The terms “smoothness” and “roughness” are generally well understood as far as a dictionary meaning goes; however, the use of the terms in test methods is sometimes confusing. In the title of a test method, the term “smoothness” is used when an increasing number is correlates with a smoother surface measurement. An example of this would be the Bekk method, where a smoother surface requires more time for a given volume of air to leak across the surface. Since the reporting for Bekk smoothness is in units of time (seconds), a surface that measures 500 Bekk seconds is smoother than a surface that measures 200 Bekk seconds. If the Bekk instrument was originally configured to report in Bekk flow, which is the reciprocal of Bekk time, then the test methods would categorize it as a Bekk roughness tester.
The two most common instruments that directly report airflow across the surface are the Sheffield and Bendtsen methods. A rougher surface causes higher airflow; therefore these instruments are designated as roughness testers. The Parker Print Surf method is also a roughness tester; however, the reporting (in microns roughness) is a function of the cube root of the measured air flow. In the last few decades, the zeitgeist has been to accurately name the test methods in accordance with the function and not to continue using the manufacturer’s earlier assigned name, if it was not technically correct.
Roughness Testers: Bendtsen, Parker Print Surf & Sheffield
Smoothness Tester: Bekk
Future posts will include information on the relationship between paper roughness (smoothness) and the following items:
- Papermaking Process
- Printing processes
- Parker Print Surf Test
- Sheffield Test