About viscosity

Rotary viscometers

The principle of operation of rotational viscometers: a spindle (cylinder or disk) is submerged in the sample to be tested, measuring the force applied to overcome the resistance against rotation or flow. A spring is connected between the spindle (cylinder or disk) and the motor shaft which is rotating on a certain speed. The deviation angle of the spindle with respect to the measuring spring is measured electronically obtaining a torque value. The torque value measured with the Viscometer is based on the rotating speed and the geometry of the spindle; the result is a direct reading of the viscosity value in mPas/cP (dPas/P).

 

Depending on the viscosity, the resistance to the movement of a substance changes proportionally to the speed or size of the spindle. The viscometer has been calibrated to obtain viscosity readings in mPas or cP (dPas/P), considering speed and spindle type. The combination of different speeds and spindles allows optimal viscosity measurements within the wide range of the instrument.

About viscosity

Viscosity is a distinctive property of the fluids. It is the measure of internal friction of a fluid when a layer of this fluid is forced to move in relation to another layer. Viscosity is a value highly dependant on temperature.

The standard units for dynamic viscosity measurements are mPa.s (S.I) or cP (C.G.S).

1mPas=1cP (centi-Poise)

1dPas=1P (Poise)

 

Shear Stress:

 

It is the force per unit/area required to produce movement in one layer of fluid in relation to another layer (internal friction). Standard units for shear stress values are N/m2 (S.I) or dynes/cm2 (C.G.S).

 

16 . User Manual v.7.09

 

Shear rate:

 

It is the measure of the speed at which layers of fluid move with respect to one another.

 

Standard unit for shear rate values is the “reciprocal second” written as sec-1 or 1/sec.

Important notes

Shear Stress

 

It is the force per unit/area required to produce movement in one layer of fluid in relation to another layer (internal friction). Standard units for shear stress values are N/m2 (S.I) or dynes/cm2 (C.G.S).

Shear rate

 

It is the measure of the speed at which layers of fluid move with respect to on another. Standard unit for shear rate values is the “reciprocal second” written as sec-1 or 1/sec.

Laminar flow

 

It is the ideal movement between layers without transfer of mass from one to the other. It is the base to calculate dynamic viscosity.

Turbulent flow

 

There is a certain speed from which a transfer of mass between layers occurs. Result is an apparently greater shear stress and an erroneously high viscosity reading. Turbulent flow is characterized by a sudden and notorious increase in viscosity above a certain speed.
Fluids can, generally speaking, be classified considering relation between shear stress and shear rate.

Newtonian fluids

 

In Newtyonian fluids shear stress and shear rate are in direct proportion.
Viscosity in Newtonian fluids at a given temperature, remain constant regardless of viscometermodel, spindle and speed being used. The most common Newtonian fluids are water and thin motor oils.

Non-Newtonian fluids

 

This kind of fluids do not show a lineal relation between shear stress and shear rate. Differentworking conditions have as a result different viscosity values.
Apparent viscosity is defined as the result of a fluid analysis. This result can be reproduced in another viscometer only if analysis is carried out maintaining identical working conditions and following a defined working process. Variables below influence results:

  • Viscometer model.
  • Dimensions of sample container.
  • Filling level.
  • Sample temperature.
  • Spindle.
  • Rotating speed.
  • Spindle protector, Yes or Not.
  • Duration of test (time dependant fluids).

Generally speaking each modification in the working method and working process will indefectibly lead to variations in final analysis results.

There are different behaviors within the non-newtonian fluids:

Pseudoplastic

 

Samples whose viscosity decreases when increasing shear rate. It is also called “shearthinning” flow behavior. Most common pseudoplastic fluids are coatings, milk, ink and jam.

Plastic

 

Under static conditions they might have a similar behaviour to a solid. For a correct evaluation of the fluid it is necessary to reach the “yield value” to make fluid flow so that product later shows any of the possible material characterizations: newtonian, pseudoplastic or dilatant.

Examples: toothpaste, chocolate, grease..

Dilatant

 

Viscosity of dilatant fluids increases when shear rate increases. It is also called “shearthickening” flow behaviour.

 

Examples: solutions of sugar and water and mixtures of sand and water.

Time depending fluids

 

Apparent viscosity depends not only on shear rate but also on the time elapsed under conditions of shear.

Thixotropic

 

Those fluids in which viscosity and shear stress decrease, maintaining a constant shear rate, with time.
Examples: Ketchup, honey, anti-drop paints, mayonnaise.

Rheopeptic

 

Those fluids in which viscosity and shear stress increase, maintaining a constant shear rate,with time.
Lubricants and some paints types are rheopeptic fluids.

Spindles

 

These accessories are made with the maximum accuracy to ensure reliable measurements according to the instrument’s specifications, as long as, the instrument is kept in good operating conditions.

Reference substances: viscosity values

SubstanceAprox. viscosity (mPas)
Motor oil SAE 10 65
Olive oil84
Paint sprayer 100
Yoghourt 150
Sugar solution 70%400
Lubricant oil 50 – 1.000
Concentrated fruit juice1.500
Inks550 – 2.200
Honey10.000
Nanocellulose (general purpose additive)8.000 – 10.000
Toothpaste100.000