ISO 8791-5:2020

INTERNATIONAL ISO
STANDARD 8791-5
First edition
2020-05
Paper and board — Determination
of roughness/smoothness (air leak
methods) —
Part 5:
Oken method
Papier et carton — Détermination de la rugosité/du lissé (méthodes
du débit d'air) —
Partie 5: Méthode Oken
Reference number
©
ISO 2020
© ISO 2020
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Published in Switzerland
ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus . 2
5.1 Oken Smoothness apparatus (two types) . 2
5.2 Water column type . 2
5.3 Electronic sensor type . 6
6 Sampling . 7
7 Conditioning . 7
8 Preparation of test pieces . 7
9 Calibration . 7
10 Procedure. 8
11 Expression of results . 8
11.1 Reporting the results . 8
11.2 Standard deviation and coefficient of variation . 8
12 Test report . 8
Annex A (informative) Measurement principle of Oken smoothness . 9
Annex B (informative) Precision data .11
Bibliography .13
Foreword
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electrotechnical standardization.
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different types of ISO documents should be noted. This document was drafted in accordance with the
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps, Subcommittee
SC 2, Test methods and quality specifications for paper and board.
A list of all the parts in the ISO 8791 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

INTERNATIONAL STANDARD ISO 8791-5:2020(E)
Paper and board — Determination of roughness/
smoothness (air leak methods) —
Part 5:
Oken method
1 Scope
This document specifies the Oken method for determining the smoothness of paper and board using an
apparatus which complies with the Oken method, as defined in this part of ISO 8791. It is applicable to
all printing papers and boards.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 48, Rubber, vulcanized or thermoplastic — Determination of hardness (hardness between 10 IRDH and
100 IRDH)
ISO 186, Paper and board — Sampling to determine average quality
ISO 187, Paper, board and pulps — Standard atmosphere for conditioning and testing and procedure for
monitoring the atmosphere and conditioning of samples
ISO 4662, Rubber, vulcanized or thermoplastic — Determination of rebound resilience
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
oken smoothness
time required for a specific volume of air to leak from the gap between the flat measuring lands of the
Oken tester and the surface of a test piece under specified conditions
Note 1 to entry: Oken smoothness is expressed in seconds per 100 ml (s/100 ml).
4 Principle
A test piece is placed between a rubber plate and measuring lands.
The air pressure on the inlet side of a narrow capillary of controlled dimensions is maintained at a
constant value above atmospheric pressure. The air passes through the capillary and then leaks
between the surface of the test piece and the measuring lands. The air pressure between the outlet
side of the capillary and a test piece depends on the rate of flow of air between the measuring lands
and the surface of the test piece. It is measured and converted to Oken smoothness. For further details
regarding the measurement principle, see Annex A.
For details regarding the test method precision, see Annex B.
5 Apparatus
5.1 Oken Smoothness apparatus (two types)
Use either the water column type described in 5.2 or the electronic sensor type described in 5.3.
5.2 Water column type
5.2.1 The water column type, see Figure 1, consists of an air compressor, filter, pressure regulator,
water column regulator, capillary, water column manometer, measuring head, and scale plate.
5.2.1.1 The measuring head, see Figure 2, consists of a loading weight, rubber plate, and sensing head.
5.2.1.1.1 The loading weight and rubber plate, the loading weight has a circular even surface of not
less than 48 mm in diameter and a mass that clamps the test piece at a pressure of (98,1 ± 1,0) kPa.
The rubber plate has a diameter of not less than 48 mm, a thickness of (4 ± 0,2) mm, a hardness of
(50 ± 5) IRHD in accordance with ISO 48, and a rebound resilience of not less than 62 % in accordance
with ISO 4662.
5.2.1.1.2 The sensing head, has 10 concentric measuring lands and nine grooves. The outermost
measuring land has a width of (0,5 ± 0,03) mm, and an outside diameter of (46 ± 0,03) mm. The width
of the other nine measuring lands is (1 ± 0,03) mm. The inside diameter of innermost measuring land
is (9 ± 0,03) mm. All grooves have a width of (1 ± 0,03) mm, and a depth of 0,5 mm to 1,2 mm. Five
grooves have air outlet holes and four grooves have air inlet holes arranged alternately. A tube connects
the sensing head to the pressure measuring chamber.
5.2.1.2 The air compressor and regulator, supply compressed and filtered air. The air is controlled
to approximately 10 kPa by the air regulator.
5.2.1.3 The water column regulator and capillary, provide a constant pressure. The water column
regulator consists of a water tank with an internal diameter of approximately 100 mm and a height of
approximately 700 mm, and a constant pressure chamber with a side tube. The open end of the side tube
is (500 ± 0,5) mm below the water surface.
The air at approximately 10 kPa introduced into the constant pressure chamber is controlled to
(4,90 ± 0,01) kPa (500 ± 1 mmH O) and transferred to the pressure measuring chamber through the
capillary.
The capillary consists of a narrow tube made of stainless material with an inner diameter of
approximately 0,3 mm and a length of approximately 50 mm. The capillary length shall be strictly
adjusted so that constant K in Formula (1), see 5.2.1.4, is 100.
2 © ISO 2020 – All rights reserved

5.2.1.4 The water column manometer and scale plate, comprise the pressure measurement system.
The water column manometer is connected to the water tank at a depth greater than 500 mm below
the surface of the water in the water tank by a pipe with a diameter large enough (with a minimum of
3 mm) to allow water to transfer smoothly between the manometer and water tank. The manometer is
connected to the sensing head and capillary. The scale plate for reading smoothness is marked in the
units of seconds per 100 ml. The scale of smoothness is determined by Formula (1):
KP*
T= (1)
PP−
c
T is the Oken smoothness;
K is the constant;
P is the pressure at the pressure measuring chamber;
P is the pressure at the constant pressure chamber.
c
The scale shall cover the range 0 s to infinity (∞ s) with scale readings of 0 s/100 ml at a pressure drop
of zero (0 mmH O), 100 s/100 ml at the midpoint 250 mm below the surface level in the tank and (∞)
s/100 ml at a pressure of 4,90 kPa (500 mmH O).
NOTE High or low air resistance type of Oken tester with different capillary dimensions or measurement
areas are available for shorter measurement time or higher precision. However, these types of Oken tester are
out of the scope of this part of ISO 8791.
Key
1 air compressor 9 test piece
2 filter 10 sensing head
3 pressure regulator 11 rubber plate
4 water column regulator / constant pressure chamber 12 loading weight
5 capillary 13 example of scale plate
6 water column regulator / pressure measuring chamber sensor P approximately 10 kPa
7 water tank P 4,90 kPa
8 scale plate
Figure 1 — Diagram of water column type Oken tester
4 © ISO 2020 – All rights reserved
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