ISO 21910-2:2025

International
Standard
ISO 21910-2
First edition
Fine bubble technology —
2025-05
Characterization of microbubbles —
Part 2:
In-situ dynamic image analysis
method
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units. 2
5 Principle . 3
6 Instrumentation and image acquisition . 3
6.1 Instrumentation .3
6.2 Setup .4
6.2.1 Determine the image resolution .4
6.2.2 Determine the magnification and acquisition time .4
6.2.3 Calibration . . .4
6.2.4 Other notes .5
6.3 Image acquisition .5
7 Image analysis . 5
7.1 Size classification .5
7.2 Image edge determination .5
7.3 Bubble image characteristic .5
7.4 Bubble identification .5
7.5 Bubble diameter calculation .6
8 Measurement errors . 6
9 Test report . 6
Annex A (informative) Examples of in-situ dynamic image analyser designs . 8
Annex B (normative) Considerations for instrument usage in open water areas such as rivers,
lakes and seas. .11
Bibliography .15

iii
Foreword
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Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
Fine bubbles are bubbles with volume equivalent diameters smaller than 100 μm in liquid. Fine bubbles are
divided into two subgroups: microbubbles with volume equivalent diameters in the range from equal or
greater than 1 μm to smaller than 100 μm and ultrafine bubbles with volume equivalent diameters smaller
than 1 μm.
In recent years, many unique properties related to microbubbles have been revealed related to their large
specific surface area, instability, and motion characteristics. Microbubble technology with bubbles formed
with various gases, such as air, oxygen and hydrogen, is widely used in the fields of agriculture, aquatic and
food industry, energy and minerals, processing and manufacturing, environmental, cleaning, and health, etc.
Many properties of microbubble dispersion systems in liquid depend largely on the composition, geometry
and size distribution of the bubbles. Because of their thermodynamic instability in a dynamic evolution
process, rapid in-situ data acquisition and accurate identification of bubbles is the key to character the
bubbles in the measurement locations and in real time. It is of great significance to track and measure bubble
size, size distribution and dynamic evolution of microbubbles in liquid in real time in order to understand
the properties of microbubbles and to optimize and control the microbubble generation process. The in-situ
dynamic image analysis is such a method to satisfy the above needs.

v
International Standard ISO 21910-2:2025(en)
Fine bubble technology — Characterization of
microbubbles —
Part 2:
In-situ dynamic image analysis method
1 Scope
This document specifies the principle, devices and operations for measuring bubble size distribution of
microbubbles in liquid media using the in-situ dynamic image method.
This document is applicable to microbubbles, as well as to bubbles in the size range of 100 μm to 500 μm,
dispersed in transparent liquid media, mostly in water.
2 Normative references
There are no normative references in this document.
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
acceptable depth of field
region where the sharpness of the edges of the images reaches the pre-set optimum and is accepted for
segmentation
[SOURCE: ISO 26824:2022, 3.8.15, modified — "" has been removed, "depth with
respect to focal depth" has been revised to "region," "particle images" revised to "images" and Note 1 to
entry has been removed.]
3.2
binary image
digitized image consisting of an array of pixels (3.13), each of which has a value of 0 or 1, whose values are
normally represented by dark and bright regions on the display screen or by the use of two distinct colours
[SOURCE: ISO 13322-1:2014, 3.1.2]
3.3
bubble size
linear dimension of a bubble determined by a specified measurement method and under specified
measurement conditions
Note 1 to entry: Different methods of analysis are based on the measurement of different physical properties.
Independent of the bubble property actually measured, the bubble size is reported as a linear dimension, e.g. as the
equivalent spherical diameter.

