The xerography and inkjet processes are common examples of processes where the PPS tester does not replicate the loading factors. The Sheffield test subjects the paper to loading pressures of 0.09 mPa at zero Sheffield units, and 0.154 mPa at 400 Sheffield units. The reason for the non-constant loading is related to the design of the air system, and the “hovercraft effect” of the variable pressure between the measurement lands. When the PPS instrument is set to measure at the lowest loading pressure, it is still about 4 to 5 times higher than the Sheffield loading.
With the introduction of digital Sheffield testers in the late 1980’s, the Sheffield method maintained its prominence for these grades, at least in the USA. There are many regions of the world where the Bendtsen test is used; however, the correlation between Bendtsen and Sheffield for these grades is excellent. There are many grades where the Sheffield method gives more uniform results than Bendtsen; those grades being the higher basis weight and stiffer grades, where the Bendtsen deadweight is not heavy enough to fully flatten the sheet under test.
Related posts include information on the relationship between paper roughness (smoothness) and the following items:
- Papermaking Process
- Printing processes
- Parker Print Surf Test
- Sheffield Test