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Monday, April 27, 2015

A Story from the Testing Lab: What is the top of the Sheffield Roughness scale?

Sheffield Measurement Head
According to TAPPI Test Method T 538, "Roughness of paper and paperboard (Sheffield method)", "This method is a measurement of the air flow between the specimen (backed by flat glass on the bottom side) and two pressurized, concentric annular lands that are impressed into the sample from the top side.  The rate of air flow is related to the surface roughness of paper or paperboard."

Older Sheffield testers had three tubes for the different recommended ranges:
  + Tube #1 (SU) 170 - 400
  + Tube #2 (SU) 56 - 170
  + Tube #3 (SU) 0 - 56

Modern Sheffield testers just provide a digital display of the current Sheffield roughness measurement.

In the Summary, the Test Method states: "The effective measuring range of this method is from 0 to approximately 445 Sheffield units." In the description of the Apparatus, the method defines "An air flowmeter, having the capability to measure the range of flow required for the samples to be tested. A range of 0 to 3400 mL/min (0-400 SU) is recommended."

Generally speaking, most modern instruments will display a value with the wording "out of range" or a flashing display if the values are above 400 or 420 SU.