ISO 24261-1:2026

• Abstract:

  This document specifies the evaluation procedure of fine bubble elimination for fine bubble dispersion in water. This document is applicable only to fine bubbles without shell.

• Status: Published
• Publication Date: 03-Nov-2026
• ICS: 13.060.60 - Examination of physical properties of water
• Technical Committee: ISO/TC 281 - Fine bubble technology
• Drafting Committee: ISO/TC 281/WG 2 - Fine bubble characterization and measurement
• Current Stage: 6060 - International Standard published
• Start Date: 04-Nov-2026
• Completion Date: 04-Nov-2026

• Revises: ISO 24261-1:2020 - Fine bubble technology — Elimination method for sample characterization — Part 1: Evaluation procedure
• Effective Date: 30-May-2026

ISO 24261-1 is an ISO standard that specifies the evaluation procedure for eliminating fine bubbles from fine bubble dispersion in water. It is intended for fine bubbles without shell, and it gives a common way to judge how well an elimination technique reduces fine bubbles while checking whether contaminants have been aggregated instead of removed.

What does ISO 24261-1 specify?

ISO 24261-1 specifies how to evaluate fine bubble elimination for fine bubble dispersion in water, from sample preparation through reporting. It focuses on measuring the change in fine bubbles before and after an elimination process, while also checking whether small contaminants have formed larger aggregates.

The document is organized into:

• Scope
• Normative references
• Terms and definitions
• Requirements
• Environment
• Evaluation of fine bubble elimination efficiency
• Test report
• Informative annexes with examples and explanations

Annex	What it covers
Annex A	Simulations of the fine bubble elimination process
Annex B	Decrease of concentration caused by fine bubble elimination and aggregation

What are the key requirements of ISO 24261-1?

ISO 24261-1 requires a controlled comparison between the sample before and after elimination. The main practical point is that a drop in the number of particles alone is not enough - the evaluation must also look at volume concentration to detect aggregation.

Sample preparation

ISO 24261-1 requires the fine bubble dispersion to be generated by cleaned fine bubble generating systems using pure water and pure gas such as air, nitrogen or oxygen. The dispersion shall not contain stabilizing agents. In high-accuracy work, Grade 1 water according to ISO 3696 is recommended.

In practice, this means the sample has to be as clean and reproducible as possible so the measured change can be attributed to the elimination method, not to the make-up of the liquid.

Measuring instruments

ISO 24261-1 requires the chosen instruments to cover the relevant concentration and size ranges for the sample. The document calls for measurement of total number concentration and/or total volume concentration, and for coverage of the total size range including fine bubbles, contaminants and aggregates.

In practice, the operator must confirm that the measuring system can see both the fine bubbles of interest and any larger aggregates that might appear during the process.

Environment

ISO 24261-1 requires clean air conditions and stable ambient temperature and atmospheric pressure during generation and measurement. For high-accuracy work, ISO Class 7 air cleanliness according to ISO 14644-1 is recommended.

In practice, this helps prevent contamination and keeps bubble behaviour stable enough for comparison between tests.

Evaluation of elimination efficiency

ISO 24261-1 evaluates efficiency using a number reduction rate and a volume reduction rate. The number reduction rate is used for the specified fine-bubble size range, while the volume reduction rate is used for a larger size range to check aggregation.

In practice, this means a good result is not just a lower particle count. The operator must also confirm that the apparent reduction is not offset by growth of larger particle groups.

Test report

ISO 24261-1 requires reporting the reduction rates, the size ranges used, the elimination technique, the elimination efficiency, a reliability statement, and the document number. It also requires reporting the gas and liquid media, environmental conditions, date and time, instrument identification, operator identification and test location.

In practice, the report should be detailed enough for another laboratory or buyer to understand what was tested and under which conditions.

What terms does ISO 24261-1 define?

• Elimination of fine bubbles - the process of decreasing the concentration index of fine bubbles.
• Fine bubble dispersion (FBD) - a liquid that contains fine bubbles.
• Number concentration index - the count-based measure used to compare bubble or particle amounts in a specified size range.
• Volume concentration index - the volume-based measure used to detect changes in total particle volume, especially when aggregation may occur.
• Reduction rate - the percentage decrease between the values before and after elimination.
• Contaminants - solid or liquid particles other than the fine bubbles being evaluated.
• Fine bubbles without shell - the type of fine bubbles covered by this document.

Who uses ISO 24261-1?

ISO 24261-1 is used by laboratories, researchers, equipment suppliers and quality teams working with fine bubble technology. Typical uses include comparing elimination methods, selecting operating conditions, validating sample-preparation steps and checking whether a treated water sample can serve as blank water for later measurements.

It is relevant in areas mentioned in the introduction such as cleaning, water treatment, agriculture, aquaculture and biomedical fields.

What changed in ISO 24261-1 from the previous edition?

ISO 24261-1 is a second edition that cancels and replaces ISO 24261-1:2020. The stated changes are:

• in 7.1, the reporting item “Number of this document” was added
• in Figure A.1 and Figure A.2, the unit “million/ml” was revised to “×10^6 particles/ml”

Which standards are used with ISO 24261-1?

