Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2005

Converting Kreb's Stormer Viscosity to Seconds for Paint Coatings

Q. How do you convert viscosity measure by 200RPM (Kreb Stormer) viscometer to weight and second?

A. The Kreb's Stormer viscometer uses shear rate to measure the viscosity of a paint coating. In the case you mentioned, it is the shear rate of a paddle that rotates at 200 RPM. Most paint coatings do not exhibit a simple relationship between shear rate and gravitational viscosity, such as is measured on a Ford #4 or Zahn cup. For instance, often the viscosity decreases as the shear rate increases, and that is why a constant shear rate is used on a Kreb's Stormer viscometer. On the other hand, with gravitational viscosity the weight of the coating in the viscosity cup determines how fast the coating will efflux from the orifice at the bottom of the cup. As you know, the smaller the orifice diameter, the longer it will take for the paint coating to drain. Therefore, the Kreb's Stormer viscometer measures viscosity very differently from a Ford #4 or Zahn cup.

I too have looked for conversions of KU viscosity to seconds on a Zahn cup, but have not found anything. If you are looking for a conversion, I suggest that you take your specific coating and have a paint laboratory measure the KU value at a standard temperature. Then measure the gravitational viscosity of the same coating using either a Ford #4 or a Zahn cup. You might do the calibrations at a few different temperatures.

Best wishes,

Ron Joseph