ISO/FDIS 5627

• Abstract:

  Cancels and replaces the first edition (1984). Specifies a method of measuring the smoothness of paper and board, called the Bekk method. Especially recommended for smooth paper and boards. However for very smooth samples, measurement times may be impractically long. Not recommended for materials greater than 0,5 mm thick or very permeable papers and board, since the amount of air passing through the test piece can influence the result. Not recommended for newsprint and not suitable for rough papers and boards.

• Status: Not Published
• ICS: 85.060 - Paper and board
• Technical Committee: ISO/TC 6/SC 2 - Test methods and quality specifications for paper and board
• Drafting Committee: ISO/TC 6/SC 2 - Test methods and quality specifications for paper and board
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 11-Sep-2026
• Completion Date: 11-Sep-2026

• Revises: ISO 5627:1995 - Paper and board — Determination of smoothness (Bekk method)
• Effective Date: 12-Aug-2023
• Revises: ISO 5627:1995/Cor 1:2002 - Paper and board — Determination of smoothness (Bekk method) — Technical Corrigendum 1
• Effective Date: 12-Aug-2023

Overview

ISO/FDIS 5627:2026 specifies the Bekk method for determining the smoothness of paper and board. Published by the International Organization for Standardization (ISO), this standard cancels and replaces the first edition (1984), incorporating significant technical revisions. The Bekk method is particularly recommended for measuring the smoothness of high-quality and smooth paper and board but is not suitable for surfaces that are rough, thick (over 0.5 mm), highly permeable, or for newsprint materials.

Smoothness is a critical property affecting the printability, appearance, and performance of paper and board products. The Bekk method quantifies smoothness by measuring the time taken for a specified volume of air to pass between a test surface and a glass plate under defined pressure conditions.

Key Topics

• Principle of Measurement: The standard outlines the process of placing a test sample on a polished glass plate, applying a consistent pressure, and measuring the time required for a partial vacuum to decrease by a specified amount due to airflow through surface irregularities.
• Equipment Requirements: It describes the use of specialized laboratory apparatus-specifically, a specimen clamping device, glass plate with central hole, rubber pad, vacuum containers, and manometers with specified precision.
• Sample Selection and Conditioning: Only properly sized, defect-free samples are tested. Test specimens must be conditioned according to ISO 187 to ensure consistent results.
• Procedure: Measurement is performed on at least 10 test pieces per side. The Bekk smoothness is represented as the arithmetic mean time (in seconds) required for a defined vacuum drop.
• Limitations: The method is not recommended for materials thicker than 0.5 mm, highly permeable or rough papers, or newsprint, as these characteristics introduce significant variability.
• Data Reporting: Results should include standard deviation or coefficient of variation, the type of vacuum container used, and any observed anomalies or deviations.

Applications

• Quality Control in Paper Manufacturing: Paper mills and board factories utilize this standard method for routine smoothness measurement in product quality assurance and conformity testing.
• Product Development and Research: R&D professionals leverage Bekk smoothness data to design papers for specific end uses such as printing, packaging, or specialty applications requiring surface uniformity.
• Supplier-Buyer Agreements: The Bekk method provides an agreed, standardized measure of surface smoothness to facilitate communication and contractual compliance between suppliers and customers.
• Laboratory Testing: Third-party and in-house laboratories employ this standard for smoothness evaluation, supporting certification, benchmarking, and process optimization.

Related Standards

• ISO 186: Paper and board - Sampling to determine average quality
• ISO 187: Paper, board, and pulps - Standard atmosphere for conditioning and testing
• ISO 48-2: Rubber, vulcanized or thermoplastic - Determination of hardness
• ISO 4662: Rubber, vulcanized or thermoplastic - Determination of rebound resilience

Additional surface smoothness and roughness methods include:

• Sheffield roughness
• Bendtsen roughness

These complementary standards help organizations comprehensively evaluate and specify the surface characteristics of paper and board, supporting a wide range of end-use requirements.

