Dynamic, Absolute and Kinematic Viscosity

An introduction to dynamic, absolute and kinematic viscosity and how to convert between CentiStokes (cSt), CentiPoises (cP), Saybolt Universal Seconds (SSU) and degree Engler

The viscosity of a fluid is a very important property in the analysis of liquid behavior and fluid motion near a solid boundary.

The viscosity of a fluid is its resistance to shear or flow and is a measure of the fluids adhesive/cohesive or frictional properties. The resistance is caused by intermolecular friction exerted when layers of fluids attempts to slide by an other.

The knowledge of viscosity is often necessary for proper design of required temperatures for storage, pumping or injection of fluids.

Common used units for viscosity:

¹ Where centistokes are greater than 50

There are two related measures of fluid viscosity which are known as dynamic (absolute) and kinematic viscosity.

Dynamic (absolute) Viscosity

is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid.

The shearing stress between the layers of non turbulent fluid moving in straight parallel lines can be defined for a Newtonian fluid as:

Equation (1) is known as the Newtons Law of Friction.

In the SI system the dynamic viscosity units are N s/m², Pa s or kg/m s where

1 Pa s = 1 N s/m² = 1 kg/m s

The dynamic viscosity is often expressed in the metric CGS (centimeter-gram-second) system as g/cm.s, dyne.s/cm² or poise (p) where

1 poise = dyne s/cm² = g/cm s = 1/10 Pa s

For practical use the Poise is to large and it's usual divided by 100 into the smaller unit called the centiPoise (cP) where

1 p = 100 cP

Water at 68.4^oF (20.2^oC) has an absolute viscosity of one - 1 - centiPoise.

• A Viscosity Converting Chart - A table converting between Centiposes, milliPascal, CentiStokes and SSU.
• Absolute or Dynamic Viscosity of Water - Absolute or dynamic viscosity of water depends on the temperature.
• A Dynamic Viscosity Converting Chart - Convert between Poiseuille- Poise - centiPoise - kg/m.h- kgf.s/m^{2 -} lbf.s/inch² lbf.s/ft^{2 -} lbf.h/ft^{2 -} lb/ft.s - lb/ft.h.

Kinematic Viscosity

is the ratio of absolute or dynamic viscosity to the density, a quantity in which no force is involved.

For the SI system the theoretical unit is m²/s or common used Stoke (St) where St = 10^-4 m²/s. Since also Stoke is an unpractical large unit, it is usual divided by 100 to give the unit called Centistokes (cSt) where 1 St = 100 cSt.

1 cSt = 10^-6 m²/s

Since the specific gravity of water at 68.4^oF (20.2^oC) is almost one it follows that the kinematic viscosity of water at 68.4^oF is for all practical purposes 1.0 cSt.

Viscosity and Reference Temperature

Viscosity is highly temperature dependent and for either dynamic or kinematic viscosity to be meaningful a reference temperature must be quoted. In ISO 8217 the reference temperature for residual fluids is 100^oC, whilst for distillate fluids it is 40^oC.

For liquids the kinematic viscosity decreases with higher temperatures. For gases the kinematic viscosity increases with higher temperature.

• Dynamic and Kinematic Viscosity of Water in Imperial and SI Units - Viscosity of water at temperatures between 0 - 100^oC or 32 - 212^oF

Other Viscosity Units

Saybolt Universal Seconds (or SUS), a unit of measure used to indicate viscosity. The efflux time in Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature, as prescribed by test method ASTM D 88. This method has largely been supplanted by the kinematic viscosity method. (This is also called the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol)).

Degree Engler is used in Great Britain, as a scale used as a conventional measure of kinematic viscosity. Unlike the Saybolt and Redwood scales, the Engler scale is based on comparing a flow of the substance being tested to the flow of another substance, namely water. Viscosity in Engler degrees is the ratio of the time of flow of 200 cubic centimetres of the fluid whose viscosity is being measured to the time of flow of 200 cubic centimeters of water at the same temperature (usually 20^oC but sometimes 50^oC or 100^oC) in a standardized Engler viscosity meter.

• Viscosity of Common Materials Viscosity of fluids as motor oil, diesel fuel, peanut oil and many more.
• Online Dynamic Viscosity Calculator Calculate dynamic viscosity between centipoise, reyn and more.
• Online Kinematic Viscosity Calculator Convert between different kinematic viscosity units - centistokes, poise, lentor and more
• Online Dynamic Viscosity Converter for Oil and Water Convert between dynamic viscosity units for oil and water

Newtonian Fluids

Fluids for which the shearing stress is linearly related to the rate of shearing strain are designated as Newtonian Fluids.

Newtonian materials are referred to as true liquids since their viscosity or consistency is not affected by shear such as agitation or pumping at a constant temperature. Fortunately most common fluids, both liquids and gases, are Newtonian. Water and oils are examples of Newtonian liquids.

Thixotropic Fluids

Shear Thinning Fluids or Thixotropic Fluids reduce their viscosity as agitation or pressure is increased at a constant temperature. Ketchup and mayonnaise are examples of thixotropic materials. They appear thick or viscous but are possible to pump quite easily.

Dilatant Fluids

Shear Thickening Fluids or Dilatant Fluids increase their viscosity with agitation. Some of these liquids can become almost solid within a pump or pipe line. With agitation, cream becomes butter and Candy compounds, clay slurries and similar heavily filled liquids do the same thing.

Bingham Plastic Fluids

Bingham Plastic Fluids have a yield value which must be exceeded before flow will start flow like a fluid. From that point on the viscosity will decrease with an increase in agitation. Toothpaste, mayonnaise and tomato catsup are examples of such products.

Example - Converting between Kinetic and Absolute Viscosity for Air

Kinematic viscosity of air at 1 bar and 40^oC is 16.97 cSt (16.97 10^-6 m²/s).

The density of air may be estimated by the Ideal Gas Law:

density = 10⁵ / 287 313 = 1.113 kg/m³

absolute viscosity = 1.113 16.97 10^-6 = 1.88 10^-5 kg/ms (Ns/m²) (P)

Viscosity Values and Specific Gravity of some Typical Liquids

¹⁾ centiPoise = centiStokes x specific gravity - where specific gravity is assumed to be 0.8 (except for water).

The exact Centipoise of can be calculated:

centiPoises (cp) = centiStokes (cSt) x Density

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