Definitions of four different viscosities

1. Absolute viscosity . After we have the two concepts of shear stress and shear rate, we can define viscosity: the viscosity coefficient η is the ratio of shear stress to shear rate.

2. Kinematic Viscosity. Kinematic viscosity (dynamic viscosity, dynamic viscosity) is the ratio of the absolute viscosity (poise) of the fluid to the density (g/cm³) of the fluid, which can be expressed by the following formula:

The units of kinematic viscosity are Stokes (Stokes—cm²/sec, it is recommended to use square meters per second, ie m²/s) and centimeters of Stokes (1/100 Stokes).

One Stokes = 1 cm²/sec = 10 ^-4 m²/sec.

The concept of kinematic viscosity needs to be introduced when using so-called kinematic viscometers (such as small hole type and bubble type), or when designing pipeline installations.

In production control, relative viscosity or conditional viscosity is often used.

3. Relative viscosity . It is the ratio of the absolute viscosity of the fluid to the absolute viscosity of a standard liquid (such as water) under the same conditions.

4. Conditional viscosity . It refers to the time for a certain amount of fluid to flow out of a small hole with a specified diameter at a certain temperature, expressed in seconds.

At 20°C (68°F), the viscosity of water is 1 C.P. The viscosity of linseed oil is about 50C.P. The viscosity of tung oil is about 300C.P. The viscosity of castor oil is about 1,000CP.

The reciprocal of viscosity is fluidity (fluidity), represented by the symbol Φ, and its relationship is as follows:

η=vρ=1/ Φ