Thermal insulation - Determination of steady-state thermal transmission properties - Calibrated and guarded hot box (ISO 8990:1994)

Lays down the principles for the design of the apparatus and minimum requirement that shall be met for determination of the laboratory steady-state thermal transmission properties of building components and similar components for industrial use. Does not specify a particular design. Describes also the apparatus, measurement technique and necessary data reporting. Primarily intended for laboratory measurements of large, inhomogeneous specimens. Does not provide for measurements where there is mass transfer through the specimen during the test.

Wärmeschutz - Bestimmung der Wärmedurchgangseigenschaften im stationären Zustand-Verfahren mit dem geregelten Heizkasten (ISO 8990:1994)

Diese Internationale Norm legt die Grundlagen für den Aufbau des Gerätes und Mindestanforderungen fest, die für die Bestimmung der stationären Wärmedurchgangseigenschaften von Bauteilen und ähnlichen Ausrüstungen für technische Zwecke im Laboratorium einzuhalten sind. Sie legt jedoch keinen speziellen Aufbau fest, da die Anforderungen, speziell an die Dimensionierung, zu einem geringeren Teil auch in bezug auf die Betriebsbedingungen, ausgesprochen vershieden sind.

Isolation thermique - Détermination des propriétés de transmission thermique en régime stationnaire - Méthodes à la boîte chaude gardée et calibrée (ISO 8990:1994)

La présente Norme internationale établit les principes de conception de l'appareillage et donne les critères minimaux à suivre pour déterminer en laboratoire les propriétés de transmission thermique en régime stationnaire, les composants de bâtiment et les composants similaires à usage industriel. Elle ne spécifie cependant pas de conception particulière, étant donné que les exigences varient, particulièrement les dimensions, et, dans une moindre mesure, les conditions de fonctionnement. La présente Norme internationale décrit également l'appareillage, la technique de mesurage et la consignation des données nécessaires. Les composants spéciaux, par exemple les fenêtres, nécessitent des procédures supplémentaires qui ne sont pas incluses dans la présente Norme internationale. Sont également exclus les mesurages de l'effet du transfert ou de la redistribution de l'humidité sur le flux thermique, mais il faut tenir compte dans la conception et le fonctionnement de l'équipement, de l'effet possible du transfert d'humidité sur l'exactitude et la pertinence des résultats d'essai. Les propriétés que l'on peut mesurer sont le coefficient de transmission thermique et la résistance thermique. Deux méthodes possibles sont spécifiées, à savoir la méthode de la boîte chaude gardée et la méthode de la boîte chaude calibrée. Ces deux méthodes conviennent à des éprouvettes horizontales telles que des plafonds et planchers. L'appareillage peut être suffisamment grand pour étudier de 1159s composants à l'échelle réelle. Les méthodes sont initialement prévues pour des mesurages en laboratoire de grandes éprouvettes non homogènes; des éprouvettes homogènes peuvent bien entendu aussi être essayées, et sont nécessaires pour l'étalonnage et la validation. L'expérience montre que pour des essais effectués sur des éprouvettes homogènes conformément à la présente Norme internationale, l'exactitude se situe dans la fourchette de + 5 %. Toutefois, l'exactitude d'un appareillage par

Toplotna izolacija - Določanje toplotne prehodnosti v stacionarnem stanju - Metoda kalibrirane in zaščitene komore (ISO 8990:1994)

General Information

Status
Published
Publication Date
20-Aug-1996
Current Stage
9093 - Decision to confirm - Review Enquiry
Due Date
13-Nov-2006
Completion Date
13-Nov-2006

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SLOVENSKI STANDARD
SIST EN ISO 8990:1997
01-december-1997
7RSORWQDL]RODFLMD'RORþDQMHWRSORWQHSUHKRGQRVWLYVWDFLRQDUQHPVWDQMX
0HWRGDNDOLEULUDQHLQ]DãþLWHQHNRPRUH ,62