[SOURCE: ISO 26824:2022, 3.1.9, modified — "particle size" has been revised to "bubble size", and Note 2 and
3 to entry have been removed.]
3.4
edge determination
method used to detect transition between objects and background
[SOURCE: ISO 13322-1:2014, 3.1.4, modified — "detection" has been revised to "determination", "methods"
has been revised to "method," and Note 1 to entry has been removed.]
3.5
field of view
field which is viewed by the viewing device
Note 1 to entry: The full image frame of a digital imaging device corresponds to its field of view.
[SOURCE: ISO 13322-1:2014, 3.1.6]
3.6
image analysis
processing and data reduction operation which yields a numerical or logical result from an image
[SOURCE: ISO 13322-1:2014, 3.1.8]
3.7
laminar flow
fluid flow characterized by the parallel movement of fluid layers (laminae) past one another in an orderly fashion
[SOURCE: ISO 8625-2:2018(en), 3.3.1]
3.8
microbubble
fine bubble with a volume equivalent diameter in the ranges from equal or greater than 1 μm to smaller
than 100 μm
[SOURCE: ISO 20480-1:2017, 3.4, modified — "range" has been revised to "ranges" and Note 1 to entry has
been removed.]
3.9
threshold
grey level value which is set to discriminate objects of interest from background
[SOURCE: ISO 13322-1:2014, 3.1.14, modified — Figure 1 has been removed.]
4 Symbols and units
Symbols Names Dimension Units
a moving distance of the centre of the bubble image metre m
n number of bubbles of diameter x dimensionless
i i
2 2
σ standard deviation of bubbles of diameter x metre squared μm
i
t strobe flash time or camera shutter time second s
v bubble moving velocity metre per second m/s
x bubble diameter metre μm
i
x average of x metre μm
mean i
A area of bubble projection metre squared μm
i
5 Principle
The in-situ dynamic image method submerges the light source, optical path and image acquisition device of
the measurement system into a transparent liquid medium containing dispersed bubbles, captures bubble
images in-situ with rapid exposure, and measures bubble sizes in real time. Annex A shows examples of the
in-situ dynamic image analyser designs. The main applications are for measuring bubble distributions in
liquid bodies, such as tanks, rivers, lakes, rice fields, etc., after the bubble generator spreads bubbles in the
liquid body. Annex B shows considerations for instrument usage in open water areas such as rivers, lakes
and seas. A schematic diagram of the measuring system is shown in Figure 1.
Key
1 bubbles dispersed in a transparent liquid (the shaded area)
2 waterproof light source
3 measurement area (acceptable depth of field)
4 waterproof front optical system
5 waterproof back optical system
6 waterproof image acquisition device
7 image analysis system
8 computer
NOTE All components in the double-line box are immersed in the liquid.
Figure 1 — Schematics of an in situ dynamic image analyser
The water body containing bubbles into which the device is immersed can be still or can have natural flow
in the regime of laminar flow. After the device is immersed into the desired location, it should wait a certain
period of time to allow the water body back to a steady state, i.e. avoiding the disturbance to the bubbles
and the distribution caused by the immersing action. Typically, the bubbles are produced outside the shaded
area in Figure 1 and the analyser measures the bubbles in-situ at the acquisition time in the measurement
area. The acquisition time per image, which is controlled by either the shutter time or the flash interval,
should be short enough so clear images of moving bubbles can be taken. The measurement can be taken at
certain time intervals to assess the change of bubble size and number over time, or taken at different regions
in different orientations to assess spatial variations of bubble size and number.
6 Instrumentation and image acquisition
6.1 Instrumentation
The analyser shall be equipped with colourless transparent glass. The light source shall be a parallel beam.
The optical design and image acquisition device shall have image resolution no less than 0,5 μm/pixel,
covering the size measurement range from 1 μm to 500 μm with the images taken at a speed no slower
than 60 frame/s, typically a few hundred frames per second. The image analysis system shall have the
following functions: illumination field distortion correction, transmitted light analysis, continuous and
reversible adjustment of thresholds, distinguishing bubbles from other pollutants, automatically calculating

parameters of individual bubble images, such as area, circumference, etc., and size statistics calculations
including maximum, minimum, average, standard deviation of the parameters of individual bubbles, and
other parameters. The instrument shall include a certified calibration ruler with the minimum scale of 2 μm.
6.2 Setup
6.2.1 Determine the image resolution
The resolution of image acquired depends not only on its optical system (optical magnification and camera
resolution), but also on the lighting system and the moving velocity of the bubbles. When a bubble with a
diameter of x moves at a velocity, v, the centre of bubble projection area moves a distance a in time t, where t
is the strobe flash time or the camera shutter time, as given by Formula (1):
a= v·t (1)
To meet the desired resolution and accuracy, a shall not exceed a certain percentage of the smallest bubble
to be characterized.
The resolution of the entire system shall be set according to the bubble size distribution and the preset
confidence intervals.
6.2.2 Determine the magnification and acquisition time
The final settings and calibration of the image capture device needs to be established via an iterative
approach. For unknown test samples, the bubble size range determines the settings required for the image
acquisition device, which remain unknown until the first image is taken. The necessary adjustments to the
image capture device to achieve the desired accuracy of bubble size measurement are to be made based on
the image obtained.
The magnification should be such as to provide a minimum number of pixels for the smallest bubble
consistent with the accuracy demanded and set to achieve a sharp focus. The magnification should also be
selected to ensure that the maximum dimension of the largest bubble does not exceed one-third of the short
side of the rectangular image frame in the test area. If the size range is so broad that the current instrument
cannot cover the
...