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Definition Of Viscosity

- Nov 13, 2017 -

Viscosity is the internal friction of the liquid. Lube oil by external forces and the relative movement occurs, the resistance of the oil molecules so that the oil can not flow smoothly, the size of the resistance called viscosity. It is the main technical indicator of lubricating oil flow performance. Most of the oil is based on the size of its sub-viscosity grades, therefore, viscosity is the main basis for a variety of mechanical equipment, oil selection.

Viscosity measurement methods are divided into two categories of absolute viscosity and relative viscosity. Absolute viscosity is divided into two kinds of dynamic viscosity and kinematic viscosity; relative viscosity Engle viscosity, Sayed viscosity and Ray's viscosity and other means of representation.

1, dynamic viscosity η


In the fluid to take two area for each 1m2, 1m away from each other, the relative movement speed of 1m / s when the resulting resistance is called dynamic viscosity. Unit Pa.s (Pascal seconds). Kinetic viscosity units used in the past are Poise or Centipoise, Poise or Centipoise as non-statutory units of measurement.


1Pa.s = 1N.s / m2 = 10P poise = 103cp

ASTM D445 standard provides kinematic viscosity to calculate the dynamic viscosity, that η = ρ.υ

Where η-dynamic viscosity, Pa.s


ρ-density, kg / m3

υ-kinematic viscosity, m2 / s

  China's national standard GB / T506-82 for low temperature lubricating oil viscosity measurement. The method is used to determine the lubricating oil and dark petroleum products at low temperature (0 ~ -60 ℃) dynamic viscosity. Under strictly controlled temperature and pressure conditions, a certain volume of the sample was measured in a calibrated capillary viscometer through the required time, in seconds. The dynamic viscosity, in Pa.s., is calculated from the product of the capillary flow constant and the capillary calibration constant and mean pressure at the time that the sample is flowing through the capillary. The method for determining the difference between two results repeatedly should not exceed ± 5% of its arithmetic mean value.

2, kinematic viscosity υ

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

  Kinematic viscosity is usually measured with a capillary viscometer. In the strict temperature and reproducible drive head, the determination of a certain volume of liquid under gravity flow through a calibrated capillary viscometer time, in order to measure the kinematic viscosity, we must first control the temperature of the measured fluid, temperature measurement Accuracy requirements of 0.01 ℃; Second, we must choose the appropriate size of the capillary to ensure that the outflow time can not be too long nor too short, thick liquid with thicker capillary, thinner liquid with thinner capillary, flow time should be Not less than 200 seconds; to be regularly calibrated viscosity tube; and the viscosity of the tube must be installed vertically. Kinematic viscosity of the national standard GB / T256-88, equivalent to ASTM D445-96 / IP71 / 75.

3, Engler viscosity 0E

China's national standards for oil products Engler viscometry GB / T266-88.


This is a commonly used relative viscosity in the past, defined as the time (s) required for 200 ml of liquid to flow through the Engler viscometer at the specified temperature, with the same volume of distilled water as required by a viscometer at 20 ° C The ratio of time (s) is called Engler viscosity.

4, Ray's viscosity (Redwood)

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


5, Saybolt Universal Viscosity


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

6, several kinds of viscosity conversion


1) Engler viscosity and kinematic viscosity conversion

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

2) Ray's viscosity and kinematic viscosity conversion

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

When R> 225s, then use

υ (mm2 / s) = 0.26R

3) Sayai viscosity and kinematic viscosity conversion:


υ (mm2 / s) = 0.225S

When S> 285s when using the above formula.


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