Thermal insulation - Determination of steady-state thermal transmission properties -

Calibrated and guarded hot box (ISO 8990:1994)
Wärmeschutz - Bestimmung der Wärmedurchgangseigenschaften im stationären
Zustand-Verfahren mit dem geregelten Heizkasten (ISO 8990:1994)

Isolation thermique - Détermination des propriétés de transmission thermique en régime

stationnaire - Méthodes a la boîte chaude gardée et calibrée (ISO 8990:1994)
Ta slovenski standard je istoveten z: EN ISO 8990:1996
ICS:
27.220 Rekuperacija toplote. Heat recovery. Thermal
Toplotna izolacija insulation
SIST EN ISO 8990:1997 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 8990:1997
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SIST EN ISO 8990:1997
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SIST EN ISO 8990:1997
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SIST EN ISO 8990:1997
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SIST EN ISO 8990:1997
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SIST EN ISO 8990:1997
INTERNATIONAL
IS0
STANDARD
8990
First edition
1994-09-01
Thermal insulation - Determination of
steady-state thermal transmission
properties - Calibrated and guarded hot
box
lsola tion thermique - 06 termination des proprig tt% de transmission
thermique en r6gime s ta tionnaire - M&hodes ;i la boife chaude gardge
et calibr6e
Reference number
IS0 8990:1994(E)
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SIST EN ISO 8990:1997
IS0 8990: 1994(E)
Contents
Page

General ............................................................................

Section 1

1.1 Scope ...........................................................................................

1.2 Normative reference ....................................................................

1.3 Definitions ....................................................................................

........................................... 2
1.4 Symbols, units and relationships

..................................................................................

1.5 Principle
..........................................
1.6 Limitations and sources of errors
.....................................................................
Section 2 Apparatus
............................................................................
2.1 Introduction
..............................................................
2.2 Design requirements
..........................................................................
2.3 Metering box
...............................................................................
2.4 Guard box
...................................................................
2.5 Specimen frame

2.6 Cold side chamber ...............................................................

................................................ 10
2.7 Temperature measurements
....................................................................
2.8 Instrumentation
...............................
2.9 Performance evaluation and calibration
...........................................................
Section 3 Test procedure
..........................................................................
3.1 Introduction
....................................................
3.2 Conditioning of specimen
....................................... 13
3.3 Specimen selection and mounting
.....................................................................
3.4 Test conditions
..........................................................
3.5 Measurement periods

3.6 Calculations ..........................................................................

............................................................................
3.7 Test report
0 IS0 1994

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced

or utilized in any form or by any means, electronic or mechanical, including photocopying and

lisher.
microfilm, without permission in writing from the pub
International Organization for Standardization
and
Case Postale 56 l CH-1211 Geneve 20 l Switzerl
Printed in Switzerland
---------------------- Page: 8 ----------------------
SIST EN ISO 8990:1997
0 IS0
IS0 8990: 1994(E)
Annexes
A Heat transfer at surfaces and environmental temperatures
. 16
B Bibliography

