Viscosity
Viscosity
More Details on on Viscosity are to be found in the links below the table
ViscosityA plate of area "A" moving under the action of a force "F" with a resulting velocity "V" over a fixed plate separated by a film of fluid "h" thick. The shear stress on the fluid is "F / A". For a Newtonian fluid the shear stress is directly proportional to the velocity gradient in the fluid- "dV /dh" .
P / A = m . dV /dy
The proportionality constant " m " is the viscosity and has units
of ( N/m 2) .( m ). ( m / s ) = N . s / m 2...
The coherent SI unit of dynamic viscosity is the pascal second (Pa s). This equates to a Newton second per square metre
(N.s / m 2 ). In the cgs unit the poise is the corresponding unit which has dimensions
of dyne seconds per cm 2. The centipoise (cP = 10 -2 poise) is the most common
unit used in tables and handbooks.... 1 Pas = 1000. cP. The viscosity of water at room temperature
is approximately 1 cP
This is the ratio of the dynamic viscosity and the fluid density r ( kg /cubic m ) The SI unit of kinematic viscosity
is the m 2 / s. The equivalent cgs unit is the stoke (St) which has the unit of
cm 2 / s. The unit commonly used is the centistoke(cSt)....... 1 m 2 / s = 10 6 cSt
Absolute Kinematic Viscosity (m 2 / s ) = m ( Pa s ) / r (kg / m3 )
Kinematic viscosity (centiStokes) = m (centiPoise) / r ( grammes /cubic cm )
For water with a density of about 1000kg/m3 the conversion is simply 1 cST = 1 cP
Viscosity conversion table Viscosity conversion table
Saybolt Universal Viscosity
A method of determining a lubrication fluids viscosity is to measure the rate of flow of the fluid through a test device. The rate is measured in seconds the greater the number of seconds the more viscous the fluid. This viscosity is seconds t = Saybolt universal Viscosity - seconds. The dynamic viscosity relates to the Saybolt universal viscosity (t) by the following formula
Viscosity m (Poise) = (0.00022 .t — 0.18 / t )
SAE Viscosity grading
SAE (Society of automobile engineers ) has allocated numbers for specifying the viscosity of Engine Oils..
The SAE grades 0W through 25W, where W stands for Winter, have a maximum viscosity
specified at low temperatures (—5 through —35C), to ensure easy starting under
low temperature conditions, and a minimum viscosity requirement at 100C to ensure
satisfactory lubrication at the final operating temperature.
The SAE grades 20 through 60 only have limits set at 100C as these grades are not
intended for use under low temperature conditions.
For marine applications,
monograde oils of SAE 30 or SAE 40 are used because of the steady operating conditions
in a ship's engine room.
On the other hand, automotive oils are normally
formulated with Viscosity Index Improvers (VI Improvers) to provide multigrade performance.
VI Improvers are very large molecules, which are chemically made by linking together smaller
molecules in a process called polymerization. The resulting products,
called polymers, may have molecular weights 1000 times or more greater than the base
stock molecules. The use of these special polymers makes it possible to
meet both the low temperature viscosity requirements of the W grades as well as the high
temperature requirements of the non-W grades. Thus a SAE 20W 40 has the low temperature
viscosity value of a SAE 20W oil at low temperatures and the high temperature viscosity of a SAE 40 oil
at the higher temperature.
Different tests are completed to establish the viscosities for different operating regimes.
Tests are completed for low temperature