The Viscosity Measuring Dip Cup That You Use Matters
If you search for “Zahn cups” using your web browser you’ll find multiple supplier listings. 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.
All Zahn type cups are not the same. A Zahn type cup bought online is almost guaranteed to not be the same as one purchased from a reputable supplier/manufacturer.
Zahn-type cup copies abound. The efflux time in seconds, production lot, or production run may greatly vary from one makers’ cup to another makers cup. Efflux times in seconds have been found to vary in excess of 50%.
Most of the available cups are not certified as liquid calibrated. Some of the cups have orifices drilled off-center and/or out of round. Buying an uncalibrated cup is said to be like buying a yardstick without measurements.
Viscosity measurements taken with Zahn cups are not particularly accurate categorically. 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. It even gets worse when multiple test persons are involved. The problem lies with reaction time, how the person is trained and a cups condition.
Most reputable cup manufacturers supply NIST Zahn cups that are guaranteed to comply with ASTM D4212. These cups are calibrated with standard G series oils.
In practice, a calibrated cup is used as a standard for comparison to cups used 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.
It’s well known that cups can take a beating on the shop floor. They are subject to not being cleaned properly between viscosity checks, being dropped or banged against hard surfaces damaging the cup physically. 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.
Zahn cups are a neat tool for obtaining a quick estimate of fluid viscosity. Viscosity describes a fluid’s internal resistance to flow. Viscosity is “thickness,” therefore, water is “thin” with a low viscosity, and honey is “thick” with a high viscosity.
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.
Zahn cups feature a stainless-steel shell with a tiny drain hole (orifice) in the center of the cups bottom. A stain-less steel handle is affixed for handling. There is a series of five cups (Zahn #1 thru Zahn #5) to accommodate a large range of viscosity testing.
Zahn cups use gravity 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. Therefore, the stoke or centistokes, units of kinetic viscosity, factor in the density of a fluid and gravity are used to measure viscosity.
Seconds efflux time for any cup can be correlated to a viscosity value in centistokes and vice versa. Therefore, efflux times between various cups can be converted using charts. This can result in greater accuracy, but the differences are not large for aqueous products.
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. The Zahn #3 viscosity of an aqueous coating can vary as much as 60 seconds when measured at 50° F and 100° F.
Comparing Zahn S90/Signature & EZ™ Dip Cups
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. Gardner Co. Inc.
The former is equivalent to the original GE Zahn Cup. The latter designed to meet earlier specifications. They differ in efflux time in seconds. Both Dip Cup series are CALIBRATED using standard oils assuring that efflux time in seconds is comparable from cup to cup. Paul N. Gardner Co. alone is authorized to use the Zahn name.
EZ™ Zahn cups are calibrated within 3% tolerance complying with and exceeding ASTM D4212 & D816 specifications. The S90/Zahn Signature cups are calibrated within a 5% tolerance.
Differences are seen in viscosity measurements between the S90/Zahn Signature dip cup and the EZ™ Zahn (ASTM) dip cup series. They are similar but not exactly the same. A S90 #2 cup has centistoke (cSt) range of 39 to 238 while the EZ™ #2 cup has a range of 19 to 56 cSt.
S90/Zahn efflux time does not meet ASTM specifications, but tables are available to convert S90/Zahn efflux time to EZ™ Zahn efflux time to comply with ASTM D4212.
The GARDCO EZ™ cup orifice is machined rather than drilled for exact burr free center positioning to produce a correct length and symmetrical efflux stream. Each EZ™ cup is supplied with a table to allow conversion from efflux time in seconds to the closest tenth of a second to viscosity in centistokes (cSt).EZ™ cups can be supplied with a Certificate of Calibration. The GARDCO EZ™ dip cup series is an improvement over the GARDCO S90/Signature dip cup series.
Viscosity Measurement with Zahn Dip Cups
- Select the proper viscosity range cup & inspect it for damage and cleanliness.
- Stir & adjust the test fluids temperature to match the products Technical Data Sheet information.
- Submerge the cup at an angle into foam free fluid for 30 seconds bringing the cup to the fluids’ temperature.
- Lift the cup vertically using the finger ring, starting the stopwatch as the cup first breaks the fluids surface.
- Stop the timer when the fluid stream breaks, recording time, Zahn cup # & temperature.
- Optionally, read the viscosity from the appropriate conversion chart.
- Clean & flush the cup promptly, using a suitable solvent, and a nylon fish line to clean the orifice, then dry thoroughly.
Our business at Cork is the development and formulation of Aqueous, energy curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives. Cork thrives on its ability to formulate novel, useful specialty products that offer the graphic arts industry printer/coater a competitive advantage.