ISO 18941:2011

INTERNATIONAL ISO
STANDARD 18941
First edition
2011-12-01
Imaging materials — Colour reflection
prints — Test method for ozone gas
fading stability
Matériaux pour l’image — Tirages par réflexion en couleurs — Méthode
d’essai de la stabilité de la décoloration à l’ozone
Reference number
©
ISO 2011
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ii © ISO 2011 – All rights reserved

Contents Page
Foreword . v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 3
5 Target selection . 3
6 Measurements . 3
6.1 Use of replicates and reference samples . 3
6.2 Holding and measurement conditions . 4
6.3 Measured attributes . 5
7 Calculations and computations . 6
7.1 Computation of densitometric attributes . 6
7.2 Density change in d patches . 6
min
7.3 Percentage density change in primary colour patches . 6
7.4 Percentage density change in secondary (mixed) colour patches . 6
7.5 Percentage density change in composite neutral patch . 6
7.6 Colour balance shift in composite neutral patch . 6
7.7 Colour balance shift in secondary (mixed) colour patches . 7
7.8 Colour balance in d patches by colorimetry . 7
min
7.9 Effects of colorant fading and stain formation on colour photographs . 7
8 Test methods — Gas fading (ozone) . 7
8.1 General . 7
8.2 Apparatus . 7
8.3 Test procedure .14
9 Test environment conditions .15
9.1 Humidity control calibration .15
9.2 Relative humidity .15
9.3 Temperature .15
9.4 Ozone concentration .15
10 Test report .16
10.1 General reporting requirements .16
10.2 Ozone test reporting .16
Annex A (informative) Method for interpolation .18
Annex B (normative) Reciprocity considerations.19
Bibliography .21
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 18941 was prepared by Technical Committee ISO/TC 42, Photography.
iv © ISO 2011 – All rights reserved

Introduction
In image permanence testing, there are four environmental variables known to affect the stability of a
[14][15][16][17][18][19][20][21][22][23][24]
photographic image: heat, light, moisture and air pollution, such as ozone
[25][26][27]
. Although natural ageing under “real-world” environmental levels of these variables is considered
the only certain test for image permanence, the high stability of most modern photographic products makes
testing under ambient conditions too lengthy a process to be of practical use. Thus, a widely used alternative to
natural ageing is accelerated ageing, whereby a sample specimen is exposed to each environmental variable
individually, and at levels considerably greater than ambient, forcing degradation of the image, by that single
factor, in a far shorter length of time.
This International Standard covers the equipment, methods and procedures for generating a known ozone
exposure and the subsequent measurement and quantification of the amount of change produced within a
photographic image due to that exposure. It is important to note that, if predictions of absolute product longevity
are of concern to the experimenter, then further knowledge needs to be gained regarding the reciprocal
behaviour of the test product under the experimental accelerated ozone conditions. See Annex B for more
information on reciprocity.
Additionally, there are other known variables in an ozone test setup that can affect the rate at which an image
will degrade in the presence of ozone. These include air flow over the sample, the nature of the chemical
reaction that is occurring, the relative quantities of the reactants (ozone and colorant molecules) and the
humidity content and the pH of the image recording layer. Each of these variables can affect the reciprocal
response and needs to be understood for a clear analysis of the accelerated data.
In some products, such as most dyes on swellable inkjet media and in silver halide products in gelatine,
the ozone reaction can be considered to be “diffusion-controlled,” whereby ozone first needs to permeate a
protective surrounding matrix before coming in contact with a colorant molecule and reacting. Further, the
reacted components then need to be desorbed and removed from the surface before fresh, unreacted molecules
can again diffuse, adsorb and react. In this type of process, a simple increase in ozone concentration might or
might not yield a proportional increase in reaction rate as diffusion, adsorption and, in some cases, desorption
may be the dominant factor controlling the rate of reaction.
