Viscosity is the resistance that a fluid offers to flow. It can be defined as the internal friction resulting from the movement of a layer of fluid in relation to another.
A simple flow is shown in the figure below to illustrate the definition of viscosity.
If the force per unit of area on the upper plate were to be measured, we would find F/A = V/d, that is, the shearing stress F/A is equal to viscosity times the deformation rate, V/d, with d being the distance between the plates. This relation essentially defines viscosity.
A fluid responding to the shearing stress (F/A) in this manner is called a Newtonian fluid: it has the property of the viscosity being independent of the speed. Many of the fluids for which measurement of the speed is desired are Newtonian (ex.: water, light oils), but others are non-Newtonian, such as paints, polymeric fluids, etc.
TYPES
There are two types of viscosity: dynamic viscosity and kinematic viscosity.
Dynamic viscosity (m) is given in terms of the force required to move a unit of area to a unit of distance. The most widely used unit is the Poise, g/(cm.s). For convenience, viscosity is expressed as centiPoise (cP), which is equal to 10-2 Poise.
Kinematic viscosity, however, is dynamic viscosity divided by the density of the fluid, n =m/r.In the International System, the unit is m²/s, but it is usually measured in centiStokes, cSt. The Stokes is cm²/s; therefore, to obtain viscosity in m²/s, multiply the viscosity in cSt by 10-6.
MEASURING VISCOSITY
Viscosity is measured in viscosimeters, which can be classified in two groups: primary and secondary. The primary group consists of instruments making direct measurements of the stress and deformation rate of the fluid. Instruments with several layouts can be conceived for this purpose, such as: disk, the cone-disk and revolving cylinder, all them aiming at the reproduction of the flow between parallel flat plates seen above.
Viscosimeters of the secondary group infer the ratio between the stress applied and the deformation rate by indirect means, that is, without measuring the stress and deformation directly. This category contains the capillary viscosimeter, in which the viscosity is obtained by measuring the pressure gradient of a laminar flow in a tube, and the Stokes viscosimeter, in which it is determined by measuring the free fall time of a ball in a stationary fluid.
Some examples of fluid and gas viscosities:
| Hydrogen @ 20°C - 0,0086 cP |
Glycol @ 20°C - 19,9 cP |
| Ether @ 20°C - 0,233 cP |
Water @ 99°C - 0,2848 cP |
| Oil SAE 20 - 125 cP |
Acetone - 0,3 cP |
| Oil SAE 50 - 540 cP |
Chloroform @ 20°C - 0,58 cP |
| Glycerin @ 20°C - 1490 cP |
Methanol @ 20°C - 0,597 cP |
| Oil SAE 70 - 1600 cP |
Benzene @ 20°C - 0,652 cP |
| Water @ 20°C - 1,002 cP |
Ethyl alcohol @ 20°C - 1,2 cP |
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UNIT CONVERSION
UNIT CONVERSION TABLE |