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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SIST EN ISO 8990:1997
0 IS0
IS0 8990: 1994(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work
of preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 8990 was prepared by Technical Committee
lSO/TC 163, Therma/ insulation, Subcommittee SC 1, Test and measure-
ment methods.
Annex A forms an integral part of this International Standard. Annex B is
for information only.
---------------------- Page: 10 ----------------------
SIST EN ISO 8990:1997
0 IS0
IS0 8990: 1994(E)
Introduction
Data on the thermal transmission properties of insulants and insulated
structures are needed for various purposes including judging compliance
with regulations and specifications, for design guidance, for research into
the performance of materials and constructions and for verification of
simulation models.
Many thermal insulating materials and systems are such that the heat
transfer through them is a complex combination of conduction, convection
and radiation. The methods described in this International Standard
measure the total amount of heat transferred from one side of the speci-
men to the other for a given temperature difference, irrespective of the
individual modes of heat transfer, and the test results can therefore be
applied to situations when that is the property required. However, the
thermal transmission properties often depend on the specimen itself and
on the boundary conditions, specimen dimensions, direction of heat
transfer, temperatures, temperature differences, air velocities, and relative
humidity. In consequence, the test conditions must replicate those of the
intended application, or be evaluated if the result is to be meaningful.
It should also be borne in mind that a property can only be assessed as
useful to characterize a material, product or system if the measurement
of the steady-state thermal transmission properties of the specimen and
the calculation or interpretation of the thermal transmission characteristics
represent the actual performance of the product or system.
Further, a property can only be characteristic of a material, product or
system if the results of a series of measurements on a number of speci-
mens from several samples provide sufficient reproducibility.
The design and operation of the guarded or calibrated hot box is a complex
subject. It is essential that the designer and user of such apparatus has a
thorough background knowledge of heat transfer, and has experience of
precision measurement techniques.
Many different designs of the calibrated and the guarded hot box exist
worldwide conforming to national standards. Continuing research and de-
velopment is in progress to improve apparatus and measurement tech-
niques. Also the variation of structures to be tested may be so great, and
the requirements for test conditions so different, that it would be a mis-
take to restrict the test method unnecessarily and to confine all measure-
ments to a single arrangement. Thus it is not practical to mandate a
specific design or size of apparatus.
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SIST EN ISO 8990:1997
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SIST EN ISO 8990:1997
INTERNATIONAL STANDARD 0 IS0 IS0 8990:1994(E)
Thermal insulation - Determination of steady-state
thermal transmission properties - Calibrated and
guarded hot box
Section 1: General
also be tested, and these are necessary for calibration
1.1 Scope
and validation.
When testing homogeneous specimens in accord-
This International Standard lays down the principles
ance with this International Standard, experience has
for the design of the apparatus and minimum re-
shown that an accuracy within + 5 % can generally
quirement that shall be met for determination of the
be achieved. However, the accuracy of each individual
laboratory steady-state thermal transmission proper-
apparatus shall be estimated with reference homo-
ties of building components and similar components
geneous specimens of thermal conductance extend-
for industrial use. It does not, however, specify a
ing over the range to be measured using the
particular design since requirements vary, particularly
apparatus.
in terms of size, and also to a lesser extent in terms
The estimation of accuracy for nonhomogeneous
of operating conditions.
specimens will be more complex and involve an
analysis of the heat flow mechanism in the particular
This International Standard describes also the appara-
types of inhomogeneous specimens being tested.
tus, measurement technique and necessary data re-
Such analyses are not covered by this International
porting. Special components, for example windows,
Standard.
need additional procedures which are not included in
this International Standard. Also excluded are meas-
The method does not provide for measurements
urements of the effect on heat flow of moisture
where there is mass transfer through the specimen
transfer or redistribution but consideration shall be
during the test.
given in the design and operation of the equipment
as to the possible effect of moisture transfer on the
accuracy and the relevance of test results. The
properties which can be measured are thermal
1.2 Normative reference
transmittance and thermal resistance. Two alternative
The following standard contains provisions which,
methods are included: the calibrated hot box method

and the guarded hot box method. Both are suitable for through reference in this text, constitute provisions

vertical specimens such as walls and for horizontal of this International Standard. At the time of publi-

specimens such as ceilings and floors. The apparatus cation, the edition indicated was valid. All standards

can be sufficiently large to study full-scale compo- are subject to revision, and parties to agreements

based on this International Standard are encouraged
nents.
to investigate the possibility of applying the most re-

