![]() The latter designed to meet earlier specifications. The former is equivalent to the original GE Zahn Cup. The gold standard in viscosity dip cups are the GARDCO EZ™ Zahn (ASTM) Dip Cups and the S90/Zahn Signature Dip Cups manufactured by Paul N. Comparing Zahn S90/Signature & EZ™ Dip Cups The Zahn #3 viscosity of an aqueous coating can vary as much as 60 seconds when measured at 50° F and 100° F. Temperature is critical in the measurement of viscosity, especially regarding operating viscosity stabilization. A rise in the temperature of a coating drops the coating’s viscosity and its flow increases, while a drop in a coatings’ temperature causes a rise in viscosity and a decrease in flow. This can result in greater accuracy, but the differences are not large for aqueous products. Therefore, efflux times between various cups can be converted using charts. ![]() Seconds efflux time for any cup can be correlated to a viscosity value in centistokes and vice versa. Therefore, the stoke or centistokes, units of kinetic viscosity, factor in the density of a fluid and gravity are used to measure viscosity. The poise or centipoise, units of dynamic viscosity, do not consider the role of gravity in measuring a fluids’ resistance to flow. Zahn cups use gravity to measure viscosity. There is a series of five cups (Zahn #1 thru Zahn #5) to accommodate a large range of viscosity testing. A stain-less steel handle is affixed for handling. Zahn cups feature a stainless-steel shell with a tiny drain hole (orifice) in the center of the cups bottom. Zahn cups (#2 Zahn or #3 Zahn) are very commonly used to measure the viscosity range of flexo inks, gravure inks, and coatings in the graphic arts industry. Viscosity is “thickness,” therefore, water is “thin” with a low viscosity, and honey is “thick” with a high viscosity. Viscosity describes a fluid’s internal resistance to flow. Zahn cups are a neat tool for obtaining a quick estimate of fluid viscosity. A build-up on the cups surface or damage will have an affect on the cups ability to provide an accurate viscosity reading, as will dents that change the cups volume. They are subject to not being cleaned properly between viscosity checks, being dropped or banged against hard surfaces damaging the cup physically. It’s well known that cups can take a beating on the shop floor. This guarantees that each cup can read as close to the same as possible and further identifies cups that are damaged beyond use. ![]() In practice, a calibrated cup is used as a standard for comparison to cups used on the shop floor. These cups are calibrated with standard G series oils. Most reputable cup manufacturers supply NIST Zahn cups that are guaranteed to comply with ASTM D4212. The problem lies with reaction time, how the person is trained and a cups condition. ![]() It even gets worse when multiple test persons are involved. It’s well known that test results are not often reproducible even when using the same cup, the same test sample and with the same person doing the testing. Viscosity measurements taken with Zahn cups are not particularly accurate categorically. Buying an uncalibrated cup is said to be like buying a yardstick without measurements. Some of the cups have orifices drilled off-center and/or out of round. Most of the available cups are not certified as liquid calibrated. Efflux times in seconds have been found to vary in excess of 50%. The efflux time in seconds, production lot, or production run may greatly vary from one makers’ cup to another makers cup. A Zahn type cup bought online is almost guaranteed to not be the same as one purchased from a reputable supplier/manufacturer. Looking at these you’ll notice different brand names and countries of origin, mostly China and Hong Kong. This is not to say that foreign manufacturers cannot make good products adhering to good manufacturing standards, but many are cheap knock-offs, and It’s hard to know which is which.Īll Zahn type cups are not the same. If you search for “Zahn cups” using your web browser you’ll find multiple supplier listings. The Viscosity Measuring Dip Cup That You Use Matters ![]()
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