The relative quantities of the reactants (ozone and colorant) will also affect the rate of reaction and reciprocal
behaviour. Under the assumed ambient conditions, a photographic image would undoubtedly contain a vast
excess of colorant molecules relative to the local concentration of ozone molecules in the air. Here, ozone
would likely be the limiting factor controlling the rate of reaction and, in the absence of other controlling factors,
an increase in ozone concentration will produce a proportional increase in the rate of reaction. At some precise
ozone concentration, the quantity of reactants would be equal and the reaction would proceed at a maximum
rate. At this point, however, a further increase in ozone concentration would not accelerate the reaction rate,
causing a failure in the reciprocal relationship that is required for converting accelerated data into predictions of
ambient performance. For this reason, if product longevity predictions are to be made, this ozone concentration
needs to be determined and never exceeded during testing.
This International Standard has been primarily developed via testing with inkjet images on porous “instant-dry”
photographic media, which have been shown to be susceptible to fading by oxidative gases present in polluted
[14][15][20][21][22]
ambient air . While many chemical species may be present in polluted air, it has been shown
[22][28][29]
that most of the fade observed for current inkjet systems can be explained by oxidation by ozone .
Additionally, this method may reasonably be used for colour photographic images made with other digital and
traditional “continuous-tone” photographic materials such as chromogenic silver halide, silver dye-bleach, dye
[27]
transfer , dye-diffusion-transfer “instant” and other similar systems. However, since these systems have, in
general, much greater stability to ozone, the validity of this accelerated test method has not yet been verified
in these systems.
High levels of ozone, often found outside major conurbations in summer months, together with high levels of
humidity, will greatly accelerate the fade. Since ozone is a highly reactive gas, storage of photographs in any
kind of gas-impermeable enclosure, such as framed behind glass or in an album, will greatly reduce image
degradation due to ozone. This method therefore relates primarily to the display of unprotected photographs.
INTERNATIONAL STANDARD ISO 18941:2011(E)
Imaging materials — Colour reflection prints — Test method for
ozone gas fading stability
1 Scope
This International Standard describes the equipment, methods and procedures for generating a known ozone
exposure and the subsequent measurement and quantification of the amount of change produced within
both digitally printed hardcopy images and traditional analogue photographic colour print images due to that
exposure.
The test method described in this International Standard uses increased levels of ozone to achieve an
accelerated test. If the principal “gas fading” mechanism for a system is not ozone, this method might not be
suitable and might give misleading results as to resistance of the test image to polluted air.
2 Normative references
The following referenced documents are indispensable for the application 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 5-3, Photography and graphic technology — Density measurements — Part 3: Spectral conditions
ISO 5-4, Photography and graphic technology — Density measurements — Part 4: Geometric conditions for
reflection density
ISO 1431-3, Rubber, vulcanized or thermoplastic — Resistance to ozone cracking — Part 3: Reference and
alternative methods for determining the ozone concentration in laboratory test chambers
ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic arts images
ISO 18913, Imaging materials — Permanence — Vocabulary
ISO 18944, Imaging materials — Reflection colour photographic prints — Test print construction and
1)
measurement
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18913 and the following apply.
3.1
air/gas
mixture of atmospheric air and ozone inside the test chamber
3.2
volume turnover
complete replacement of the air/gas volume within the test chamber
3.3
volumetric turnover rate
rate at which volume turnover occurs
1) To be published.
3.4
agitation
degree to which air/gas is circulated within the chamber resulting in a mixing of the air/gas at the surface of the
test sample to overcome concentration gradients
NOTE Agitation can be directly related to flow rate but inversely related to volume turnover. For a given incoming gas-
flow velocity, the actual flow across the samples, and therefore the agitation, can be affected by chamber volume, with for
example larger chamber volumes resulting in lower flow over the samples. Agitation of air/gas is important to ensure mixing
so that any reaction by-products are carried away from the test samples.
3.5
air velocity at sample
rate of flow of air/gas across the sample plane, as opposed to the flow of air/gas within the chamber volume, or
within the entering or exiting ports
-1
NOTE Expressed in reciprocal milliseconds (ms ).
