oSIST prEN ISO 4638:2026

• Abstract:

  Applicable to test pieces cut from products of cellular materials. Useful for two purposes: a) studying the structure of cellular products in connection with their physical properties and their method of manufacture and b) ensuring product quality (quality assurance). Describes principle, apparatus, test piece and conditions, procedure and calculation and expression of the results. Three figures deliver the details for testing.

• Status: Not Published
• Public Enquiry End Date: 19-Oct-2026
• ICS: 83.100 - Cellular materials
• Technical Committee: IPMA - Polimer materials and products
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 14-Aug-2026
• Due Date: 01-Jan-2027
• Ref Project: prEN ISO 4638 - Polymeric materials, cellular flexible - Determination of air flow permeability (ISO/FDIS 4638:2026)

• Revises: SIST EN ISO 4638:1999 - Polymeric materials, cellular flexible - Determination of air flow permeability (ISO 4638:1984)
• Effective Date: 14-Jan-2026

oSIST prEN ISO 4638:2026 is the SIST publication of the draft International Standard ISO/FDIS 4638:2026. It specifies a method for determining the air flow permeability of flexible cellular polymeric materials from test pieces cut from cellular products. Engineers, laboratories, and quality teams use it to relate structure and manufacture to physical properties, and to check product quality.

What does oSIST prEN ISO 4638:2026 specify?

oSIST prEN ISO 4638:2026 specifies a test method for the permeability of flexible cellular polymeric materials to air flow. It applies to test pieces cut from products of cellular material, so the result is used on prepared samples rather than on finished parts in service.

The document is organized into Scope, Normative references, Terms and definitions, Principle, Apparatus, Test pieces, Testing conditions, Procedure, Calculation and expression of results, and Test report. Figures 1 to 3 support the test principle, the apparatus, and the measurement cell, and Table 1 supports result presentation.

The introduction says the method is useful for studying cellular structure in relation to physical properties and manufacture, and for quality assurance. It also points readers to ISO 7231 when the purpose is simple quality control rather than comparing intrinsic material properties.

What are the key requirements of oSIST prEN ISO 4638:2026?

The core of oSIST prEN ISO 4638:2026 is a controlled air-flow test through a cylindrical or parallelepiped test piece, with the pressure drop measured across the specimen. In practice, the reader must control the air flow, seal the specimen properly, and record the pressure drop and thickness under test conditions.

Test principle and flow conditions

Clause 4 bases the method on Darcy's law for flow through a porous medium. For homogeneous, isotropic material, permeability is derived from the measured flow, area, thickness, viscosity, and pressure drop. For anisotropic materials, the flow direction must be defined, and the flow must remain laminar.

Apparatus and measurement cell

Clause 5 requires a measurement cell, a steady air source, a volumetric flow measurement device, a pressure-drop measuring device, and a means of measuring thickness in place. The cell may be cylindrical or parallelepipedal, and the support, seals, and tapping points must prevent leaks because leakage would distort permeability results.

Test pieces and preparation

Clause 6 requires test pieces slightly larger than the cell cross-section so the elastic material seals the sides. The faces through which air flows shall be parallel, and the material shall be cut with suitable tools, not hot wire. For permeability or specific resistance work, surface skin or unevenness is removed; the test piece is then weighed.

Testing conditions and procedure

Clause 7 requires testing under ISO 23529 conditions after property stabilization. Clause 8 then sets out the sequence: place the test piece in the cell, seal the edges, measure thickness in position, establish the flow, and take measurements at five different flow velocities that give laminar flow. For known products used in quality assurance, the multi-velocity check is not necessary.

Calculation and reporting

Clause 9 distinguishes homogeneous from non-homogeneous test pieces. For homogeneous material, the document allows calculation of permeability and, if needed, specific air flow resistance; for non-homogeneous material, air flow resistance is calculated directly. Clause 10 requires the report to include product nature, apparent density, test conditions, preparation method, specimen count and dimensions, orientation if relevant, surface skin, test thickness and density, and any deviation that could affect the result.

