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  Viscosity Explained

VISCOSITY

Definition of Viscosity

The viscosity of a substance is the shearing resistance of a liquid film which separates two horizontal plates, one of which is being moved across the other. The absolute viscosity of a substance is the force in dynes which will move one square centimeter of a plane surface with a speed of one centimeter per second relative to another parallel plane surface from which it is separated by a layer of the substance one centimeter thick. This may be expressed by the following formula:

viscosity = (force x film thickness) / (area of plate x velocity)

The Cannon-Fenske tube (46460) measures kinematic viscosity in units of centistokes and equates to time of flow multiplied by tube constant. More exacting values for the constant of a given tube can be determined using a liquid of known viscosity, measuring the time (sec.) and calculating c in the formula:

kinematic viscosity (centistokes or cSt) = c x time (sec.)

Viscosity is temperature sensitive and is normally run in constant temperature baths.

Basic Units of Viscosity

1. Poise: The unit of poise, the value for absolute viscosity, is equal to one dyne second per square centimeter. That is, when two plates have a shearing area of 1 square centimeter and a film thickness of 1 centimeter, if a force of one dyne is required to maintain a velocity of 1 centimeter per second, the fluid is said to have a viscosity of 1 poise. One poise also equals one stoke multiplied by density (at specific temperature). Centipoise: Since the poise is such a large unit it is often more convenient to use the centipoise,(1/100 poise).

2. Stoke: The kinematic viscosity of a substance is the ratio of the viscosity to the density of the substance at the temperature of measurement. The unit of kinematic viscosity is the stoke and is equal to that possessed by a fluid which has a viscosity of one poise and a density of one gram per cubic centimeter, e.g. for a fluid with viscosity of 9 poise and density of 0.9090 it is equal to 8.18 stokes (818 centistokes). The appropriate tube selection would be 46460-400 for range 240-1200 centistokes.

3. Centistoke: Again since the stoke is such a large unit it too is generally more convenient to use the centistoke (1/100 stoke).

Other Viscosity Units:

Of special importance in the field of lubrication are the so-called kinematic viscosimeters which measure the time in seconds for a given volume of liquid to flow through a definite orifice at some specified temperature. See table for conversion of kinematic viscosity in centistokes to Saybolt, viscosimeter values.