This difference in the reading depends on how the fluid behaves to shear stress. For example, a falling piston type viscometer creates significantly more shear stress during its measuring cycle. 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. The Zahn cup's reading could be less sensitive than an automated viscometer employing a higher shear methodology. The only forces at work are gravity causing the flow and the hole size and gauge of the cup. The principle operation of a Zahn cup is resistance to flow a preset volume of fluid draining through a set hole size. Misconception #4 – A Zahn cup will read the same value as an automated viscosity control system This ensures that each cup reads as close to the same as possible and identifies cups which are no longer usable. 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. 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.
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The fact is that on the shop floor, the cups take a beating. Misconception #3 – Zahn cups do not require calibration 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. 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. Misconception #2 – All Zahn cups are the sameĪ #2 Zahn cup bought online through eBay is not the same as one purchased from a reputable supplier or manufacturer. 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. 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%. Misconception #1 – Zahn cups are accurate 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. This applies to flexographic and gravure printing, as well as food and beverage can manufacturing, just to name a few applications. This is hardly ideal when precise viscosity readings are required as they are on the inks and coatings employed in printing. However, there are variables involved that can contribute to an inaccurate viscosity measurement. Other than the ability to use a stopwatch, no special skills are required to use a Zahn cup. As I am fond of saying, it's not rocket science it's fluid science, (which requires a degree of education). For purposes of this blog, I am using the Zahn cup to refer to all efflux cups). (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.
![viscosity cup viscosity cup](https://d2j6dbq0eux0bg.cloudfront.net/images/13046272/1164687215.jpg)
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." To determine the viscosity of a liquid, a Zahn cup is dipped and completely filled with the liquid to be measured.