viscosity

viscosity
Viscosity is the internal friction of a liquid. When lubricating oil undergoes relative movement under external forces, the resistance generated by oil molecules prevents the smooth flow of lubricating oil, and the magnitude of its resistance is called viscosity. It is the main technical indicator of lubricating oil flow performance. The vast majority of lubricating oils are classified based on their viscosity, therefore viscosity is the main basis for selecting oils for various mechanical equipment.
The measurement methods of viscosity are divided into two categories: absolute viscosity and relative viscosity. Absolute viscosity is divided into two types: dynamic viscosity and kinematic viscosity; There are several ways to express relative viscosity, including Engler viscosity, Saybolt viscosity, and Rayleigh viscosity.
1. Dynamic viscosity η The resistance generated when two areas of 1m2 are taken in a fluid, with a distance of 1m and a relative moving speed of 1m/s, is called dynamic viscosity. Unit Pa. s (Pa. s). The units of dynamic viscosity used in the past were poise or centipoise, and poise or centipoise were non legal units of measurement.
1Pa. s=1N. s/m2=10P, and the 3rd power cp of 10=1Kcps
The ASTM D445 standard specifies the use of kinematic viscosity to calculate dynamic viscosity, i.e η=ρ.υ In the equation η- Dynamic viscosity, Pa. s target system ρ- Density, kg/m3 υ- The kinematic viscosity, m2/s, is determined by the low-temperature dynamic viscosity measurement method for lubricating oils according to the Chinese national standard GB/T506-82. This method is used to determine the low temperature (0-60 ℃) dynamic viscosity of lubricating oils and dark petroleum products. Measure the time, in seconds, required for a certain volume of sample to flow through a calibrated constant capillary viscometer under strict control of temperature and different pressure conditions. Calculate the dynamic viscosity in Pa.s by multiplying the time the sample flows through the capillary with the capillary calibration constant and average pressure. The difference between the two results of repeated measurement by this method shall not exceed ± 5% of its arithmetic mean value.
2. Kinematic viscosity υ Dynamic viscosity of fluids η The density of the fluid at the same temperature ρ The ratio of 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 used to be the centimeter (cSt), which is equal to 10-6m2/s, i.e. 1cSt=1mm2/s.
Kinematic viscosity is usually measured using a capillary viscometer. Under strict temperature and reproducible driving pressure, the time for a certain volume of liquid to flow through a calibrated capillary viscometer under the action of gravity must be measured. In order to accurately measure the kinematic viscosity, the temperature of the measured fluid must first be controlled, and the temperature measurement accuracy must reach 0.01 ℃; Secondly, it is necessary to choose the appropriate size of the capillary to ensure that the outflow time cannot be too long or too short. For viscous liquids, a slightly thicker capillary should be used, while for thinner liquids, a slightly thinner capillary should be used, and the flow time should not be less than 200 seconds; Regularly calibrate the viscosity tube constant; And when installing the viscosity tube, it must be kept vertical. The national standard for kinematic viscosity is GB/T256-88, which is equivalent to ASTM D445-96/IP71/75.
3. The national standard of China for Engler viscosity 0E is the method for measuring Engler viscosity of petroleum products GB/T266-88. This is a commonly used relative viscosity in the past, which is defined as the time (s) required for 200ml of liquid to flow through an Engler viscometer at a specified temperature, compared to the time (s) required for distilled water of the same volume to flow through an Engler viscometer at 20 ℃.
4. Redwood viscosity is mainly used in the UK and Japan. Its definition is the time, in seconds, required for 50ml of test oil to flow through the Rayleigh viscometer at a specified temperature of 60 ℃ or 98.9 ℃.
5. Saybolt Universal Viscosity is commonly used in the United States as a unit of viscosity, defined as the time required to flow 60ml of liquid from the Saybolt viscometer at a specified temperature, measured in seconds. The American standard method is ASTM D88
6. Conversion of Several Viscosities 1) Conversion of Engler Viscosities and Kinematic Viscosities
Kinematic viscosity υ (mm2/s)=7.310E-6.31/0E 2) Conversion of Reynolds viscosity to kinematic viscosity Kinematic viscosity υ (mm2/s)=0.26R-172/R When R>225s, use υ (mm2/s)=0.26R 3) Conversion of Saybolt viscosity to kinematic viscosity: υ (mm2/s)=0.225S When S>285s, use the above equation