Efflux cups are employed in most applications as a benchmark measurement for the fluid in process; they are by no means accurate. Why, you ask? Human nature! You may have 2 or 3 operators measuring the fluids and each may get 3 disparate readings of the same fluid. This may have to do with how the operator was trained to take a cup reading. It may be due to the operator’s reaction time on the stopwatch. It may be a result of when the operator actuated the stopwatch when pulling the efflux cup from the fluid. Whatever the reason, there is less precision when taking a cup measurement.
Let’s talk about differences in shear stress and how it relates to viscosity measurement.
In order to do so it’s important to remember there are a number of different ways to measure viscosity: efflux cups, capillary tubes, plate and cone devices, vibration, torsional and falling piston/ ball and so on.
Each employing a different principle of measurement, each creating shear on the fluid, either by two surface physical contact with the ink (falling piston, plate & cone), high or low frequency dampening (torsional, vibration), and efflux cup drain time (Zahn cup, Ford cup).
Since viscosity is a measurement of how a fluid reacts to shear applied, it is safe to assume the falling piston, and plate and cone will be the most sensitive i.e. see smaller changes in viscosity, followed by torsional and vibration, the cups and tubes.
Here’s where it starts to get exciting
Let’s assume we have 3 different viscometer types on a recirculating print line, a falling piston, a vibrating reed and torsional. Now take a Zahn #2 cup measurement of the ink, which, after averaging, reads 22 seconds.
Now actuate the sensors and notice the measurements, the falling piston reads 30 seconds, the vibrating reed displays 18 seconds, the torsional indicates 24 seconds.
You may think there is something wrong with the viscometers when in fact, they are all reading correctly. Each method of measurement creates a different shear stress on the ink resulting in disparate readings. The non-Newtonian physical properties and chemical makeup of the ink have different reactions to the shear stress applied.
As another example you may have three coatings in process, all read 22 Zahn cup seconds. You may put a falling piston sensor in line with each, the first sensor read 22 seconds, the 2nd reads 32 seconds, the 3rd reads 18 seconds.
Once again, they are correct. The non-Newtonian properties react, or perform, in a different way to the shear stress applied.
Based on this fact, it is reasonable to assume that, lower shear devices will provide less shear stress on the fluid, so smaller changes in viscosity may not be noticed.
Whereas, the higher shear devices will, by virtue of the more precise nature of their measuring method, sense smaller changes in viscosity. That sensitivity, when introduced to an ink or coating system, can ensure tighter process control. That tighter control can result in more consistent coating or color, leading to less waste due to rejections. It follows that if there is less waste, there may be more profit for the job run.