3.6
effective concentration
concentration of ozone as experienced by the test object, i.e. the concentration that results in a specific change
in a specific sample after exposure for a specific time
3.7
closed-loop system
system in which the air/gas volume is recirculated within the test chamber, with ozone added as needed to
maintain the desired aim concentration
3.8
open-loop system
system where the air/gas volume continually enters, flows through and exits the system with no recirculation
3.9
ideal mixing
sufficient agitation that results in uniform concentration throughout the chamber, such that no localized
concentration gradients exist across the test samples
3.10
operational control point
set point for equilibrium conditions measured at one or more sensor locations in an exposure device
NOTE Adapted from ASTM G113.
3.11
operational fluctuations
positive and negative deviations from the setting of the sensor at the operational control set point during
equilibrium conditions in a laboratory-accelerated weathering device
NOTE 1 Operational fluctuations are the result of unavoidable machine variables and do not include measurement
uncertainty. Operational fluctuations apply only at the location of the control sensor and do not imply uniformity of conditions
throughout the test chamber.
NOTE 2 Adapted from ASTM G113.
3.12
operational uniformity
range around the operational control point for measured parameters within the intended exposure area, within
the limits of intended operational range
NOTE Adapted from ASTM G113.
2 © ISO 2011 – All rights reserved

3.13
uncertainty (of measurement)
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could be reasonably attributed to the measurement
NOTE 1 The parameter might be, for example, a standard deviation (or a given multiple of it), or the half-width of an
interval having a stated confidence level.
NOTE 2 Uncertainty of measurement comprises, in general, many components. Some of these components can be
evaluated from statistical distribution of the results of series of measurements and can be characterized by experimental
standard deviations. The other components, which can also be characterized by standard deviations, are evaluated from
assumed probability distributions based on experience or other information.
NOTE 3 It is understood that the result of the measurement is the best estimate of the value of the measurement and
that all components of uncertainty, including those arising from systematic effects, such as components associated with
corrections and reference standards, contribute to the dispersion.
NOTE 4 Adapted from ISO Guide 98-3:2008, 2.2.3.
4 Requirements
This International Standard specifies a set of recommended test methods with associated requirements for
permitted reporting. Data from these tests shall not be used to make life expectancy claims, such as time-
based print lifetime claims, either comparative or absolute. Conversion of data obtained from these methods
for the purpose of making public statements regarding product life shall be in accordance with the applicable
International Standard(s) for specification of print life.
The test methods in this International Standard can be useful as stand-alone test methods for comparing
the stability of image materials with respect to one specific failure mode. Data from the test methods of this
International Standard can be used in stand-alone reporting of the absolute or comparative stability of image
materials with respect to the specific failure mode described in this International Standard, when reported
in accordance with the reporting requirements of this International Standard. Caution shall be used when
comparing test results for different materials. Comparisons shall be limited to test cases using equipment with
matching specifications and matching test conditions.
5 Target selection
For general testing purposes, users of this International Standard are free to choose whatever target patches
and starting densities they feel are appropriate for their testing needs. An example of such a target is included
in ISO 18944 along with requirements and recommendations for sample preparation. Applicable International
Standards for specification of print life may require the use of specific targets. Other recommendations for
sample preparation are contained in ISO 18909. Image prints may also be used.
6 Measurements
6.1 Use of replicates and reference samples
At least two replicate prints are required for each test case. Replicates shall be located for testing in different
regions of the test chamber volume.
It is recommended that reference samples be included in every exposure test to track consistency of the test
procedures as well as unintended changes in test conditions (see Reference [13]).
6.2 Holding and measurement conditions
Measurements and sample holding for measurement and next test phase preparation shall be conducted in a
controlled environment with no time constraint, or in a less controlled environment with a time constraint. The
measurement environment and sample holding environment can influence measured densities.
NOTE 1 “Sample holding environment” refers to the environment in which samples are held between test phases, such
as before and after measurement, while the samples are not in the active test environment.
The controlled sample holding environment with no time constraint shall meet the following set of conditions:
samples shall be kept in the dark at (23 ± 2) °C and (50 ± 10) % RH while waiting for measurement and while
holding between test stages.
