Definition of viscosity

Viscosity

Viscosity is the internal friction of a liquid. When the lubricating oil is relatively moved by the external force, the resistance generated by the oil molecules prevents the lubricating oil from flowing smoothly. The magnitude of the resistance is called viscosity. It is the main technical index of lubricating oil flow performance. Most lubricating oils are classified according to their viscosity. Therefore, viscosity is the main basis for selecting oil for various mechanical equipment.

The measurement methods of viscosity are divided into two categories: absolute viscosity and relative viscosity. The absolute viscosity is divided into two kinds of dynamic viscosity and kinematic viscosity; the relative viscosity has several expression methods such as Entropy viscosity, Saybolt viscosity and Rayleigh viscosity.

1. Kinematic viscosity η

In the fluid, the two areas are 1m2, 1m apart, and the resistance generated when the relative moving speed is 1m / s is called dynamic viscosity. The unit is Pa.s (Pa.s). The unit of dynamic viscosity used in the past was poise or centipoise, and Poise or centipoise was an illegal unit of measurement.

1Pa.s = 1N.s / m2 = 10Ppoise = 103cp

The ASTM D445 standard specifies the use of kinematic viscosity to calculate kinematic viscosity, ie η = ρ.υ

Where η-dynamic viscosity, Pa.s

ρ-density, kg / m3

Ï…-Kinematic viscosity, m2 / s

China's national standard GB / T506-82 is a low-temperature dynamic viscosity measurement method for lubricating oils. This method is used to determine the low-temperature (0 ~ -60 ℃) dynamic viscosity of lubricating oil and dark petroleum products. Under strict control of temperature and different pressures, determine the time, in seconds, required for a certain volume of sample to flow through a capillary viscometer with a calibrated constant. The dynamic viscosity is calculated from the product of the time the sample flows through the capillary and the capillary calibration constant and the average pressure in Pa.s. This method repeatedly determines that the difference between the two results should not exceed ± 5% of its arithmetic mean.

2. Kinematic viscosity

The ratio of the fluid's dynamic viscosity η to the fluid's density ρ at the same temperature is called kinematic viscosity. It is a measure of the flow resistance of this fluid under the action of gravity. In the International System of Units (SI), the unit of kinematic viscosity is m2 / s. In the past, the unit of kinematic viscosity was usually used as cSt, which is equal to 10-6m2 / s, (ie 1cSt = 1mm2 / s.

Kinematic viscosity is usually measured with a capillary viscometer. Under strict temperature and reproducible driving head, measure the time for a certain volume of liquid to flow through the calibrated capillary viscometer under the action of gravity. In order to measure the kinematic viscosity, the temperature of the measured fluid must be controlled first. The precision requirement is 0.01 ℃; secondly, the proper capillary size must be selected to ensure that the outflow time cannot be too long or too short, that is, thicker capillaries are used for viscous liquids and thinner capillaries are used for dilute liquids. Not less than 200 seconds; the viscosity tube constant must be calibrated regularly; and must be kept vertical when installing the viscosity tube. The national standard for kinematic viscosity is GB / T256-88, which is equivalent to ASTM D445-96 / IP71 / 75.

3. Enn viscosity 0E

The national standard of China is GB / T266-88 for the determination of petroleum products Engel's viscosity.

This is a relative viscosity commonly used in the past, which is defined as the time (s) required for a 200ml liquid to flow through an Engel's viscometer at a specified temperature, and the same volume of distilled water at 20 ℃ to flow through an Engel's viscometer The ratio of time (s) is called Ens viscosity.

4. Redwood viscosity

This viscosity is mainly used in the United Kingdom and Japan. It is defined as the time required for 50ml of test oil to flow through the Rayleigh viscometer at a specified temperature of 60 ° C or 98.9 ° C in seconds.

5. Saybolt Universal Viscosity

The United States is more used to use this viscosity unit, which is defined as the time required for a 60ml liquid to flow out of the Saybolt viscometer at a specified temperature in seconds. The American standard method is ASTM D88.

6. Conversion of several viscosities

1) Conversion of Engel's viscosity and kinematic viscosity

Kinematic viscosity Ï… (mm2 / s) = 7.310E-6.31 / 0E

2) Conversion of Rayleigh viscosity and kinematic viscosity

Kinematic viscosity Ï… (mm2 / s) = 0.26R-172 / R

When R> 225s, use

υ （mm2 / s） = 0.26R

3) Conversion of Saybolt viscosity and kinematic viscosity:

υ （mm2 / s） = 0.225S

Use the above formula when S> 285s.