The methods are primarily intended for laboratory cent edition of the standard indicated below. Mem-

measurements of large, inhomogeneous specimens, bers of IEC and IS0 maintain registers of currently

although homogeneous specimens can, of course, valid International Standards.
---------------------- Page: 13 ----------------------
SIST EN ,SO 8990:1997
IS0 8990: 1994(E)
IS0 7345:1987, Thermal insulation - Physical quan-
A Area perpendicular to heat
Cm21
tities and definitions.
tlow
Density of heat flow rate
[W/m*]
d Specimen thickness
c 1
1.3 Definitions
Air temperature
t-u
For the purposes of this International Standard, the Mean radiant temperature
t-u
following definitions apply.
Environmental temperature
t-u
Surface temperature
CKI
1.3.1 mean radiant temperature, T,: Appropriate
weighting of the temperatures of surfaces “seen” by R, = A(T,i - T,,)I@,
the specimen for the purpose of determining the ra-
R, = 1 /h
diant heat flow rate to the surface of the specimen
Rsi = A(Tni - Tsi)/@l
(see annex A).
se = ACT se - Tne) I@1
1.3.2 environmental temperature, 7’“: Appropriate
Ru = l/U
weighting of air and radiant temperatures, for the
u = al/A(Tni - Tne)
purpose of determining the heat flow rate to the sur-
= q) - t& - @, [for guarded hot box]
face of the specimen (see annex A).
= a$ - a3 - a4 [for calibrated hot box]
NOTE 1 This method does not directly measure the
thermal conductivity although it can be derived in case of
opaque, homogeneous, flat specimens using the relation-
ship ;1 = d/R,.
1.4 Symbols, units and relationships
The following recommended symbols are used:
i Interior, usually hot side
e Exterior, usually cold side
1.5 Principle
Surface
n Environmental
1.5.1 General
R Thermal conductivity
[W/h-K)]
Both types of apparatus, the guarded hot box (GHB)
R Thermal resistance
[h*-U/W]
and the calibrated hot box (CHB), are intended to re-
u Thermal transmittance
[W/h*- U]
produce conventional boundary conditions of a speci-
men between two fluids, usually atmospheric air,
h Surface coefficient of heat
[W/(m2-K)]
each at uniform temperature.
transfer
@ Heat flow rate
The specimen is placed between a hot and a cold
chamber in which environmental temperatures are
Total power input, heating or
iwl
known.
cooling
Heat flow rate through speci-
@I Iv1
Measurements are made at steady-state of air and
men
surface temperatures and of the power input to the
Imbalance, heat flow rate par-
hot side chamber. From these measurements the
@2 cw
allel to specimen
thermal transfer properties of the specimen are cal-
culated. Heat exchange at the surfaces of the test
Heat flow rate through meter-
@3 WI
specimen involves both convective and radiative
ing box walls
components. The former depends upon air tempera-
Flanking loss, heat flow rate
@4 WI
ture and air velocity, and the latter depends upon the
flanking specimen
temperatures and the total hemispherical emittances
Peripheral loss, heat flow rate,
of specimen surfaces and of surfaces “seen” by the
t-w
parallel to specimen surface at
test specimen surface. The effects of the heat trans-
the edges of the specimen
fer by convection and radiation are combined in the
---------------------- Page: 14 ----------------------
SIST EN ISO 8990:1997
0 IS0
IS0 8990: 1994(E)

concept of an “environmental temperature” and a of the thermal transmittance, at conventional surface

surface heat transfer coefficient.
coefficients.
Thermal transmittance is defined between two en-
vironmental temperatures, and therefore suitable 1.5.2 Guarded hot box
temperature measurements are required to enable

these to be determined. This is particularly important In the guarded hot box (see figure I), the metering

with test specimens of low thermal resistance for box is surrounded by a guard box in which the en-

which the surface coefficients of heat transfer form vironment is controlled to minimize lateral heat flow

in the specimen, @,, and heat flow through the me-
a significant fraction of the total resistance. In case
tering box walls, a3. Ideally, when a homogeneous
of test specimens with a moderate to high thermal
specimen is mounted in the apparatus and when both
resistance, it may be sufficient to record air tempera-
inside and outside the metering box the temperatures
tures only during a test, if it can be shown that the

