ISO 3669:2026

Vacuum technology — Dimensions of knife-edge flanges

This document specifies the dimensions of fixed or rotatable bolted knife-edge flanges used in vacuum systems for pressures ranging from atmospheric to as low as 10−11 Pa.

Technique du vide — Dimensions des brides à guillotine

General Information

• Status: Published
• Publication Date: 23-Jun-2026
• ICS: 23.160 - Vacuum technology
• Technical Committee: ISO/TC 112 - Vacuum technology
• Drafting Committee: ISO/TC 112 - Vacuum technology
• Current Stage: 6060 - International Standard published
• Start Date: 24-Jun-2026
• Due Date: 30-Aug-2026
• Completion Date: 24-Jun-2026

Relations

• Revises: ISO 3669:2020 - Vacuum technology — Dimensions of knife-edge flanges
• Effective Date: 02-Sep-2023

Overview

ISO 3669: Vacuum technology - Dimensions of knife-edge flanges is an international standard developed by ISO/TC 112. This document specifies the dimensional requirements for fixed or rotatable bolted knife-edge flanges employed in vacuum systems. These flanges are critical for creating reliable, leak-tight connections in high and ultra-high vacuum environments, accommodating pressures from atmospheric pressure down to as low as 10⁻¹¹ Pa. The standard ensures consistency and interchangeability of knife-edge flanges, a design widely known through the ConFlat® trademark and commonly used across research, industrial, and scientific applications.

Key Topics

• Flange Types and Compatibility:
  ISO 3669 defines dimensional requirements for both fixed and rotatable knife-edge flanges, ensuring compatibility across international suppliers and with key industry standards such as ASTM E2734M.

• Sealing Mechanism:
  The standard provides specifications for the use of metal gaskets, typically softer than the flange material, to achieve ultra-high vacuum performance with minimal leakage.

• Materials:
  Flanges are usually made from corrosion-resistant, non-magnetic materials such as austenitic stainless steel. The selection considers factors like service temperature, sealing reliability, and compatibility with gasket materials.

• Bolt Holes and Threads:
  Detailed specifications are provided for metric and English thread bolts, recognizing both international and US-origin flange systems.

• Dimension Tables:
  Comprehensive tables specify key measurements - outside diameters, bolt circle diameters, knife-edge profiles, and recommended groove placements for leak checking.

• Harmonization:
  Recent editions focus on harmonizing with ASTM standards and including widely used flange variants (e.g., 295CF for turbomolecular pumps, and 350ACF compatible with ASTM 350CF).

Applications

Knife-edge flanges specified by ISO 3669 are essential components in:

• Research Laboratories:
  Used extensively in vacuum chambers for particle physics, material analysis, and surface science experiments requiring ultra-high vacuum integrity.

• Semiconductor Manufacturing:
  Employed in process chambers for etching, deposition, and fabrication steps that demand contaminant-free, vacuum-sealed environments.

• Industrial Vacuum Process Systems:
  Utilized in coating, degassing, and freeze-drying equipment, ensuring reliable sealing under severe conditions.

• Turbomolecular Pumps and Vacuum Plumbing:
  The standard’s inclusion of flange types for advanced pump interfaces promotes system compatibility and ease of maintenance.

• Nuclear Fusion Research and Particle Accelerators:
  Standardized knife-edge flanges support the extensive vacuum plumbing required in large-scale scientific infrastructure.

Related Standards

For a comprehensive approach to vacuum technology and flange compatibility, consider the following international standards:

• ASTM E2734M: Vacuum technology - Dimensions of knife-edge-type flanges
• ISO 1609: Vacuum technology - Dimensions of non-knife edge flanges
• ISO 286 (all parts): Geometrical product specifications (GPS) - ISO code system for tolerances on linear sizes
• ISO 965-1: ISO general purpose metric screw threads - Tolerances - Part 1

Adopting ISO 3669 helps ensure that knife-edge flanges and associated components meet consistent, internationally recognized dimensional criteria, supporting reliable vacuum performance and simplifying procurement, replacement, and system integration. For further information or updates, consult your national standards body or the ISO Online Browsing Platform.

Standards Content (Sample)

International
Standard
ISO 3669
Fourth edition
Vacuum technology — Dimensions
2026-06
of knife-edge flanges
Technique du vide — Dimensions des brides à guillotine
Reference number
© ISO 2026
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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 2
5 Requirements . 2
5.1 Materials .2
5.1.1 Flange .2
5.1.2 Bolt holes .2
5.1.3 Grooves .2
5.1.4 Gasket .2
5.2 Dimensions .2
Bibliography . 7

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,
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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 112, Vacuum technology.
This fourth edition cancels and replaces the third edition (ISO 3669:2020), which has been technically
revised.
The main changes are as follows:
— title has been updated;
— Flange type 295CF has been added as it is commonly used for turbo molecular pumps;
— Flange type 350ACF has been added which is compatible with ASTM E2734 flange type 350CF;
— knife edge angle range has been expanded to 0°-40°.
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
This edition of ISO 3669 has been revised with the intention of harmonizing its content with ASTM E2734M
in order to ensure compatibility of flanges and gaskets as defined in the two standards. This document is
designed to set the minimum requirements to ensure flange compatibility. Manufacturers’ designs may vary
within the tolerances of this standard to meet specific sealing requirements.
1)
ASTM E2734M was developed from the original ConFlat® flange dimensions and tolerances, as developed
by Varian. The intent of this document is to ensure interchangeability of flanges. It is reasonable to accept
that flanges manufactured to the original Varian specifications are compatible with flanges manufactured
according to this document, even though they might not fall within all tolerances.
1) ConFlat® is the trademark of a product supplied by Varian, Inc. This information is given for the convenience of users
of this document and does not constitute an endorsement by ISO of the product named. Equivalent products may be used
if they can be shown to lead to the same results.

v
International Standard ISO 3669:2026(en)
Vacuum technology — Dimensions of knife-edge flanges
1 Scope
This document specifies the dimensions of fixed or rotatable bolted knife-edge flanges used in vacuum
−11
systems for pressures ranging from atmospheric to as low as 10 Pa.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the
...