2)
The sample holding environment shall be ozone-free [≤2 nl/l average ozone concentration over any 24 h
period] for ozone-sensitive samples. Ozone sensitivity is determined in accordance with this International
Standard and ISO 18944. A material that is not sensitive to ozone shall have demonstrated no measurable
change in minimum density, d , or printed patch colour, at ambient ozone exposure levels and measurement
min
condition temperature and humidity, over time periods consistent with measurement and test-staging time
periods.
The controlled measurement environment with no measurement-process time constraint shall meet the
following set of conditions: ambient illuminance on the sample surface not less than 200 lx, temperature of
(23 ± 2) °C, (50 ± 10) % RH, and ozone-free (≤2 nl/l average ozone concentration over any 24 h period) for
ozone-sensitive samples.
If either sample holding or measurement is conducted in a less controlled environment, samples shall be held
or measured in the less controlled environment for a maximum of 2 h for each test stage. The less controlled
environment may be unfiltered for ozone, and shall have a maximum RH of 75 % and a maximum temperature
of 30 °C, with ambient illuminance on the sample surface up to 1 000 lx.
NOTE 2 Stray light decreases the accuracy of measurements taken in less controlled lighting environments. Shielding
the measurement instrument from direct lighting so that the actual measurement surface lighting is not less than 200 lx
can improve measurement accuracy and repeatability.
The temperature and humidity tolerances for the sample holding and measurement environments apply
specifically to the vicinities in which the samples are held and measured. Operational fluctuations, operational
uniformity and uncertainty of measurement shall be contained within the stated tolerances in those vicinities.
The measurement environment and sample holding environment with respect to temperature, relative humidity,
ozone and light levels, fluctuations and uniformity shall be reported in the test report.
The CIE colour coordinates of the d patch (unprinted paper) shall be measured in accordance with
min
ISO 13655 measurement condition M0 for the relative spectral power distribution of the flux incident on the
specimen surface. White backing is recommended in accordance with ISO 13655. Report the backing used
or the material opacity according to ISO 2471, stating that the backing has no influence on the measurement.
Measurement conditions shall be consistent throughout the test process. In accordance with ISO 13655,
calculated tristimulus values and corresponding CIELAB values shall be computed using CIE illuminant D50
and the CIE 1931 standard colorimetric observer (often referred to as the 2° standard observer).
NOTE 3 With completely opaque materials, such as the aluminium substrate used in outdoor testing, the backing has
no relevance.
Optical densities shall be measured according to ISO 5-3, with the relative spectral power distribution of the
flux incident on the specimen surface conforming to CIE illuminant A, ISO 13655 measurement condition M0,
and spectral products conforming to Status A or Status T density, as appropriate for the material under test.
2) 1 nl/l = 1 ppb (1 × 10−9). Although the notation “ppb” (parts per billion) is widely used in the measurement and reporting
of trace amounts of pollutants in the atmosphere, it is not used in International Standards because it is language-dependent.
4 © ISO 2011 – All rights reserved

White backing is recommended in accordance with ISO 5-4. ISO 5 standard reflection density as defined in
ISO 5-4 shall be used, allowing either annular influx mode or annular efflux mode. Either white or black backing
is allowed. Report the backing used. Measurement conditions shall be consistent throughout the test process.
NOTE 4 When testing in accordance with an image life specification standard, either standard status A or
status T density is selected according to that specification standard.
A single measurement instrument shall be used for all of the measurements taken pertaining to a particular
test. For example, initial patch values of a test target print and subsequent degraded patch values of that
particular test target print shall be measured using the same measurement instrument. Replicate prints may
be measured on separate measurement instruments as long as each is consistently measured on the same
instrument used for its initial readings. According to best practice, in the case of equipment failure the test
should be invalidated. A replacement instrument with a known offset, determined for the test measurement
conditions and materials such as those being measured, may be used when the original instrument is not
available. In this case, all measurements shall be corrected with the known offset.