difference in air and radiant temperatures on either are uniform and furthermore when cold side tem-

side of the test specimen is so small that the accuracy peratures and surface coefficients of heat transfer are

requirements are met. uniform, a temperature balance for air both inside and
outside the metering box would imply a balance on
A special situation arises when the hot box has a ra-
the specimen surface and vice versa, i.e. CD* = D3 =
diant panel, close to the warm side of the specimen,
0. The total heat flow through the specimen will then
as heat supply. In this case the radiant component
be equal to the heat input to the metering box.
will be more dominant in the heat transfer to the

specimen surface. This method with radiant panel can In practice, for each equipment and each specimen

be used to measure the thermal resistance of the under test, there will be a limit in detecting imbalance

specimen but is not suitable for direct measurements (imbalance resolution, see 1.6.1 .I 1.

Metering box
/ Cold box
fl Specimen
Guard box
Figure 1 - Guarded hot box
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SIST EN ISO 8990:1997
IS0 8990: 1994(E) 0 IS0
ine the corres pond ing best
1.5.3 Calibrated hot box air, respectively, def
imbalance resolu
tion.
The calibrated hot box (see figure 2) is surrounded by
The apparatus shall be designed and operated in such
a temperature-controlled space not necessarily at the
a way as to obtain optimum heat flow balance as in-
same air temperature as that inside the metering box.
dicated in a) above, i.e. apparatus geometry and guard
The heat losses through the box walls, D3, are kept
air space and air flow speed so that o3 does not ex-
low by using a construction of high thermal resist-
ceed 10 % of c#+-,.
ance. The total power input, $, shall be corrected for
the wall losses, c&, and for the flanking losses, Q&.
Inhomogeneities in the specimen will enhance non-
The flanking heat flow path is illustrated in figure3,
uniformities in local surface coefficients and in speci-
which shows details of the specimen and specimen
men surface-temperatures. Heat flow imbalance
frame with the adjacent hot and cold side box walls.
through the metering box wall and in the specimen
The correction for box wall losses and flanking losses
shall be evaluated, and when necessary corrected for.
are determined by tests on calibration specimens of
For this purpose the metering box walls shall be
known thermal resistance. For flanking loss cali-
equipped to serve as a heat flowmeter. Additionally,
bration, the calibration specimens should cover the
a thermopile across the metering area periphery can
same thickness and thermal resistance range as the
be mounted on the specimen surfaces. In routine
specimens to be measured and the temperature
testing, imbalance detection can be simplified by cali-
range of intended use.
bration and calculation.
1.6.1.2 Size of metered area
The metering area is defined:
1.6 Limitations and sources of errors
a) for a guarded hot box, as the centre-nose to
The operation of the apparatus, to a certain desired
centre-nose when the specimen is thicker or
accuracy, is limited by a number of factors related to
equal to the nose width, or if the specimen is
equipment design, calibration and operation and
thinner than the nose width, as the inner periph-
specimen properties, e.g. thickness, thermal resist-
ery of the nose;
ance and homogeneity.
b) for a calibrated hot box, as the inner periphery of
1.6.1 Limitations and errors due to
the metering box.
apparatus
The size of the metered area determines the maxi-
1.6.1.1 ILimitations in imbalance resolution in a
mum thickness of the specimen. The ratios of the
guarded hot box
metering area side to the specimen thickness and of
the guard width to the specimen thickness are gov-
In practice, even with homogeneous specimens, local
erned by principles similar to those for the guarded
surface coefficients of heat transfer are not uniform,
hot box.
especially close to the borders of the metering box.