---

Keywords: ISO FDIS 5627, Bekk smoothness, paper and board testing, surface smoothness, Bekk method, quality control, paper manufacturing standards, ISO paper standards, laboratory testing, packaging material standards.

FINAL DRAFT
International
Standard
ISO/TC 6/SC 2
Paper and board — Determination
Secretariat: SIS
of smoothness (Bekk method)
Voting begins on:
Papier et carton — Détermination du lissé (Méthode Bekk) 2026-05-07
Voting terminates on:
2026-07-02
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 6/SC 2
Paper and board — Determination
Secretariat: SIS
of smoothness (Bekk method)
Voting begins on:
Papier et carton — Détermination du lissé (Méthode Bekk)
Voting terminates on:
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus . 2
6 Sampling . 5
7 Conditioning . 6
8 Procedure . 6
9 Expression of results, calculation. 6
10 Precision . 6
11 Test report . 6
Annex A (informative) Checking and maintenance of the apparatus for measurement of
smoothness . 8
Annex B (informative) Compensation the influence of the atmospheric air pressure .11
Annex C (informative) Precision .12

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/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.
This third edition cancels and replaces the second edition (ISO 5627:1995), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 5627:1995/Cor 1:2002.
The main changes are as follows:
— Clause 5 revised;
— Example of the pressure curve during smoothness test added;
— Figure 3 redimensioned to ensure the contact area;
— possible combinations of container volume and differential pressure interval displayed in the form of a
table;
— dependence on atmospheric air pressure added;
— Formula for calculating the intake volume added;
— absolute atmospheric air pressure information added to the test report;
— Annex A revised;
— compensation of influence on atmospheric air pressure added in new Annex B;
— precision data added in new Annex C.
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
Introduction
Based on the current state of the art, it is not possible to recommend a single method for measuring the
smoothness or roughness of paper or board, and there is no exact correlation among the various methods
used for determining these properties. Instruments of the air-flow type are designed to obtain a numerical
value indicative of the smoothness or roughness of the paper or board. The results of these tests are
referenced in terms of the specific type of instrument used, such as Bendtsen roughness, Sheffield roughness,
Bekk smoothness, etc.
Bekk smoothness is dependent on the shape, total volume and distribution of the hollow spaces between the
surface of the test piece and a theoretically ideal plane under the specified conditions of contact. The greater
the Bekk smoothness number, the smoother the sample.
The air permeability of the sample being tested can also affect the results.
This method has been used in practice for a long time. However, reliable measurement results and their
reproducibility are only achieved when the apparatus and auxiliary equipment are in perfect condition, and
their maintenance and checking have been carefully performed. Therefore, details are also given for these
aspects.
v
FINAL DRAFT International Standard ISO/FDIS 5627:2026(en)
Paper and board — Determination of smoothness (Bekk
method)
1 Scope
This document specifies a method for measuring the smoothness of paper and board, called the Bekk
method.
The smoothness of a wide range of papers and boards can be measured by this method. This document
is particularly applicable for smooth papers and boards. However, for very smooth samples, measurement
times can be impractically long.
This method is not intended for material greater than 0,5 mm thick or very permeable papers and board,
since the amount of air passing through the test piece can influence the result.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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 48-2:2018, Rubber, vulcanized or thermoplastic — Determination of hardness — Part 2: Hardness between
10 IRHD and 100 IRHD
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 terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
bekk smoothness
time in seconds which, under a defined pressure differential, is required to draw a definite quantity of air at
atmospheric pressure between the surface of the test piece and a ring-shaped plane surface, under specified
conditions of contact
4 Principle
Subjection of a test piece of paper or board placed on a glass plate to a specified pressure and creation of a
partial vacuum to draw atmospheric air across the contact surface. Measurement of the time required for a
specified change in vacuum.
5 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.
5.1 General. The test device has a specimen clamping device with which the specimen can be pressed
against the flat glass plate with a contact area of 10 cm by means of a pressure plate and rubber pad with a
surface pressure of 100 kPa as shown in Figure 1.
Key
F clamping force
1 pressure plate
2 rubber pad
3 test piece
4 glass plate (measuring surface)
5 anvil
6 centric hole to vacuum container
Figure 1 — Measuring principle
The centric hole (Figure 1, key 6) of the polished glass plate is connected to a delimited volume (vacuum
container) for the duration of the measurement, and the time for a defined pressure drop is measured,
shown in Figure 2.
Key
p measurement pressure at start of test (-50,7 kPa)
p measurement pressure at end of test (-48,0 kPa)
1 switch-off point of the vacuum pump when the pre-pressure is reached
2 start of test; start of time measurement when the pressure p is reached
3 end of test; stop of time measurement when the pressure p is reached
Figure 2 — Example of the pressure curve during the smoothness test
The pre-pressure (Figure 2, key 1) should be selected so that the start of measurement (Figure 2, key 2) is
not greatly delayed and there is sufficient time for the flow to stabilize before the measurement pressure
p (Figure 2, key 2) is reached. An adjustable pre-pressure would be advantageous for adapting to products
of different smoothness (e.g. print side/reverse side).
Since at the pressure drop ∆p = p – p (distance from point 2 to point 3 in Figure 2) the incoming air mass
2 1
remains constant, the incoming air volume changes depending on the atmospheric air pressure and thus on
the location of the device. This should be compensated by an appropriate correction of the measured value
or by an adapted measuring sequence of the device (e.g. p = variable).
5.2 Glass plate, conforming to the design shown in Figure 3.
The test surface shall be circular, plane and perfectly polished and have a contact area of approximately
10 cm according to Figure 3. The central hole in the glass plate shall be capable of being connected with a
vacuum container and also of being disconnected.