What terms does oSIST prEN ISO 4638:2026 use?

• Air flow permeability, K - the parameter calculated from the test to show how easily air passes through the cellular material.
• Specific air flow resistance, R_s - a permeability-derived parameter used particularly in acoustics.
• Air flow resistance, R - the resistance related to material thickness, used directly for non-homogeneous test pieces.
• Volumetric air flow rate, q - the volume of air passing through the test piece per unit time.
• Linear air flow velocity, u - the air speed through the cross-section of the specimen.
• Pressure drop, Δp - the pressure difference measured between the two faces of the test piece.
• Apparent density - the density of the product as measured for reporting and comparison.

Who uses oSIST prEN ISO 4638:2026?

oSIST prEN ISO 4638:2026 is used by manufacturers of flexible cellular polymeric products, testing laboratories, and quality assurance teams. It is also useful in research and development when comparing materials, studying foam structure, and linking structure to manufacturing method.

The document is relevant wherever air flow behavior matters, including acoustic applications that use specific air flow resistance. Buyers and technical reviewers can use it to understand what a supplier’s permeability result actually measures and what conditions it was taken under.

What changed in oSIST prEN ISO 4638:2026 from the previous edition?

This second edition cancels and replaces ISO 4638:1984 and is described as a minor revision. The stated change is that the reference to withdrawn ISO 471 has been replaced with ISO 23529.

Which standards are used with oSIST prEN ISO 4638:2026?

• ISO 845 - provides the method for determining apparent density, which the report requires.
• ISO 23529 - gives the general procedures for preparing and conditioning test pieces for physical test methods, and is used for the testing conditions.
• ISO 7231 - is cited in the scope note as a simple quality control method based on air flow through cellular materials when intrinsic properties do not need to be calculated.

What does the oSIST prEN ISO 4638:2026 document contain?

The document contains formulas for permeability, specific air flow resistance, air flow resistance, and flow velocity, along with worked guidance for homogeneous and non-homogeneous materials. It also includes three figures showing the test principle, a schematic apparatus, and a cylindrical measurement cell.

Clause 8 gives the practical test sequence, including sealing, thickness measurement in place, and repeated measurements at different flow velocities. Clause 9 includes a summary table and a graph for presenting results, while Clause 10 lists the reporting items needed for traceable test records.

SLOVENSKI STANDARD
01-oktober-2026
Polimerni materiali, mehke pene - Določanje prepustnosti toka zraka (ISO/FDIS
4638:2026)
Polymeric materials, cellular flexible - Determination of air flow permeability (ISO/FDIS
4638:2026)
Weichelastische Schaumstoffe - Bestimmung der Luftstromdurchlässigkeit (ISO/FDIS
4638:2026)
Matériaux polymères alvéolaires souples - Détermination de la perméabilité à l'air
(ISO/FDIS 4638:2026)
Ta slovenski standard je istoveten z: prEN ISO 4638
ICS:
83.100 Penjeni polimeri Cellular materials
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

FINAL DRAFT
International
Standard
ISO/FDIS 4638
ISO/TC 45/SC 4
Polymeric materials, cellular
Secretariat: DSM
flexible — Determination of air flow
Voting begins on:
permeability
2026-08-07
Matériaux polymères alvéolaires souples — Détermination de la
Voting terminates on:
perméabilité à l'air
2026-10-30
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
ISO/CEN PARALLEL PROCESSING 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
ISO/FDIS 4638:2026(en) © ISO 2026

