Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2008

Choosing the Correct Zahn Cup

Q: We currently measure the viscosity of our coatings using a S90 #2 Zahn Cup. Typical efflux times for various products range from 13-17 seconds. As I understand this is lower than the range typically measured with a #2 Zahn cup. Should we be using a #1 cup instead? The product is a solvent borne coating. Thank you!

A: A viscosity of 13-17 seconds is so short that a small error in stopping the stop watch makes a significant difference to the result. If you were to use a smaller diameter orifice, such as a #1 Zahn cup that you suggested, you would lessen the potential error. When you go to a smaller orifice insure that toward the end of the measurement, the effluxing paint should not start, stop, start, stop, etc. You should get a clean cut-off.

Good luck,

Ron Joseph