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Surface Roughness Methods

The roughness of a certain surface is often of importance for the user and thus specified. When this is the case there is a good chance that the method used to describe the roughness is a method according to Bekk, Bendtsen, or Sheffield, a PPS method (PPS or PPS-Flex) or the Stylus Roughness method.

All but the last of these methods are air-leak methods which indicate the roughness of a certain paper surface by telling how much air passes between the edge of a measuring head and the surface in question, under specified conditions. The PPS-method has a rather limited measuring range and is mainly used for smooth grades e.g. for printing. The Bendtsen and Sheffield methods are air-leak methods with a relatively wide measuring range. They have over the years been found reliable, simple to use and good all around methods for determination of surface roughness on many grades. The stylus roughness method uses an entirely different approach.

The Bekk Method

The first instrument of this type was designed by Dr Julius Bekk in 1927. His instrument was built around a vacuum chamber. The result was presented as the time used for a certain reduction of the under pressure due to leakage of air between an annular glass ring and the paper sample.

Bekk Smoothness measuring head

Bekk Smoothness measuring head

Bendtsen developed a sensing head in which the width of the ring is 0,15 mm. The rubber behind the test piece, used in the Bekk instrument, is replaced by a glass plate. The pressure against the paper is 98 kPa as in the Bekk instrument. The measuring results are expressed in terms of air-flow rate, Bendtsen units, at a given differential pressure.


The Bendtsen Method

The Bendtsen method is an old and well known method but it is not the first air-leak method. In 1940, Mr C. Bendtsen, pointed out that the annular glass ring of the Bekk tester was much bigger than the cavities in the paper surface.

Bendtsen roughness measuring head

Bendtsen roughness measuring head


The Sheffield Method

A North American improvement of the Bendtsen method is the Sheffield tester developed by Mr W.I. Wilt and Mr N.E. Emmons. The air pressure as well as the contact pressure are much higher than in the Bendtsen case. The results are expressed in standard cubic centimetres (SCCM) or in Sheffield units.

Sheffield roughness measuring head

Sheffield roughness measuring head


The PPS Method

The above instruments were used successfully for many years but in 1965 the time was due for a new instrument, the PPS Tester, invented by Dr J.R. Parker. In this instrument the major drawbacks of the earlier instruments had been eliminated. The measuring pressure is increased to 490–1960 kPa which corresponds with the actual nip pressure in a real printing press. The permeability error was minimized by a unique design of the measuring head. Another unique feature was that the instrument was calibrated in absolute units namely µm.

PPS roughness measuring head

PPS roughness measuring head


The PPS Flex Method

By redesigning the head and backing of the original PPS the PPS flex instrument has been greatly improved. Roughness may be measured over a wider range, at lower clamping pressures and without the spuriously high results previously obtained on papers more than 100 µm thick or with a bending stiffness of more than a few mNm.

PPS flex measuring head

PPS flex measuring head

To overcome the PPS problems, changes were made to the backing and to the sensing head.

The fluid supported diaphragm backing
The old blanket is replaced by a thin rubber diaphragm supported by a fluid. More consistent results may be obtained and paper roughness measured at lower pressures.

The Linear PPS Head
One advantage is that both land length and land width can be altered, making it possible to alter the measuring range of a roughness tester simply by changing the head. Heads can now be provided to measure either smoother or rougher papers outside the 0.6 to 6.0 µm range of the PPS.

The PPS Flex is basically similar to the PPS. It gives similar readings at comparable clamping pressures. Also, because the test piece is held flat, it may be used for both thick (stiff) and ordinary papers. The new backing and head open new fascinating possibilities for understanding and predicting the relation between surface roughness and the printing properties of paper.


Stylus Roughness

The stylus type measurement allows a great deal more information to be gathered about the paper surface than does any air leakage measurement. The stylus system takes and records a stylus vertical position every tenth of a mm it travels across the surface. This information is processed to reflect printability numbers and the lower the number, the better the print quality.

Stylus roughness measuring head

Stylus roughness measuring head

All air leakage instruments measure the amount of air escaping under a precision seal pressed against the paper surface. If the surface topography consists of wide valleys, a lot of air can escape beneath the seal, but if the valley is wide and shallow, printing may well be accomplished across the surface of the valley. The difficult areas to print consist of steep narrow valleys which may not leak a lot of air volume.

The instrument records the vertical position of the stylus. The result is reported as Micro Average. The Standard Deviation Number, which is basically a measure of the general roughness of the profile closely follows most air leakage numbers.