Dimensions in mm
Key
1 support cylinder
Figure 3 — Glass plate
Glass plates with a 45° phase on the outer edge may also be used, but the outer diameter of the specimen
support surface shall always maintain the dimension given in Figure 3. Glass plates with a phase on the
inner edge are not allowed.
A supporting cylinder (metal plug) located in the hole is intended to prevent the test piece from being
pressed into the hole by the applied vacuum. This cylinder should have a central hole of 1,5 mm to 2,0 mm
in diameter. Its base should have four radial channels through which the air can pass without obstruction.
NOTE Some instruments have a glass plate without a supporting cylinder. For most papers, this does not
significantly affect the results obtained, but the use of a plug is desirable.
The test surface shall be kept clean and shall not be touched by bare hands. Before each measurement, any
fibres and the like that can have adhered to the plate shall be removed. When not in use, the plate shall be
protected by a cover. Scratches or cracks render it unusable.
5.3 Clamping device, to clamp the test piece against the glass plate.
A pressure plate having a circular, even surface of not less than 45 mm diameter and which shall be connected
to the device that presses the pressure plate against the glass plate at (100 ± 2) kPa.
5.4 Rubber pad, placed between the pressure plate and the test piece.
When examined by incident light under a magnifying glass. the surface of the rubber pad shall be free from
marks, scratches and tears and shall have the following dimensions:
a) thickness: (4 ± 0,2) mm; the maximum thickness variation of one pad shall be ±0,05 mm;
b) surface: circular with a diameter of not less than 45 mm, or rectangular with sides not less than 50 mm
in length.
The rubber pad shall have the following mechanical properties:
— hardness: (40 ± 5) IRHD (measured in accordance with ISO 48-2:2018, method N);
— rebound resilience: at least 62 % (m
...