As a consequence, neither the specimen surface- The size of the specimen can also limit possibilities for

temperature nor the air temperature are uniform close
a representative section of the construction to be
to the periphery of the metering box both inside and
tested and thus allow errors and difficulties in inter-
outside. This has two consequences:
pretation of the result.

a) It can be impossible to reduce to zero at the same Measurement errors in testing to the hot box meth-

time both the lateral heat flow, a2, through the ods are in part proportional to the length of the per-

specimen, and the heat flow, $, through the imeter of the metering area. The relative influence of

this diminishes as metering area is increased. In the
metering box walls;
guarded hot box, the minimum size of the metered

b) The temperature nonuniformity close to the me- area is 3 times specimen thickness or 1 m x 1 m,

tering box, on the specimen surface, and in the whichever is the greater.
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SIST EN ISO 8990:1997
IS0 8990:1994(E)
Metering box A
/Cold box
T Specimen
Figure 2 - Calibrated hot box
,kq----------
1 Cold s ide
Hot side 1 1
Specimen
-------- Isothermal
Heat flow
Figure 3 - Heat flow path in specimen and frame
---------------------- Page: 17 ----------------------
SIST EN ISO 8990:1997
IS0 8990: 1994(E)

For the calibrated hot box, minimum specimen size is 1.6.2.2 Specimen inhomogeneity

I,5 m x I,5 m.
The perimeter error in the guarded hot box is due to
Most test specimens representative of building and
the heat flow rate, a2, along the surface of the
industrial components will generally be inhomogene-
specimen, due to imbalance between metering and
ous. Inhomogeneities in the test specimen will affect
guarded area, or by inhomogeneities. The perimeter
the pattern of the density of heat flowrate in such a
errors in the calibrated hot box are due to the flanking
manner that it is neither one-dimensional nor uniform.
heat flow, a4, which includes the distortion of the
Also variations of the thickness throughout the speci-
heat flow rate at the edges of the specimen.
men can cause significant local modifications of the
pattern of the density of heat flowrate. The effects
of these are nonuniformities in temperatures and local
1.6.1.3 Minimum power input
transfer coefficients making the following more diffi-
cult or even impossible:
Total power input, OP, to the metering box is the sum
of the power supplied to heaters, fans, transducers,
a) the definition of a mean surface temperature;
actuators, etc. Some of these cannot be reduced to
zero thus defining a minimum heat flow which has to
b) the detection of imbalance in the guarded hot box
pass through the specimen.
apparatus;
This limit can be lowered by cooling the hot chamber,
c) the definition of the metering area;
but that will cause further uncertainty connected with
the measurement accuracy of the cooling rate.
d) the error analysis of test results for a given in-
homogeneous specimen.
The minimum power is also limited by the uncertainty
of total power input to the metering box including
Specific examples include:
@3*
facings having a high thermal conductivity. These
All the above factors set a lower limit for the ratio
form easy paths for imbalance heat flow rate, CD*,
(T,i - TsJ~Ru~
and flanking heat losses, D4. It can help to cut the
facing along the metering box periphery. When
1.6.1.4 Maximum power input
layers are homogeneous, an alternative solution is
to run independent tests on each layer with test
Maximum power input is limited by required tem-
methods using a guarded hot plate or a heat flow
perature uniformity and surface coefficients. Large
meter;
heat flowrates imply large air mass flow across the
specimen surface if a high degree of air temperature
horizontal and vertical structural members like
uniformity is to be maintained; this will affect the heat
studs. Their effect is in most cases symmetrical;
transfer mechanism of the surface. In the case of the
guarded hot box decreasing the specimen resistance,
sections of the specimen made of different ma-
this imposes stricter requirements on the equivalence
terials. The temperature differences through the
of convective and radiative heat transfer in the me-
materials are not the same. A heat flow exists
tering and guard box to obtain a given accuracy.
close to the interface of the different materials.
When this interface is not far from the metering
box periphery, this implies a temperature nonuni-
1.6.2 Limitations and errors due to specimen
formity that affects both imbalance detection and
the ambiguity in the definition of the metering

