Viscosity Cups

Q. I am a technician in a paint manufacturer’s quality control lab. I was disturbed to discover that viscosity cups of the same type made by different suppliers do not give the same values, and did not agree with their respective data sheets as to volume. Can you answer the following questions? 

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1. Must these viscosity cups conform to certain specs?
2. What is the degree of tolerance?
3. How important is their accuracy?
4. Should cups similar cups made by different manufacturers give the same readings? D.M.

A. You raised some interesting questions about viscosity cups which could also be applied to viscometers in general. Most of the cup types were developed for paint finishing line use, where painters can quickly adjust paint viscosities for spraying.
This means they were not intended to be used as laboratory instruments for absolute viscosity measurements, owing to the many variables involved such as paint temperature, Newtonian flow, etc. The answers to your questions are:

1. Because many of the cups you described are not covered by federal or ANSI Specifications, they need only comply with their manufacturer’s specifications.
2. Because there is so little control over the manufacture of these cups, it follows that there will be little control over the degree of tolerance.
3. Because these cups were never intended for use as laboratory instruments, speed is more import than accuracy. Furthermore, in many painting specifications, viscosities are given as a range or as a minimum or maximum, e.g. “20–25 sec on a particular cup,” “not less than 30 sec or not more than 40 sec.”
4. My personal feeling is that cups made by different companies should give similar readings.

For your purposes in the quality control laboratory, I suggest you investigate more precise laboratory instruments. These are listed under Viscometers on Page 400 of the 2007 PRODUCTS FINISHING DIRECTORY AND TECHNOLOGY GUIDE.