Draft ISO/FDIS 5627:
ISO/TC 6/SC 2
Secretariat: SIS
Date: 2026-03-1204-23
Paper and board — Determination of smoothness (Bekk method)
Papier et carton — Détermination du lissé (Méthode Bekk)
FDIS stage
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.orgwww.iso.org
Published in Switzerland
ii
Draft ISO/FDIS 5627:2026(en)
Contents
Foreword . iv
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 2
6 Sampling . 8
7 Conditioning . 8
8 Procedure . 8
9 Expression of results, calculation . 9
10 Precision . 9
11 Test report . 9
Annex A (informative) Checking and maintenance of the apparatus for measurement of
smoothness . 11
Annex B (informative) Compensation the influence of the atmospheric air pressure . 15
Annex C (informative) Precision . 16

Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Apparatus . 1
6 Sampling . 6
7 Conditioning . 6
8 Procedure . 6
9 Expression of results, calculation . 6
10 Precision . 6
11 Test report . 7
Annex A (informative) Checking and maintenance of the apparatus for measurement of
smoothness . 8
Annex B (informative) Compensation the influence of the atmospheric air pressure . 11
Annex C (informative) Precision . 12

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents.www.iso.org/patents. ISO shall not be held responsible for identifying any or all such
patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.htmlwww.iso.org/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.
This third edition cancels and replaces the second edition (ISO 5627:1995), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 5627:1995/Cor 1:2002.
The main changes are as follows:
— — Clause 5 has been thoroughlyClause 5 revised ;
— — Example of the pressure curve during smoothness test was added;
— — Figure 3 has beenFigure 3 redimensioned to ensure the contact area;
— — Possiblepossible combinations of container volume and differential pressure interval is displayed in
the form of a table;
— — The dependence on atmospheric air pressure has been added;
— — Formula for calculating the intake volume has been added;
— — The absolute atmospheric air pressure information has been added to the test report;
— — Annex A has been thoroughlyAnnex A revised;
iv
Draft ISO/FDIS 5627:2026(en)
— — The compensation of influence on atmospheric air pressure has been added in newly created Annex
Bnew Annex B;
— — Precisionsprecision data has been added in newly created Annex Cnew Annex C.
—
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.

v
Introduction
Based on the current state of the art, it is not possible to recommend a single method for measuring the
smoothness or roughness of paper or board, and there is no exact correlation among the various methods
used for determining these properties. Instruments of the air-flow type are designed to obtain a numerical
value indicative of the smoothness or roughness of the paper or board. The results of these tests are referenced
in terms of the specific type of instrument used, such as Bendtsen roughness, Sheffield roughness, Bekk
smoothness, etc.
Bekk smoothness is dependent on the shape, total volume and distribution of the hollow spaces between the
surface of the test piece and a theoretically ideal plane under the specified conditions of contact. The greater
the Bekk smoothness number, the smoother the sample.
The air permeability of the sample being tested can also affect the results.
This method has been used in practice for a long time. However, reliable measurement results and their
reproducibility are only achieved when the apparatus and auxiliary equipment are in perfect condition, and
their maintenance and checking have been carefully performed. Therefore, details are also given for these
aspects.
vi
DRAFT International Standard Draft ISO/FDIS 5627

Paper and board — Determination of smoothness (Bekk method)
1 Scope
This document specifies a method offor measuring the smoothness of paper and board, called the Bekk
method.
The smoothness of a wide range of papers and boards can be measured by this method. This document is
particularly applicable for smooth papers and boards. However, for very smooth samples, measurement times
can be impractically long.
This method is not intended for material greater than 0,5 mm thick or very permeable papers and board, since
the amount of air passing through the test piece can influence the result.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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--2:2018, Rubber, vulcanized or thermoplastic — Determination of hardness — Part 2: Hardness between
10 IRHD and 100 IRHD
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 terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obphttps://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/https://www.electropedia.org/
3.1 3.1
bekk smoothness
time in seconds which, under a defined pressure differential, is required to draw a definite quantity of air at
atmospheric pressure between the surface of the test piece and a ring-shaped plane surface, under specified
conditions of contact
4 Principle
Subjection of a test piece of paper or board placed on a glass plate to a specified pressure and creation of a
partial vacuum to draw atmospheric air across the contact surface. Measurement of the time required for a
specified change in vacuum.
5 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.
5.1 5.1 General. The test device has a specimen clamping device with which the specimen can be
pressed against the flat glass plate with a contact area of 10 cm by means of a pressure plate and rubber pad
with a surface pressure of 100 kPa as shown in Figure 1.Figure 1.
2 © ISO #### 2026 – All rights reserved
Draft ISO/FDIS 5627:2026(en)
Key
F clamping force
1 pressure plate
2 rubber pad
3 test piece
4 glass plate (measuring surface)
5 anvil
6 centric hole to vacuum container
Figure 1 — Measuring principle
The centric hole (Figure 1,(Figure 1, key 6) of the polished glass plate is connected to a delimited volume
(vacuum container) for the duration of the measurement, and the time for a defined pressure drop is
measured, shown in Figure 2.Figure 2.

