![]() ![]() ![]() 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. For example, a lubricant with an ISO grade of 32 has a viscosity within the range of 28.8-35.2, the midpoint of which is 32.Viscosity Definitions and Methods of Measurement Each ISO viscosity grade number corresponds to the mid-point of a viscosity range expressed in centistokes(cSt) at 40☌. Many petroleum products are graded according to the ISO Viscosity Classification System, approved by the international standards organization (ISO). ![]() SAE 90 to 250 and 20 to 50 specified at 100☌. Viscosities based on 96 VI single grade oils.Įquivalent viscosities for 100° & 210☏ are shown Viscosities can be related horizontally only ![]()
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