NOTE 5 It is useful to retain freezer check print samples of the measurement materials so that instrument offsets can
be measured if needed. Offset measurements from materials matched to those under test are preferred to measurements
using BCRA tiles. See ISO 18920 for print storage methods.
6.3 Measured attributes
6.3.1 Definition of density terms
The symbol for measured density is d.
6.3.2 Density attributes to be measured
The following Status A or Status T densities of the specimens shall be measured before and after the treatment
interval.
a) dN(R) , dN(G) , dN(B)
t t t
The red, green and blue Status A or Status T densities of neutral patches that have been treated for time, t,
where t takes on values from 0 to the end of the test.
b) dC(R) , dM(G) , dY(B)
t t t
The red, green and blue Status A or Status T densities of cyan, magenta and yellow colour patches that have
been treated for time, t, where t takes on values from 0 to the end of the test.
c) dR(G) , dR(B) , dG(R) , dG(B) , dB(R) , dB(G) ,
t t t t t t
The red, green and blue Status A or Status T densities of the composite secondary R, G, B colour patches that
have been treated for time t, where t takes on values from 0 to the end of the test.
6.3.3 Definitions of colorimetry terms
L* is CIELAB lightness, a* and b* are the CIELAB a* and b* coordinates respectively, as defined in ISO 11664-4.
6.3.4 Colorimetry values to be measured
The following colorimetry values of the specimens, prepared as described in Clause 5, shall be measured
before and after the treatment interval: L* , a* , b* , which are the lightness, red-green and blue-yellow colour
t t t
coordinates, respectively, for the unprinted areas of specimens (paper white) that have been treated for time, t,
where t takes on values from 0 to the end of the test.
7 Calculations and computations
7.1 Computation of densitometric attributes
Calculations for 7.2 to 7.8 shall be performed for selected patches with a range of initial densities.
7.2 Density change in d patches
min
a) Red density change: Δ d (R) = d (R) − d (R)
min t min t min 0
b) Green density change: Δ d (G) = d (G) − d (G)
min t min t min 0
c) Blue density change: Δ d (B) = d (B) − d (B)
min t min t min 0
7.3 Percentage density change in primary colour patches
a) Cyan patch: %ΔdC(R) = {[dC(R) − dC(R) ] ÷ dC(R) } × 100
t t 0 0
b) Magenta patch: %ΔdM(G) = {[dM(G) − dM(G) ] ÷ dM(G) } × 100
t t 0 0
c) Yellow patch: %ΔdY(B) = {[dY(B) − dY(B) ] ÷ dY(B) } × 100
t t 0 0
7.4 Percentage density change in secondary (mixed) colour patches
a) Magenta in Red patch: %ΔdR(G) = {[dR(G) − dR(G) ] ÷ dR(G) } × 100
t t 0 0
b) Yellow in Red patch: %ΔdR(B) = {[dR(B) − dR(B) ] ÷ dR(B) } × 100
t t 0 0
c) Cyan in Green patch: %ΔdG(R) = {[dG(R) − dG(R) ] ÷ dG(R) } × 100
t t 0 0
d) Yellow in Green patch: %ΔdG(B) = {[dG(B) − dG(B) ] ÷ dG(B) } × 100
t t 0 0
e) Cyan in Blue patch: %ΔdB(R) = {[dB(R) − dB(R) ] ÷ dB(R) } × 100
t t 0 0
f) Magenta in Blue patch: %ΔdB(G) = {[dB(G) − dB(G) ] ÷ dB(G) } × 100
t t 0 0
7.5 Percentage density change in composite neutral patch
a) Cyan in neutral patch: %ΔdN(R) = {[dN(R) − dN(R) ] ÷ dN(R) } ×100
t t 0 0
b) Magenta in neutral patch: %ΔdN(G) = {[dN(G) − dN(G) ] ÷ dN(G) } × 100
t t 0 0
c) Yellow in neutral patch: %ΔdN(B) = {[dN(B) − dN(B) ] ÷ dN(B) } ×100
t t 0 0
7.6 Colour balance shift in composite neutral patch
Contrast and colour balance distortions brought about by differential fading of the three image colorants
can result in significant visually degrading effects. These can be measured as shifts in colour balance from
highlights to shadows and are especially noticeable in a scale of neutrals; for example, a shift from magenta to
green due to fading of the photograph’s magenta image colorant, or from yellow to blue or cyan to red due to
fading of the yellow or cyan colorant.
