When the fluid is flowing, there is relative movement between adjacent fluid layers, so the frictional resistance between the two fluid layers will be generated, which is called viscous force.Viscosity is used to measure the viscosity of a physical data.Its size is determined by the kind of substance, temperature, concentration and other factors.Viscosity is generally short for dynamic viscosity, which is measured in Pa·s (Pa·s) or mPa·s (MPa ·s).Viscosity is divided into dynamic viscosity, kinematic viscosity, relative viscosity, the three have a difference, can not be confused.Viscosity can also be measured by applying 4 or 1 cup in seconds (s).
The dynamic viscosity symbol is μs and the unit is PASCAL second (pA ·s), defined by the following equation: L= μs ·μ0/h
μ0 — The velocity of a flat plate moving parallel to a fixed flat wall in its own plane
H — Distance from flat plate to fixed flat wall.However, the distance should be small enough so that the flow of the fluid between the plate and the fixed flat wall is laminar
L — Fluid friction force acting on unit area of the flat plate during the movement of the flat plate
The symbol of kinematic viscosity is V. In engineering calculation, kinematic viscosity is the ratio of the dynamic viscosity of a substance to its density, and its unit is (m2/s).The units are quadratic meters per second (m2/s), v=μ/p
Viscosity has dynamic viscosity, its unit: PASCAL second (Pa·s);It is also used in the petroleum industry as a viscosity, which is not the viscosity concept described above.It is the reading of the fluid directly measured in the Englor viscometer.
Viscosity measurement methods are divided into viscosity and relative viscosity two categories.The viscosity is divided into two kinds: dynamic viscosity and kinematic viscosity.The relative viscosity can be expressed by several methods, such as Engelwald viscosity, Sayelwald viscosity and Rayleigh viscosity.
1, dynamic viscosity η in the fluid take two areas of 1m2, the distance between 1m, the relative moving speed of 1m/s when the resistance is called dynamic viscosity.Unit Pa. S (Pa.Seconds).The unit of dynamic viscosity used in the past is poises or centipoises, and Poise or centipoises is a non-legal unit of measurement.1Pa. S =1N. S /m2=10P Poor =10 to the third CP = 1Kcps ASTM D445 standard stipulates that the kinematic viscosity is used to calculate the dynamic viscosity, i.e. η=ρ.M2 / S China’s national standard GB/T506-82 lubricating oil low temperature dynamic viscosity measurement method.This method is used to determine the dynamic viscosity of lubricating oil and dark petroleum products at low temperature (0 ~ -60℃).Under the conditions of strict control of temperature and different pressures, the time required for a certain volume of sample to flow through the calibrated constant capillary viscometer is measured in seconds.The dynamic viscosity is calculated by multiplying the time of the sample flowing through the capillary with the calibration constant of the capillary and the mean pressure, and the unit is pA.s.The difference between the two results of repeated determination by this method should not exceed ±5% of their arithmetic mean.
- Kinematic viscosity The ratio of the dynamic viscosity η of the υ fluid to the density ρ of the fluid at the same temperature is called kinematic viscosity.It is a measure of the resistance of such a fluid to the action of gravity.In the system of units (SI), kinematic viscosity is measured in m2/s.In the past, the unit for kinematic viscosity was commonly used as cSt, which was equal to 10-6m2/s (i.e., 1cSt=1mm2/s).
Kinematic viscosity is usually measured with a capillary viscometer.Under the strict temperature and reproducible driving head, the flow time of a certain volume of liquid through the calibrated capillary viscometer under the action of gravity is measured. In order to accurately measure the kinematic viscosity, the temperature of the measured flow body must be controlled first, and the temperature measurement accuracy must reach 0.01℃.Secondly, the appropriate capillary size must be selected to ensure that the outflow time can not be too long or too short, that is, thicker liquid with a slightly thicker capillary, thinner liquid with a slightly thinner capillary, flow time should not be less than 200 seconds;The constant of the viscosimeter must be calibrated regularly;And the installation of the viscosity tube must be kept vertical.The national standard for kinematic viscosity is GB/T256-88, equivalent to ASTM D445-96/IP71/75.
- The national standard of our country is the measurement of the viscosity of petroleum products. GB/T266-88.This is a relative viscosity commonly used in the past. It is defined as the time (s) required for a 200ml liquid to flow through the Ensler viscometer at a specified temperature. The ratio of the time (s) required for the same volume of distilled water to flow through the Ensler viscometer at 20 ° C is called the Ensler viscosity.
4, Redwood viscosity (Redwood) This viscosity is mainly used in England and Japan.It is defined as the time required for 50ml of oil test to flow through the Reiter viscometer at a specified temperature of 60℃ or 98.9℃, in seconds.
- This unit of Viscosity, commonly used in the United States, is defined as the time, in seconds, required for the flow of 60 mL of liquid from the Saybolt Universal Viscosity Meter at a specified temperature.The standard method in the United States is ASTM D88
6, several viscosity conversion
1) Conversion between Engl viscosity and kinematic viscosity υ (mm2/s) =7.310E-6.31/0E
2) Conversion of Raynesian viscosity to kinematic viscosity υ (mm2/s) = 0.26r-172 /R When R BBB 0 225s, υ (mm2/s) = 0.26r
3) Conversion of Saywell viscosity and kinematic viscosity :υ (mm2/s) = 0.225s. The equation is used when S>285s