Instruments of the capillary type, such as the Ostwald, Bingham, and Ubbelohde viscosimeters are used primarily for fluids of low viscosity, such as water. A definite volume of fluid is allowed to flow through a capillary tube or orifice of specified proportions and the time of efflux noted. Viscosity is measured by an instrument called a viscosimeter. When the shear stress to shear ratio rate is constant for all shear rates at any given instant of time, but increases with time, a fluid is said to be rheopectic. When the ratio of shear stress to shear rate decreases as shear rate increases and is time dependent in that this ratio increases back to its "rest" value gradually with lapse of time at zero shear rate and stress, and decreases to a limit value gradually with lapse of time at constant shear rate, a fluid is said to be thixotropic. When the ratio of shear stress to shear rate decreases as shear rate increases, reversibly and independent of time, and zero shear rate occurs only at zero shear stress, a fluid is said to be pseudo-plastic. When the shear stress to shear rate ratio is constant for shear rates above zero, is independent of time, but when shear occurs only for shear stress above a fixed minimum greater than zero, a fluid is said to be plastic. When the ratio of shear stress to shear rate increases as the shear rate increases, reversibly and independent of time, a fluid is said to be dilatent. Thus the ratio of shear stress to shear rate is a constant for all shear rates, is independent of time, and zero shear rate exists only at zero shear stress such a fluid is said to be Newtonian. The viscosities of fluids, such as mineral oil and water, are unaffected by the magnitude and kind of motion to which they may be subjected as long as the temperature remains constant. The influence of change in pressure usually is negligible. The viscosities of most fluids vary appreciably with changes in temperature. Some useful relationships are as follows:ġ square foot = 929.034 square centimetersġ dyne-second per sq cm = 1 poise = 100 centipoiseġ lb-sec/sq ft = 478.801 poises = 47,880.1 centipoise The distinction between the dynamic and the kinematic viscosity should be carefully noted so that the correct parameter will be used as required in computations. Therefore, the dimensions of kinematic viscosity are The foregoing may be expressed by the equations
The proportionality factor (μ) is the dynamic viscosity. The velocity distribution will be linear over the distance (d) and experiments show that the slope of the velocity line (v/d) will be directly proportional to the unit shearing force ( τ = F/A) for all "true" or "Newtonian" fluids. A force (F) is applied to and in the plane of the upper surface, causing it to move with a velocity (v) parallel to the lower fixed surface. 1 which shows two parallel plane surfaces of area (A) separated a distance (d) and the space between completely filled with fluid. (See the Kinematic and Dynamic Viscosity Conversion Tool to perform viscosity conversions.) The dynamic viscosity may be defined with the aid of Fig. There are two basic viscosity parameters: dynamic ( or absolute) viscosity and kinematic viscosity. ANSI/HI 9.6.7 acts as a guideline that explains these effects. In rotodynamic pumps, fluid viscosity can have a significant impact on performance. For instance, molasses having the same specific gravity (1.48) and the same Brix rating (90) may vary in viscosity from 128,000 to 303,000 Seconds Saybolt Universal (SSU). There is no relation between the viscosity and the specific gravity of most liquids. Since motion or flow of a fluid is produced by shearing forces, viscosity is associated with fluid motion. The viscosity of a fluid (liquid or gas) is that property which tends to resist a shearing force. Viscosity Definitions and Methods of Measurement
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