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. Large number cup sizes are used when viscosity is high, while low number cup sizes are used 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.
- 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 = 23t
Note the above equations are for a Brookfield series of Zahn cups. Gardco signature Zahn cups and Gardco EZ Zahn cups use different conversions. There seem to be no published conversions available for Weschler Instrument Zahn cups.