Key
p 𝑝𝑝 measurement pressure at start of test (-50,7 kPa)
p 𝑝𝑝 measurement pressure at end of test (-48,0 kPa)
1 switch-off point of the vacuum pump when the pre-pressure is reached
2 start of test; start of time measurement when the pressure p1 is reached
3 end of test; stop of time measurement when the pressure p2 is reached
Figure 2 — Example of the pressure curve during the smoothness test
4 © ISO #### 2026 – All rights reserved
Draft ISO/FDIS 5627:2026(en)
The pre-pressure (Figure 2,(Figure 2, key 1) should be selected so that the start of measurement (Figure
2,(Figure 2, key 2) is not greatly delayed and there is sufficient time for the flow to stabilize before the
measurement pressure (Figure 2,𝑝𝑝 (Figure 2, key 2) is reached. An adjustable pre-pressure would be
p
advantageous for adapting to products of different smoothness (e.g. print side/reverse side).
Since at the pressure drop ∆p = p – p 𝛥𝛥𝑝𝑝 = 𝑝𝑝 – 𝑝𝑝 (distance from point 2 to point 3 in Figure 2)Figure 2)
2 1
2 1
the incoming air mass remains constant, the incoming air volume changes depending on the atmospheric air
pressure and thus on the location of the device. This should be compensated by an appropriate correction of
the measured value or by an adapted measuring sequence of the device (e.g. p 𝑝𝑝 = variable).
5.2 5.2 Glass plate, conforming to the design shown in Figure 3.Figure 3.
The test surface shall be circular, plane and perfectly polished and have a contact area of approximately 10 cm
according to Figure 3.Figure 3. The central hole in the glass plate shall be capable of being connected with a
vacuum container and also of being disconnected.

Dimensions in mm
Key
1 support cylinder
Figure 3 — Glass plate
Glass plates with a 45° phase on the outer edge may also be used, but the outer diameter of the specimen
support surface shall always maintain the dimension given in Figure 3.Figure 3. Glass plates with a phase on
the inner edge are not allowed.
A supporting cylinder (metal plug) located in the hole is intended to prevent the test piece from being pressed
into the hole by the applied vacuum. This cylinder should have a central hole of 1,5 mm to 2,0 mm in diameter.
Its base should have four radial channels through which the air can pass without obstruction.
NOTE Some instruments have a glass plate without a supporting cylinder. For most papers, this does not
significantly affect the results obtained, but the use of a plug is desirable.
The test surface shall be kept clean and shall not be touched by bare hands. Before each measurement, any
fibres and the like that can have adhered to the plate shall be removed. When not in use, the plate shall be
protected by a cover. Scratches or cracks render it unusable.
5.3 5.3 Clamping device, to clamp the test piece against the glass plate.
6 © ISO #### 2026 – All rights reserved
Draft ISO/FDIS 5627:2026(en)
A pressure plate having a circular, even surface of not less than 45 mm diameter and which shall be connected
to the device that presses the pressure plate against the glass plate at (100 ± 2) kPa.
5.4 5.4 Rubber pad, placed between the pressure plate and the test piece.
When examined by incident light under a magnifying glass. the surface of the rubber pad shall be free from
marks, scratches and tears and shall have the following dimensions:
a) a) thickness: (4 ± 0,2) mm; the maximum thickness variation of one pad shall be ±0,05 mm;
b) b) surface: cir
...