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IS0
FOR STAND A RD I2 AT I ON
I NT ERN AT I ON AL ORGANIZATION
IS0 RECOMMENDATION
R 483
PLASTICS
METHODS FOR MAINTAINING CONSTANT
RELATIVE HUMIDITY IN SMALL ENCLOSURES
BY MEANS OF AQUEOUS SOLUTIONS
1st EDITION
April 1966
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 483, Plastics - Methods for Maintaining Constant
Relative Humidity in Small Enclosures by Means of Aqueous Solutions, was drawn up by
Technical Committee ISO/TC 61, Plastics, the Secretariat of which is held by the American
Standards Association, Inc. (ASA).
Work on this question by the Technical Committee began in 1958 and led, in 1962,
to the adoption of a Draft IS0 Recommendation.
In July 1963, this Draft IS0 Recommendation (No. 597) was circulated to all the
IS0 Member Bodies for enquiry. It was approved by the following Member Bodies:
Argentina Germany New Zealand
Australia Hungary Poland
India Portugal
Austria
Be 1 g i u m Israel Romania
Canada Italy Sweden
Colombia Japan Switzerland
Czechoslovakia Korea, Rep. of U.S.A.
Netherlands
Finland U.S.S.R.
Two Member Bodies opposed the approval of the Draft:
France
United Kingdom
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council which decided, in April 1966, to accept it as an IS0 RECOMMENDATION.
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IS0 / R 483 - 1966 (E)
IS0 Recommendation R 483 April 1966
PLASTICS
METHODS FOR MAINTAINING CONSTANT RELATIVE HUMIDITY
IN SMALL ENCLOSURES BY MEANS OF AQUEOUS SOLUTIONS
1. SCOPE
1.1 This IS0 Recommendation establishes methods for obtaining atmospheres of constant
relative humidity in small enclosures using aqueous salt solutions or glycerol-water mixtures.
Chambers large enough to accommodate personnel or enclosures requiring frequent opening
to ambient atmosphere are specifically excluded from the scope of these methods.
1.2 Techniques for obtaining relative humidities at the temperatures specified in IS0 Recom-
mendation R 291 - Plastics - Standard Atmospheres for Conditioning and Testing, are
described.
1.3 Information is given and reference is made to a bibliography (page 1 I) concerning the methods
of producing desired humidities in these enclosures at temperatures from 5 to 60 OC.
1.4 The procedures described are intended for conditioning small quantities of plastics materials
prior to test and for such tests as may be carried out entirely within a small enclosure, e.g.,
electrical tests.
2. DEFINITIONS
2.1 Relative humidity. Ratio of the actual pressure of water vapour existing in the atmosphere
to the maximum possible (saturation) pressure of water vapour in the atmosphere at the
same temperature, expressed as a percentage.
2.2 Dew point. Temperature to which water vapour should be reduced to obtain saturation
vapour pressure, that is, 100 per cent relative humidity.
NOTE.- These definitions should be considered as tentative. Any decision reached by
* deviating from this wording will be valid.
ISO/ATCO
3. APPARATUS
The apparatus consists of the following:
3.1 A suitable container for volumes less than 14 dm3.
Container of simple shape which is easily cleaned and inert with regard to the saturated
solutions which are used; the temperature should be kept constant and uniform at all points
by placing it in an oven, a water bath or a laboratory conditioned at constant temperature
(see 3.2.2).
Air circulation is generally recommended, particularly when hygroscopic materials are being
tested. A fan whose blades are located just above the aqueous solution should preferably
be used.
* Co-ordinating Committee on Atmospheric Conditioning for Testing.
I
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lSO/ R 483 - 1986 (E)
However, if the enclosure is intended only for conditioning practically nonabsorbent
products or for conditioning over a long enough time in relation to the water absorption
capacities of the specimens that are placed there, air circulation need not be installed.
Without air circulation, the height of the container should not exceed the smallest dimension
of the free surface of the solution. In containers with air circulation, this height may reach
1.5 times the smallest dimension of this free surface.
3.2 A conditioning enclosure for volumes exceeding 14 dm'.
Conditioning enclosure constructed on the inside from material nonabsorbent and inert with
regard to the saturated solutions which are used, and thermally insulated on the outside.
It is fitted with thermostatic control of temperature and a fan for forced circulation of air
through the enclosure and past a tray of saturated sait solution. Cooling coils may be fitted,
if required, but their temperature should not be below the dew point.
3.2.1 The temperature should be kept constant in accordance with the tolerances required by
specifications on the temperature itself, or according to the tolerances on this tempera-
ture determined by the tolerances on the relative humidity, whichever is the closer. The
free surface of the solution should always be as large as possible. A means of measuring
temperature should be provided, as well as means of measuring relative humidity, such
as wet and dry bulb thermometers. If wet and dry bulb thermometers are used, the air
shoiild circulate around them at not less than 3 m/s.
3.2.2 Temperature variations and fluctuations should usually be controlled to 50.5 "C or
better since, without any change in the moisture content of the air, differences in air
temperature from point to point in the working space will produce differences in relative
humidity. The relative humidity will be low at points where the temperature is high,
and it will be high at points where the temperature is low.
For example, for the standard test atmospheres,* a difference of 0.5 "C causes a differ-
ence of nearly 2 per cent relative humidity. At 90 per cent relative humidity, the corres-
ponding difference is approximately 3 per cent. Fluctuations with time in the tempera-
ture at a given point will tend to produce a similar effect. If the fluctuations are slow,
the changes in relative humidity will be reduced by the action of the solutions.
3.3 A refractometer
Refractometer which covers the range of 1.330 to 1.470 with an accuracy of fO.OO1, for
use only with glycerol solutions.
4. CHEMICAL REAGENTS
4.1 Glycerol, industrial grade.
4.2 Salts, as required, of industrial grade.
4.3 Distilled water.
* See IS0 Recommendation R 291.
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iSO/ R 483 - 1966 (E)
5. PROCEDURE
5.1 The duration of conditioning indicated in specifications for the material is counted, or
testing started, only from the time at which the enclosure has regained the specified con-
ditions after the introduction of the specimens.
5.2 Overload in a small enclosure can lower the rate of establishing the humidity in the tray so
that it may be necessary to wait a very long time for the specified conditions to be reached.
In view of the fact that the load limit depends on the conditioned material's water absorption
capacity and its moisture content, this limit cannot be specified.
volume
In enclosures without air circulation and with specimens having a high ratio of
surface area
Y
(-), the tota
...