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A **Zahn cup** is a viscosity measurement device widely used in the paint industry. It is commonly a stainless steel cup with a tiny hole drilled in the center of the bottom of the cup. There is also a long handle attached to the sides. There are five cup specifications, labeled Zahn cup #x, where x is the number from one through five. Use large number cup sizes when viscosity is high, use low number cup sizes when viscosity is low.

To determine the viscosity of a liquid, the cup is dipped and completely filled with the substance. After lifting the cup out of the substance the user measures the time until the liquid streaming out of it breaks up, this is the corresponding "efflux time".

On paint standard specifications, one denotes viscosity in this manner: *efflux time*, *Zahn cup number*.

**Conversion**

One can convert efflux time to kinematic viscosity by using an equation for each cup specification number, where *t* is the efflux time and *ν* is the kinematic viscosity in centistokes.

- Zahn Cup #1:
*ν*= 1.1(*t*- 29) - Zahn Cup #2:
*ν*= 3.5(*t*- 14) - Zahn Cup #3:
*ν*= 11.7(*t*- 7.5) - Zahn Cup #4:
*ν*= 14.8(*t*- 5) - Zahn Cup #5: ν = 23
*t*

- Patton, Temple C. (1979) (in English).
*Paint Flow and Pigment Dispersion*(2nd ed. ed.). John Wiley & Sons Inc. ISBN 0-471-03272-7.