Viscosity - Newtonian and Non-Newtonian Fluids.
Viscosity is defined as the degree to which a fluid resists flow under an applied force, measured by the tangential friction force per unit area divided by the velocity gradient under conditions of streamline flow; coefficient of viscosity. The falling piston method of measuring viscosity is superior because it accurately reflects the ratio of the Shear Stress/Shear Rate of a fluid. For a detailed discussion of Non-Newtonian fluid viscosity and the Norcross Falling Piston method of measuring viscosity click here: 

Figure 3 shows what happens when the force is increased even more. Note that while the force was doubled in each case the increase in speed is not the same in Figures 2 and 3.

Process viscometers apply more force than efflux cups and thus will measure this non-linear response.

It is for this reason that when process viscometers are to display viscosity, as efflux cup seconds, they need to be calibrated to the specific efflux cup and process fluid being measured.

Once calibrated they will track changes in viscosity and can display viscosity in efflux cup seconds.

Figure 4 shows how the NORCROSS Falling Piston is a very close approximation to the basic method of measuring viscosity. This is one reason the NORCROSS Viscometer is so successful.

Figure 2 shows what happens when measuring a Non-Newtonian fluid, such as an ink or coating. The change in velocity is non-linear.

Figure 1 shows the two surfaces with a fluid between them. A force is applied to the top surface and thus it moves at a certain velocity. The ratio of the Shear Stress / Shear Rate will be the viscosity.

Note that as the force is doubled then the velocity doubles. This is indicative of a Newtonian fluid, such as motor oil.

VISCOSITY CONVERSION TABLES

A PROUD TRADITION IN PROCESS VISCOSITY CONTROL

Norcross Corporation is making available free viscosity conversion tables and free viscosity monitoring and control application notes. Norcross’s comprehensive viscosity conversion tables facilitate conversion among Shell and Zahn Cup seconds, centipoises, SSU, SSF, and other viscosity units.

For comprehensive viscosity conversion tables, click HERE to download document 8.2.2 as an Adobe Acrobat PDF file.

For flexographic viscosity conversion tables, click HERE to download document 8.2.1 as an Adobe Acrobat PDF file.

Viscosity - Newtonian and Non-Newtonian Fluids continued.