FINAL DRAFT
ISO/FDIS 4638:2026(en)
International
Standard
ISO/FDIS 4638
ISO/TC 45/SC 4
Polymeric materials, cellular
Secretariat: DSM
flexible — Determination of air flow
Voting begins on:
permeability
Matériaux polymères alvéolaires souples — Détermination de la
Voting terminates on:
perméabilité à l'air
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
ISO/CEN PARALLEL PROCESSING
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
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Phone: [phone]
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Published in Switzerland Reference number
ISO/FDIS 4638:2026(en) © ISO 2026

ii
ISO/FDIS 4638:2026(en)
Contents  Page
Foreword .iv
Introduction .v
1 Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Principle . 1
4.1 General .1
4.2 Air flow permeability .2
4.3 Specific air flow resistance.2
4.4 Air flow resistance .2
5 Apparatus .3
5.1 General .3
5.2 Measurement cell .3
5.3 Means of providing air flow .3
5.3.1 Source .3
5.3.2 Characteristics of the flow .3
5.4 Means of measuring volumetric flow rate .4
5.5 Means of measuring pressure drop .4
5.6 Means of measuring test piece thickness .4
6  Test pieces .4
6.1 Shape .4
6.2 Dimensions .4
6.2.1 Lateral dimensions .4
6.2.2 Thickness .5
6.3 Preparation .5
6.4 Number of test pieces .5
7  Testing conditions . 5
8  Procedure . 5
9  Calculation and expression of results .6
9.1 For homogeneous test pieces .6
9.2 For non-homogeneous test pieces .7
9.3 Precision . . .7
10  Test report .8
Bibliography .11

iii
ISO/FDIS 4638:2026(en)
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
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in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents 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).
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This document was prepared by Technical Committee ISO/TC 45, Rubber and rubber products, Subcommittee
SC 4, Products (other than hoses), in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 249, Plastics, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 4638:1984), of which it constitutes a minor
revision. The change is as follows:
— reference to the withdrawn document ISO 471 replaced with ISO 23529.
A list of all parts in the ISO 4638 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/FDIS 4638:2026(en)
Introduction
The air flow permeability of cellular materials indicates, in an indirect manner, some of their structural
properties. It can be used to establish correlations between the structure of these materials and some of
their physical properties. It also enables identification of the modifications to cellular structures produced
by chemical agents used in foaming, for example catalysts or surfactants.
This document is, therefore, useful for two purposes:
a) in studying the structure of cellular products in connection with their physical properties and their
method of manufacture;
b) in ensuring product quality (quality assurance).
NOTE Details of publications relating to flow behaviour in both laminar and turbulent conditions are given in
[1] [2] [3] [4] [5]
References   .
v
FINAL DRAFT International Standard ISO/FDIS 4638:2026(en)
Polymeric materials, cellular flexible — Determination of air
flow permeability
1 Scope
This document specifies a method for the determination of the permeability of flexible cellular polymeric
materials to air flow.
It is applicable to test pieces cut from products of cellular material.
[6]
NOTE ISO 7231 specifies a simple quality control method which is also based on the flow of air through cellular
materials. This can be used when it is not the intention to calculate the intrinsic properties of various materials in
order to compare them, but merely to control the quality of a given cellular material.
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 845, Cellular plastics and rubbers — Determination of apparent density
ISO 23529, Rubber — General procedures for preparing and conditioning test pieces for physical test methods
3  Terms and definitions
No terms and definitions are listed in this document.
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/
4  Principle
4.1  General
Passing air under controlled conditions through a test piece in the form of a cylinder or parallelepiped.
Measurement of the pressure drop between the two free faces of the test piece.

ISO/FDIS 4638:2026(en)
4.2  Air flow permeability
K, is given by Darcy's law (see Figure 1), which describes the air flow in a homogeneous and isotropic (see
4.2, Note 1) porous medium under laminar flow conditions (see 4.2, Note 2) by Formula (1):
(1)
where
u is the linear air flow velocity, in metres per second;
q is the volumetric air flow rate crossing the test piece, in cubic metres per second;
V
A is the right cross-sectional area of the test piece, in square metres;
K is the flow permeability of the porous medium, in square metres;
Δp is the pressure drop across the test piece, in pascals;
η is the dynamic viscosity of ai
...