• ISO 20480-1 - provides the terminology for fine bubble technology used in Clause 3.
• ISO 3696 - gives the water purity grade recommended for high-accuracy generation of FBD.
• ISO 14644-1 - provides the air cleanliness classification used for the recommended environment.
• ISO 20298-1 - supplies the source definition of fine bubble dispersion used in Clause 3.
• ISO/TR 23015 - points to measurement techniques that can be used for FBD in water.
• ISO 20480-2 - provides categorization of fine-bubble attributes.
• ISO 21910-1 - covers off-line evaluation of size index for microbubbles.

What does the ISO 24261-1 document contain?

ISO 24261-1 contains a step-by-step evaluation procedure, formulas for number and volume reduction rates, and guidance on how to confirm whether aggregation has affected the result. It also includes a figure that explains the size ranges used for the reduction-rate calculations.

Annex A gives worked examples and simulations of bubble-peak decrease and particle aggregation. Annex B explains, with figures, why volume concentration is useful when aggregation can make number concentration misleading. These annexes support method development, testing and interpretation of results.

International
Standard
ISO 24261-1
Second edition
Fine bubble technology —
2026-11
Elimination method for sample
characterization —
Part 1:
Evaluation procedure
Technologie des fines bulles — Méthode d'élimination pour la
caractérisation de l'échantillon —
Partie 1: Mode opératoire d'évaluation
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 1
4.1 Sample .1
4.2 Measuring instruments .2
5 Environment . 2
6 Evaluation of fine bubble elimination efficiency . 2
6.1 General .2
6.2 Reduction rate .2
6.3 Confirmation of contaminants aggregation .3
6.4 Evaluation procedure of fine bubble elimination .3
7 Test report . 4
7.1 Report of the testing results.4
7.2 Report of the testing conditions .4
Annex A (informative) Simulations of fine bubble elimination process . 5
Annex B (informative) Decrease of concentration caused by fine bubble elimination and
aggregation . 7
Bibliography . 9

iii
Foreword
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of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
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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 281 Fine bubble technology.
This second edition cancels and replaces the first edition (ISO 24261-1:2020), of which it constitutes a minor
revision.
The changes are as follows:
— in 7.1, the following list item was added "f)  Number of this document";
— in Figure A.1 and Figure A.2, the unit "million/ml" was revised to "×10⁶ particles/ml".
A list of all parts in the ISO 24261 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
Introduction
Fine bubble technology has recently seen growth in its application to markets in cleaning, water treatment,
agriculture, and aquaculture as well as biomedical fields. Now, methods are required to evaluate the
generation systems. Characteristics like number concentration index and the size index of fine bubbles are
especially indispensable for those evaluations.
Furthermore, fine bubble dispersion water may contain other solid and liquid particles. Due to this concern,
it may be impossible to evaluate the characteristics of fine bubbles. Therefore, a method to address the
presence of other particles is urgently needed.
There are several measurement methods widely used to evaluate the number concentration index and the
size index of particles. However, there are few methods to distinguish bubbles in fine bubble dispersions
from other particles.
This issue can be resolved, using the phenomenon by which the bubbles can be eliminated without any
residues after dissolution and flotation. If a method that eliminates fine bubbles in specific size range is
known, it is possible to distinguish fine bubbles from other solid and liquid particles. The eliminated particles
can be fine bubbles. A solution in which fine bubbles are not present or have been almost completely removed
can be used as a blank solution for background measurements. Standardizing elimination techniques and
evaluation methods are required as fine bubble elimination techniques will continue to develop further.
This document is intended to specify the evaluation method for elimination efficiency of fine bubbles from
fine bubble dispersions in water.
A standard method to evaluate fine bubble elimination efficiency enables easy and clear comparison among
several techniques and presents optimal conditions for respective elimination techniques.

v
International Standard ISO 24261-1:2026(en)
Fine bubble technology — Elimination method for sample
characterization —
Part 1:
Evaluation procedure
1 Scope
This document specifies the evaluation procedure of fine bubble elimination for fine bubble dispersion in
water. This document is applicable only to fine bubbles without shell.
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 20480-1, Fine bubble technology — General principles for usage and measurement of fine bubbles — Part 1:
Terminology
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 20480-1 and the following 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
elimination of fine bubbles
process for the decrease of concentration index of fine bubbles
3.2
fine bubble dispersion
FBD
liquid which contains fine bubbles
[SOURCE: ISO 20298-1:2018, 3.1]
4 Requirements
4.1 Sample
The fine bubble dispersion to be evaluated shall be generated by cleaned fine bubble generating systems
using pure water and pure gas such as air, nitrogen and oxygen.
The purity level of water and gas depends on the sample whose size and concentration indices of fine
bubbles should be evaluated because fine bubble elimination is one of the evaluating process for specified

samples. So, the purity level cannot be generally determined as measurement condition for evaluations of
the elimination efficiency.
The fine bubble dispersion shall not contain stabilizing agents.
If the measurement accuracy of size and concentration indices is very important (e.g. for accreditation
purposes), a water purity level of Grade 1 according to ISO 3696 is recommended for the water used for
generation of FBD.
4.2 Measuring instruments
When the measuring instruments are selected for evaluating fine bubble elimination for FBD in water
the following parameters about concentration and size range shall be confirmed, which depends on the
characteristics of the sample to be evaluated:
a) Total number concentration and/or total volume concentration including fine bubbles and contaminants
such as solid particles and liquid particles can be measured. Water diluent can be used for diluting the
FBD in water when the concentration is over the measuring range.
b) Total size range including fine bubb
...