To determine the viscosity of a liquid, a Zahn cup is dipped and completely filled with the liquid to be measured. After lifting the cup out of the liquid, the user measures the time it takes for the liquid streaming from the opening in the bottom of the cup to begin “breaking up.” This is the corresponding "efflux time."
(Quick note: Efflux is defined as “the flowing out of a particular substance or particle.” Cups of this nature may be more recognizable by names like Zahn Cup, Shell Cup, or Ford Cup. For purposes of this blog, I am using the Zahn cup to refer to all efflux cups). As I am fond of saying, it's not rocket science it's fluid science, (which requires a degree of education).
Sounds simple right?
Other than the ability to use a stopwatch, no special skills are required to use a Zahn cup. However, there are variables involved that can contribute to an inaccurate viscosity measurement. This is hardly ideal when precise viscosity readings are required as they are on the inks and coatings employed in printing. This applies to flexographic and gravure printing, as well as food and beverage can manufacturing, just to name a few applications. These inconsistencies, which may contribute to inaccurate readings, include temperature swings, operator error and maintenance issues such as a cup with a plugged or damaged hole.
Misconception #1 – Zahn cups are accurate
It is well known that you can have three people using the same Zahn cup in the same ink and get three different readings varying by as much as 30%. It is not that anyone has done anything wrong, it all has to do with reaction time, how someone has been trained and the condition of the cup.
Misconception #2 – All Zahn cups are the same
A #2 Zahn cup bought online through eBay is not the same as one purchased from a reputable supplier or manufacturer. There are a variety of different manufacturers across the globe, some of which adhere to strict manufacturing guidelines and some of which are cheap knock offs. It's not easy to tell the difference. This becomes a problem when you are comparing one cup that has the correct volume and orifice to another cup which has a different volume and/or orifice.
Misconception #3 – Zahn cups do not require calibration
The fact is that on the shop floor, the cups take a beating. In addition to not being cleaned between measurements, they are sometimes dropped, hit or otherwise not handled with "kid gloves." Any buildup or damage will affect the cup reading which ultimately affects the ink viscosity in process. Since most reputable cup manufacturers offer NIST Zahn cups, (cups guaranteed to comply with ASTM D4212), it is imperative that the cups used on the floor are compared to the calibrated cup. This ensures that each cup reads as close to the same as possible and identifies cups which are no longer usable.
Misconception #4 – A Zahn cup will read the same value as an automated viscosity control system
The principle operation of a Zahn cup is resistance to flow; a preset volume of fluid draining through a set hole size. The only forces at work are gravity causing the flow and the hole size and gauge of the cup. The Zahn cup's reading could be less sensitive than an automated viscometer employing a higher shear methodology.
It is possible to use a #2 Zahn cup to measure the ink viscosity and get a reading of 22 seconds while an automated device may read that same ink at 15 or even 35 cup seconds. For example, a falling piston type viscometer creates significantly more shear stress during its measuring cycle. This difference in the reading depends on how the fluid behaves to shear stress. In the case of the falling piston, it is the shear stress applied by the surface area of the piston against the wall of the cylinder in which it falls that generates a more accurate reading.
(This is getting into the contrast between Newtonian and non-Newtonian fluids and can be a pretty abstract concept for the un-initiated. We did a 3-part series of blogs that do a nice job of describing Newtonian and non-Newtonian fluids. It is treated lightly here, yet it’s a very important topic. Click here to review Part 1, Part 2, and Part 3 of the series.)
Misconception #5 – A Zahn cup is good enough to control the viscosity of a print job
That may have been true 20 years ago, but is not the case anymore. Manually controlling the viscosity by using a Zahn cup is a good start and is helpful in benchmarking an ink. However, controlling the viscosity during the print job requires a vigilant effort on the part of the operator to ensure that viscosity stays within a certain range. This is not always possible due to other press concerns.
Invariably, the operator may be too late in noticing a problem with the color and try to add either solvent or makeup fluid to bring the color back. By that time, there may be hundreds of feet of printed film that could be rejected. Since customers are demanding better end product and are scrutinizing every inch of film, it is critical that the viscosity of the fluid is maintained throughout the print job. In these instances, manual control using a Zahn cup is just not going to make the grade.
Zahn cups are a nice tool for generating a quick and rough estimate of fluid viscosity. Whether they are sufficient to manage the quality of your print jobs is a decision only you can make. There have been many developments in the measurement and control of fluid viscosity since the emergence of the Zahn cup. If you are at all uncertain, (or if you are losing ground to your competition), it may make sense to look at some more cost effective options.
EDITOR'S NOTE: This post was originally published in March 2016, but due to its popularity and relevance, we're sharing it again for anyone who may not have seen it.
Download our FREE Viscosity Conversion Table to facilitate accurate conversion among Shell and Zahn Cup seconds, centipoises, SSU, SSF, and other viscosity units.