anywhere a laboratory setting is not practical. In many onsite and in-the-field applications, viscometers are needed to provide quick, approximate viscosity readings usually for the purpose of warranting further analysis. Small Onsite Viscometers For more accurate viscosity readings, small onsite viscometers are marketed for laboratory settings but at a reasonable price. Such viscometers include those of the U-tube varieties or the Brookfield. While there are several variations of the U-tube viscometer, three of the most well-known are the Ostwald, Cannon-Fenske and Ubbelohde. Full-service Viscometers In order to obtain the highest degree of accuracy in viscosity testing, a full-service viscometer must be employed. These units typically have the capability to automatically test kinematic and absolute viscosity in a wide range of temperatures as well as provide density and specific gravity calculations. One common challenge oil analysis laboratories face with samples requiring multiple tests is 62% of lubrication professionals use some type of viscometer for oil analysis, according to a recent poll at machinerylubrication.com the ability to perform these tests with as little sample as possible. Several of the more sophisticated full-service viscometers can make accurate measurements within 0.1-percent repeatability using a sample size of less than 0.1 milliliter. In large laboratories that test hundreds of samples per day, this is a necessary advantage of fullservice viscometers. Inline/Online Viscometers While most oil viscosity testing is conducted after the acquisition of a contained sample, some viscometers can provide viscosity readings continuously in active flow lines. This usually is accomplished by means of a sensor or transducer that is directly inline (part of the pipeline) or online (bypass stream or transducer). Viscosity measurements are taken and then sent 50 | July - August 2013 | www.machinerylubrication.com