Neutral colour balance shift is calculated as the difference in percentage change between any two primary
colours of a neutral patch. The percentage change of individual primary colours in a neutral patch is defined
in 7.5.
a) Cyan-magenta shift: %ΔdN(R-G) = |%ΔdN(R) − %ΔdN(G) |
t t t
b) Magenta-yellow shift: %ΔdN(G-B) = |%ΔdN(G) − %ΔdN(B) |
t t t
c) Yellow-cyan shift: %ΔdN(B-R) = |%ΔdN(B) − %ΔdN(R) |
t t t
6 © ISO 2011 – All rights reserved

7.7 Colour balance shift in secondary (mixed) colour patches
Secondary colour balance shift is calculated as the difference in percentage change between the two primary
colours of each secondary colour patch. The percentage change of the individual primary colours in each
secondary colour patch is defined in 7.4.
a) Cyan-magenta shift in Blue patch: %ΔdB(R-G) = |%ΔdB(R) − %ΔdB(G) |
t t t
b) Magenta-yellow shift in Red patch: %ΔdR(G-B) = |%ΔdR(G) − %ΔdR(B) |
t t t
c) Yellow-cyan shift in Green patch: %ΔdG(B-R) = |%ΔdG(B) − %ΔdG(R) |
t t t
7.8 Colour balance in d patches by colorimetry
min
Colour balance in the d patches is calculated using the following equation:
min
* * * 2 * * 2 * * 2
ΔE = (L − L ) +(a −a ) +(b −b ) .
ab t 0 t 0 t 0
where L*, a*, and b* are the colour coordinates of the d patch at the initial time 0 and at time t, as defined by
min
ISO 11664-4.
7.9 Effects of colorant fading and stain formation on colour photographs
Any change in density, contrast or stain, whether due to colorant fading, changes in colorant morphology, or
discolouration of residual substances, will change the appearance of the photograph.
The most damaging change tends to be contrast balance distortions brought about by differential fading of the
three image colorants.
The second most consequential change is that caused by an increase in stain. The result may simply be a
discolouration of the d areas or a change in the d colour balance.
min min
8 Test methods — Gas fading (ozone)
8.1 General
For the purpose of predicting fade rates, it is assumed that increasing ozone concentration should proportionally
[14][16][17]
increase the rate of fading. This has been generally shown to be the case, but exceptions are known
[18][24]
(see also Annex B).
8.2 Apparatus
WARNING — Attention is drawn to the highly toxic nature of ozone. Efforts should be made to minimize
the exposure of workers at all times. In the absence of more stringent or contrary national safety
regulations in member body countries, it is recommended that 0,1 microlitres of ozone per litre of
air of the surrounding atmosphere by volume be regarded as an absolute maximum concentration to
which a worker shall be exposed, while the maximum average concentration should be appreciably
lower.
NOTE An exhaust vent to remove ozone-laden air is advised.
8.2.1 Ozone test device
8.2.1.1 General
Two general types of ozone test device can be used, each having unique systems to deliver ozone to the test
samples. Both designs may be combined with either open- or closed-loop circuits for feeding with ozonized air.
Additionally, there are inherent differences in the ozone control strategies for each (see 8.2.5). It is critical that
any chamber design result in turbulent, not laminar, flow to maintain consistent ozone concentrations at the
sample surface (see 8.2.5 for additional information on turbulent flow).
8.2.1.2 Design 1 (chamber circulation design)
This design consists of an enclosed chamber at atmospheric pressure into which multiple test samples can be
simultaneously placed and ozonized air (i.e. air whose oxygen content has been partially converted to a specific
ozone concentration) can be delivered at a given concentration, temperature, relative humidity (RH), volume
turnover and agitation. Preliminary tests should be run to ensure that this regime is not in the concentration-
sensitive region of the materials being tested (see 8.2.5). It is recommended that the laboratory run reference
samples periodically to maintain this condition.
