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.
Fig 107-1-154
Figure 2 shows what happens when measuring a Non-Newtonian fluid, such as an ink or coating. The change in velocity is non-linear.
Fig 107-2-155
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.
Fig 107-3-156
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.
Fig 107-4-157
For a full explanation, click here to download an Adobe Acrobat PDF file.
