Viscometer(redirected from Falling Sphere Viscometer)
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an instrument for the determination of viscosity. The most common types of viscometer are capillary, rotary, falling-sphere, and ultrasonic.
The determination of viscosity with a capillary viscometer is based on Poiseuille’s law and consists in the measurement of the flow time of a known quantity (volume) of liquid or gas through narrow tubes of circular cross section (capillaries) with a given pressure drop. Capillary viscometers are used to measure viscosity from 10-5 newton · sec/m2 (N·sec/m2) for gases to 104 N·sec/m2 for solid lubricants. The relative error of master capillary viscometers is ± 0.1-0.3 percent; of working instruments ± 0.5-2.5 percent. The construction of various types of glass viscometer is shown in Figure 1. In capillary viscometers of the types indicated, the flow of liquid occurs under the influence of gravity—at the starting moment the level of fluid in one arm is higher than in the other. The discharge time of the measuring reservoir is defined as the interval between the moments when the liquid level passes the marks at the upper and lower ends of the reservoir. In automatic (continuous-action) capillary viscometers the fluid enters the capillary from a constant-output pump. The pressure drop in the capillary, measured with a manometer, is proportional to the unknown viscosity.
In rotary viscometers, the viscous medium being investigated is in the space between two coaxial bodies (cylinders, cones, spheres, or a combination of them); one of these bodies (the rotor) turns, and the other is fixed. The viscosity is determined by the torque moment at a given angular velocity or by the angular velocity at a given torque moment. Rotary viscometers are used to measure the viscosity of lubricating oils (at temperatures up to -60° C), petroleum products, molten silicates and metals (up to 2000° C), highly viscous varnishes and cements, and clay mortars. The relative error of the most common rotary viscometers is between 3 and 5 percent. The construction of the RV-7 rotary viscometer, with limits of measurement from 1 to 105 N·sec/m2 and an error of ±3 percent, is shown in Figure 2.
The action of a viscometer with a sphere that moves in the medium under investigation is based on Stokes’ law; viscosity is determined according to the speed of passage of the falling ball between the marks on the tube of the viscometer. The widely used Hoeppler universal viscometer, with a “sliding” sphere (Figure 3), belongs to this type of instrument. The limits of measurement for viscometers of this type are 6 x 10-4 to 250 N-sec/m2, with an error of ± 1-3 percent.
The operation of ultrasonic viscometers is based on the measurements of the rate of decay of vibrations in a plate made of magnetostrictive material immersed in the medium under investigation. The vibrations arise from short (10-30 microsec) impulses of current in a coil wound onto the plate. When the plate vibrates, electromotive force is induced in the coil that decreases with a speed that is dependent on the viscosity of the medium. When the electromotive force decreases to a certain threshold value, a new excitational impulse enters the coil. The viscosity of the medium is deter-mined according to the frequency of the succession of impulses. Ultrasonic viscometers are used to measure viscosities in the range from 10-3 to 500 N·sec/m2, with a relative error of ± 5 percent.
In addition to viscometers that permit the expression of the results of measurements in units of dynamic or kinematic viscosity, there are also viscometers for measuring the viscosity of liquids in arbitrary units. This kind of viscometer consists of a vessel with a calibrated discharge tube; viscosity is evaluated according to the time of discharge for a certain volume of liquid. For example, by means of VZ-1 and VZ-4, viscometers, which are designed for research on varnishes and paints, viscosity is expressed in seconds, and with VU (Engler) viscometers, for petroleum products, it is ex-pressed in Engler degrees. The conversion of arbitrary units into units of viscosity of the International System of Units (N·sec/m2 and m2/sec) is possible but inaccurate.
REFERENCESSoveshchanie po viazkosti zhidkostei i kolloidnykh rastvorov: [Trudy], vols. 1-3. Edited by E. A. Chudakov and M. P. Volarovich. Moscow-Leningrad, 1941-45.
Volarovich, M. P. Viazkost’ smazochnykh masel pri nizkikh ternperaturakh, part 1. Moscow, 1944.
Belkin, I. M., G. V. Vinogradov, and A. I. Leonov. Rotatsionnye pribory. Moscow, 1968.
L. P. STEPANOV