In our last post, we discussed how the non-Newtonian properties of most coatings affect how they perform in our process (click here if you missed it or want to review). In this segment, we look at how these properties make real-time measurement of fluid viscosity difficult.
A Brief Review
In our last post we reviewed how the shear sensitive nature of non-Newtonian fluids makes them harder to manage in our process. We also looked at how shear is ever-present in virtually all coating processes. Perhaps this explains the increasing trend toward in-line viscosity measurement in the coating world.
How Shall I Measure Viscosity? Let Me Count the Ways…
There are almost as many ways to measure viscosity as there are engineers to design viscometers! Everybody wants to have something to set them apart from the pack. For the purpose of this discussion, however, we are going to focus primarily on in-line, real-time viscometers.
The No-Moving-Parts Trend
In the effort to set themselves apart, many viscometer manufacturers have settled on a “no-moving-parts” strategy for their in-line process viscometers. This is based on the need to keep these clean, or easy to clean, so that they function properly on a continuing basis, but the peculiar thing is that almost all of these viscometers utilize moving parts as their measuring elements. In all fairness, the trick is that these are often comprised of a smooth, rotating (or more accurately, oscillating) probe that attempts to measure the resistance to the rotation exerted by the fluid under test – but the parts are moving just the same.
The reality is that the only true “no-moving-parts” sensors are the ultrasonic type that measures viscosity by generating and measuring the movement of “waves” through the fluid. These are often referred to as “acoustic viscometers” even though their frequency of operation may be well outside the “audible range”.
No Movement – No Shear!
When measuring non-Newtonian fluids, like coatings, shear is an essential component in getting a reliable, accurate, and applicable measurement that we can use to manage our coating process. Without shear in the equation, we can only guess what the fluid is going to do in our process. The problem is, these “no-moving-parts” viscometers don’t produce the shear necessary to get an accurate measurement of our coating.
But didn’t I just say that most of the “no-moving-parts” viscometers have moving measurement elements?
Yes, I did.
The case of the acoustic or ultrasonic viscometers is simple, the waves propagated in the fluid simply don’t create shear. But in the case of other “no-moving-parts” viscometers – the ones with moving probes – they often restrict their motion to 90° rotations, or other oscillations, that don’t create the kind of shear that the coating will experience in the application system.
Measuring With Shear in the Lab
To measure non-Newtonian fluids in the lab, a rotating plate or cone squeezes the fluid against a stationary plate, placing it in shear between the two surfaces. In fact, these are some of the oldest and most renowned systems out there. But in reality, this measurement scheme closely recreates the kind of shear that a coating will experience in a dispensing system and will produce a reading that we can actually use. But taking samples to the lab for measurement doesn’t provide the kind of real-time results that we need to manage our process.
So what are we to do?
Putting the Shear Back in the In-Line Measurement System
As with those lab systems, some of the oldest in-line measurement systems utilize shear as an integral part of their measurement process. Though discredited by those who would have us believe that their “new technologies” are superior, if we look at what is really important to effectively managing our coating system, it makes sense to turn to those technologies, regardless of their age, that provide the key measurement parameters that we need.
Maybe that’s why “vintage” is so popular…