NOTE Agitation can be directly related to flow rate but inversely related to volume turnover. For a given incoming
gas flow velocity, the actual flow across the samples, and therefore the agitation, can be affected by chamber volume,
with, for example, larger chamber volumes resulting in lower flow over the samples. Therefore, it is important to meet the
requirements of 8.2.5 with respect to effective mixing within the chamber and equilibration into the media.
The chamber shall be lined with, or constructed from, a material (such as stainless steel, anodized aluminium,
or ozone-resistant polycarbonate) that does not readily decompose ozone. Dimensions shall be such that the
requirements of Clause 9 can be met.
The chamber may be provided with a window through which the test specimen can be observed. A light to
examine the test specimen may be installed. Light entering the test chamber shall be limited so as not to
confound results. If an ultraviolet (UV) source is used for ozone generation, the equipment design shall prevent
any UV radiation from entering the test chamber.
Figures 1 and 2 provide illustrative examples of loop-system chamber designs, in which the loop is either
closed or open. In the case of a closed-loop system (Figure 1), ozonized air for feeding the chamber is only
partially replenished, whereas in an open-loop system (Figure 2), ozonized air is continuously prepared fresh
from purified laboratory air. In any case, replacement of air is necessary to remove reaction products. For
Figure 1, make-up air would be added just ahead of the ozonizer.
Ozone concentration
measurement device
Purifying Exhaust
Purifying column
column dehumidifier
Temperature / humidity controller
M
Fan
Blower
Heating unit
Test chamber
Flow meter
Velocity of ozone gas
Preconditioning
0,3 m/s < v < 0,6 m/s
chamber
Cooling unit
Purifying
column
for NO
Fan
Vaporizer
Heat exchanger
Ozonizer
Figure 1 — Example of a closed-loop system
8 © ISO 2011 – All rights reserved

c
b
17 18 19 20
M
a
M
Key
1 activated carbon filter 11 temperature sensor
2 dust filter 12 humidity sensor
3 blower 13 plate with holes
4 air dryer 14 circulation blower
5 ozone generator 15 extra blower
6 flow meter 16 valve and exhaust hood
7 valve 17 ozone analyser
8 valve 18 pressure meter
9 temperature control 19 flow meter
10 humidity control 20 pump
a
Fresh air.
b
Circulating air.
c
Exhaust.
Figure 2 — Example of an open-loop system
8.2.1.3 Design 2 (local injection or direct-impingement design)
This design consists of a delivery system that supplies aim-concentration ozonized air directly, and uniformly,
to the entire surface of each individual test sample. Conceptually, each sample is contained within its own test
chamber and isolated from the effects of ozone quenching by other samples in the test device.
The chamber shall be lined with, or constructed from, a material (such as stainless steel, anodized aluminium,
or ozone-resistant polycarbonate) that does not readily decompose ozone. Dimensions shall be such that the
requirements of Clause 9 can be met.
Figure 3 describes a typical direct-impingement design.
Exhaust
Make-up
air
Ozone
Plenums to impinge
chamber
ozone-enriched air
directly on targets
Control
valve
Temperature / RH
UV light
measurement
source Control
valve
Rotameter
Gas dryer
PRV
Ozone
measurement
Variable speed
Steam
Cooling Heating
recirculation fan
humidifier
coil element
Figure 3 — Example of a direct-impingement system
8.2.2 Source of ozonized air
Any methodology that can generate ozone with sufficient quantity, purity and flow rate may be used as an
ozone source. Examples of such apparatus include:
a) an ultraviolet lamp;
b) a corona discharge unit.
It is known that corona discharge units can generate other pollutants (such as oxides of nitrogen) when air,
rather than purified oxygen, is used as the ozone source, and that this feature can change as the unit ages. In
addition, if the air is not dried before ozonation, significant quantities of acids from oxides of nitrogen can be
3)
produced. In any case, total oxides of nitrogen in the chamber shall not exceed 2 nl/l when testing at 1 μl/l or
0,2 % of ozone concentration when testing at ozone concentrations greater than 1 μl/l.