1.6.2.1 Specimen thickness and thermal area. Also, local heat transfer coefficients are af-

resistance
fected by these inhomogeneities;
For a given apparatus design, specimen thickness can
cavities within the specimen. Natural convection

be limited for reasons depending upon specimen can create an unknown imbalance heat flow rate,

properties and boundary conditions, an upper limit for D2. The effect of installing barriers shall be evalu-

the thickness is due to edge losses 05, or flanking ated.
losses CD,, which, although decreasing with increas-

ing specimen thickness can become significant in It is not possible to provide immediate solutions to all

comparison to @, and degrade measurement accu- types of problems. The operator is advised to be fully

racy. aware of te effects of anomalies.
---------------------- Page: 18 ----------------------
SIST EN ISO 8990:1997
0 IS0 IS0 8990: 1994( E

Calculations of the importance and effects if inhomo- quirement, the method of conditioning shall be re-

geneities are of great help to predict the thermal per-
ported. For most specimens, it is normally impossible,

formance of the test specimen. If significant without derating measurement accuracy to an un-

differences exist between predicted and measured
acceptable level, to reduce temperature differences
specimen performance which cannot be explained, as
so much that moisture transfer is so slow that
a minimum requirement, where such divergences
steady-state mass transfer can be assumed during

exist, a careful inspection of the specimen should be measurement time. It should also be realized that not

performed to identify any difference between actual only moisture transfer through the specimen, but also

and specified sizes, dimensions, materials, etc. Any moisture redistribution in the specimen and phase

irregularities from the original specification shall be change, will affect the results.

reported.
1.6.2.4 Temperature correlation
1.6.2.3 Moisture content in specimen
Specimen thermal resistances or thermal transmit-

Moisture transfer during the test may have a signif- tances are often a function of temperature differences

icant effect on test results. It is not possible to specify across the specimen itself. Care shall then be taken

a standard pre-test conditioning. As a minimum re- in reporting and interpreting measurement results.

---------------------- Page: 19 ----------------------
SIST EN ISO 8990:1997
IS0 8990: 1994(E)
Section 2: Apparatus
The metered area shall be big enough to provide a
2.1 Introduction
representative test area. For modular components the

As stated in 1 .I, it is impractical to impose specific metered area should preferably span exactly an inte-

design details for an apparatus. However, this section gral number of modules.
gives mandatory requirements and the aspects which
The ratio of metered area to perimeter of the metered
must be considered.
area influences accuracy in both types of boxes be-

Figures 1 and 2 show typical arrangements of the test cause one-dimensional heat flow cannot be main-

specimen and major elements of the apparatus. Fig-
tained at the perimeter of the metered area. These
ures 4 and 5 show alternative arrangements. Other
error heat flows at the perimeter of the metered area,
arrangements, accomplishing the same purpose, may
measured as a fraction of the metered heat flow, will
be used. The effect on the heat transfer through the
increase with decreasing metered area.
specimen of the box walls in figure 1 and of the frame
Imbalance heat flow, Q>,, in the guarded hot box is due
in figure2 depends upon the wall or frame shape and
to nonuniformities both in surface coefficients and air
material, upon the specimen thickness and resistance
temperatures close to the periphery of the metered
and such test conditions as temperature differences
area.
and air velocities. The apparatus design and con-
struction should be made compatible with the ex-
An amount of heat enters the specimen through the
pected types of specimen to be tested and expected
nose of the metering box in the guarded hot box.
testing conditions.
Deviation from one-dimensional heat flow is caused
by the finite thickness of the nose seal.
Both edge insulation and edge boundary conditions
2.2 Design requirements
affect peripheral losses, c&, for the guarded hot box
and flanking losses, Q4, for the calibrated hot box.
The size of the apparatus shall be commensurate with
the intended use, taking the following points into
All these problems are made more complex by non-
...

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