Manufacturers who dispense fluids in their process (i.e. – paints and coatings, sealers and adhesives, potting compounds and encapsulants, etc.) all understand that the viscosity of that fluid is fundamental to the process out
And for years, we’ve all been measuring that viscosity with a Zahn Cup. But while we say “Zahn Cup”, in reality this is a lot like saying “Kleenex”, it applies to any of the various cups used to measure viscosity in process control applications, as shown at right, like DIN cups, Ford Cups, Fisher Cups, and a host of others. In this three-part series we take a close look at viscosity and viscosity measurement to figure out where the issues lie.come (read: quality).
A Known Measurement Problem
Ask any of these manufacturers and they will likely tell you that the cup measurement process is fraught with problems. Unfortunately, being a manual process, most of these are laid at the feet of the operator – variations in how the cup is filled and withdrawn from the fluid, when the stopwatch is started and stopped, how the cup is cleaned, and the list goes on.
But in reality, the problems begin even before the operator picks up the cup. Let’s start with the choice of cup. Different cups are designed to measure different viscosity ranges – determined by cup volume and orifice size. It is very important to select the proper cup for the fluid you are measuring.
But how many cups do most manufacturers usually have on hand? If you guessed “one” you’re right about 99% of the time.
And then there’s the temperature of the measurement. All cups are designed to measure over a fairly limited temperature range – often right around 25°C (77°F) – the temperature at which they are calibrated. Remember, virtually all fluids change viscosity as a function of temperature. Furthermore, most cups are metal. As their temperature changes, so does their volume and more importantly the diameter of the orifice, which can have a significant impact on the measurement. It’s also very important that the cup be at the same temperature as the fluid. Otherwise, the fluid and cup are changing as the measurement progresses, introducing unpredictable error. This can be a greater problem with a cup of high mass (like a Ford or DIN Cup) than with a cup with a lower mass (like a Zahn Cup).
In short, it’s just not perfect…
So, you have to ask yourself, “Why are we trusting such a critical control parameter to such an unreliable measurement process?” Or, “It’s been used and trusted for years, is it really that unreliable?”
In order to answer these questions, we need to start with the fundamentals of viscosity.
What is Viscosity?
As defined by none other than Sir Isaac Newton, viscosity is the resistance of a fluid to flow. There are two types of viscosity: Kinematic and Dynamic.
Kinematic viscosity is the measure of a fluid’s resistance to flow when no external force, except gravity, is acting on it. A good example is how honey or water spread out on a tabletop when you spill them. The water spreads out quickly because it has very little resistance to flow (low viscosity) whereas the honey spreads out slowly because it has very high resistance to flow (high viscosity). Often measured in the unit centistokes (cSt) to honor Sir George Gabriel Stokes, Kinematic measurements are dependent on the density of the fluid.
Dynamic Viscosity is a fluid’s resistance to flow when an external force, like pumping or compressing, is applied to it. This is the reason that fluids require pressure to get them to flow through a pipe. The thicker (more viscous) the fluid, the higher the pressure (force) required to get it to flow. The standard unit is the Poise, so named to honor Jean Léonard Marie Poiseuille, but it is most commonly measured in centipoise (cP). Unlike Kinematic viscosity, density is not a factor in Dynamic viscosity measurements.
But there is another important fact that we must consider in this discussion: thixotropy – and that refers to how a fluid behaves when it is placed “under stress”.
In the next installment in this series, we will look at Newtonian and Non-Newtonian fluids and how they affect our measurement process.