For corona discharge systems that are designed to eliminate or exclude oxides of nitrogen or their precursors
(for example ones that start with pure oxygen instead of air), the measuring and reporting of oxides of nitrogen
is not necessary. It should be understood that, for some systems, very small levels of oxides of nitrogen can
cause rapid image degradation and would confound the measurement of change attributable to ozone. In any
case, the reporting shall reference the manufacturer’s specification for ozone purity.
Air used for generation of ozone shall first be purified, for example by passing it over activated charcoal. It shall
be free from any contaminants likely to affect the ozone concentration, sample fading, or estimation of ozone
concentration.
The ozonized air shall be fed from the generator into the air delivery system. A heat exchanger shall be
provided to adjust the gas temperature to that required by the test and shall be brought to the specified relative
humidity level by the introduction of moisture into the gas flow.
3) 1 μl/l = 1 ppm (1 × 10−6). Although the notation “ppm” (parts per million) is widely used in the measurement and
reporting of trace amounts of pollutants in the atmosphere, it is not used in International Standards because it is language-
dependent.
10 © ISO 2011 – All rights reserved

8.2.3 Means for adjusting, controlling and maintaining ozone concentration
8.2.3.1 General
Adjustment of the concentration of ozone may be, but does not have to be, automatic. The voltage/drive signal
to the lamp or corona discharge unit can be varied to control the production of ozone. Alternatively, a shield can
be used to mask part of the lamp. The adjustment shall be such that the concentration and tolerances given in
9.4 can be maintained.
Each test device design described in 8.2.1 has unique control considerations, which are outlined in 8.2.3.2 and
8.2.3.3.
8.2.3.2 Design 1
In a multi-sample chamber, it is common for a group of samples to begin quenching significant quantities of
ozone as they are placed into the chamber. As each individual sample reacts with the surrounding ozone, the
effective concentration to which all other samples are exposed would consequently be lower than the entering
ozone concentration. If the entering concentration is on aim, then the effective chamber concentration would
be lower than aim. This is not an acceptable testing condition and would yield erroneous results.
In this chamber design, the effective test ozone concentration is best represented by the exiting chamber
ozone concentration. It is important to note that if the exiting ozone concentration is at the desired aim and if
significant quenching is occurring, then the entering ozone concentration would be greater than aim. In this
situation, testing accuracy is highly dependent upon ideal (or near-ideal) mixing, to ensure that a concentration
gradient does not exist across the chamber. Any concentration gradient would result in some samples receiving
a greater exposure than others, depending on their proximity to the incoming air supply. Additionally, the
greater the difference between entering and exiting ozone concentrations, the greater the potential of a gradient
existing, and the greater the amount of mixing that would be required to diminish the effect. A test piece carrier,
like that described in 8.2.6, can greatly aid in equalizing the effects of a concentration gradient present within
a chamber. Concentration gradients can also be minimized by layout and choice of sample positions such that
no shielding of agitated air flow occurs.
Another solution would be to have a sufficient bulk quantity of ozone in the chamber so that ozone quenching
by the sample specimens is virtually undetectable relative to the total bulk ozone quantity. This can be achieved
by removing samples from the chamber until quenching cannot be detected from measurement noise, or by
increasing the total bulk volume of aim-concentration air flow into the chamber. It should be noted that, with
fixed piping dimensions in the air/ozone delivery system, an increase in the total bulk air volume will increase
the velocity of flow across the samples. If the test chamber is not already operating at a level that is insensitive
to changes in flow, then this control method will yield inconsistent results.
Therefore, in a multi-sample chamber design, the exiting chamber ozone concentration shall be equal to the
desired aim ozone concentration, with an operational fluctuation within ±4 % of aim, for the duration of testing,
and mixing shall be sufficient to demonstrate that an ozone concentration gradient does not exist across the
chamber. If rapidly fading reference samples (see 8.3) placed at different points within the chamber fade at the
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