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Beyond the Zahn Cup

SpecialChem / Dec 10, 2008

The standard coatings method for reading batch viscosity usually involves a Zahn or Efflux cup (Figure 1). This tried and true method involves testing for kinematic viscosity by dipping the cup into the batch and filling it with material. The user then draws the cup out of the material. When the cup breaks the surface, a stop watch is used to time the flow out of the hole in the bottom of the cup until the cup is empty. From this method, kinematic viscosity is then computed. Sounds simple, and it is; no special skills are required to use a Zahn or Efflux cup. However, there are variables involved that can contribute to an inaccurate viscosity measurement. This is hardly ideal when an accurate viscosity is required on materials being produced in large and sometimes expensive quantities. Variables that can contribute to inaccurate readings include temperature considerations, operator error and maintenance issues such as a cup with a hole that has not been cleaned. Also, what does one do if rheological data (flow behavior) is required?.

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