Thermal insulation - Determination of steady-state thermal transmission properties of thermal insulation for circular pipes (ISO 8497:1994)

Specifies apparatus performance requirements, but does not specify apparatus design. Applies to circular pipes, generally operating at temperatures above ambient. The type of specimen, temperatures and test conditions to which the standard applies are specified in the standard in detail.

Wärmeschutz - Bestimmung der Wärmetransporteigenschaften im stationären Zustand von Wärmedämmungen für Rohrleitungen (ISO 8497:1994)

Diese Internationale Norm beschreibt ein Verfahren zur Bestimmung der stationären Wärmedurchgangseigenschaften von Wärmedämmungen für Rohre, die im allgemeinen bei Temperaturen über der Umgebungstemperatur eingesetzt werden.Sie legt Anforderungen an die Leistungsparameter des Prüfgerätes, jedoch nichtdie Geräteausführung fest. In den Abschnitten 5 und 6 sind die Art des Probekörpers, die Temperaturen und Prüfbedingungen angegeben, in deren für die diese Internationale Norm gilt.

Isolation thermique - Détermination des propriétés relatives au transfert de chaleur en régime stationnaire dans les isolants thermiques pour conduites (ISO 8497:1994)

La présente Norme internationale prescrit une méthode de mesure des propriétés thermiques relatives au transfert de chaleur en régime stationnaire à travers des isolants pour conduites, pour des températures supérieures à la température ambiante. Elle normalise la méthode de mesure, y compris les modes opératoires et le fonctionnement de l'appareillage, mais elle ne normalise pas la conception de l'appareillage. Les types d'éprouvettes, les températures et les conditions d'essai auxquels s'applique la présente Norme internationale sont décrits en détail aux articles 5 et 6.

Toplotna izolacija - Določanje toplotne prevodnosti v stacionarnem stanju pri materialih za izolacijo okroglih cevi (ISO 8497:1994)

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SIST EN ISO 8497:1997
Thermal insulation - Determination of steady-state thermal transmission properties of
thermal insulation for circular pipes (ISO 8497:1994)
Wärmeschutz - Bestimmung der Wärmetransporteigenschaften im stationären Zustand
von Wärmedämmungen für Rohrleitungen (ISO 8497:1994)
Isolation thermique - Détermination des propriétés relatives au transfert de chaleur en
régime stationnaire dans les isolants thermiques pour conduites (ISO 8497:1994)
Ta slovenski standard je istoveten z: EN ISO 8497:1996
27.220 Rekuperacija toplote. Heat recovery. Thermal
Toplotna izolacija insulation
SIST EN ISO 8497:1997 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 8497:1997

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SIST EN ISO 8497:1997

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SIST EN ISO 8497:1997

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SIST EN ISO 8497:1997

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SIST EN ISO 8497:1997

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SIST EN ISO 8497:1997
First edition
Thermal insulation - Determination of
steady-state thermal transmission
properties of thermal insulation for circular
Isolation thermique - Dbtermina tion des propri&& relatives au trans fert
de chaleur en r6gime stationnaire dans les isolants thermiques pour
Reference number
ISO 8497:1994(E)

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SIST EN ISO 8497:1997
ISO 8497:1994(E)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Symbols and units
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General considerations
Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test specimens
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 End cap corrections
11 Calculations
12 Test precision and accuracy
13 Test report
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Bibliography
0 ISO 1994
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronie or mechanical, including photocopying and
microfilm, without Permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-l 211 Geneve 20 l Switzerland
Printed in Switzerland

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SIST EN ISO 8497:1997
ISO 8497:1994(E)
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. Esch 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.
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 ISO 8497 was prepared by Technical Committee
lSO/TC 163, Thermal insulation, Subcommittee SC 1, Test and measure-
ment methods.
Annex A of this International Standard is for information only.
. . .

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SIST EN ISO 8497:1997
ISO 8497: 1994(E)
The thermal transmission properties of pipe insulation generally have to
be determined using pipe test apparatus rather than flat specimen appa-
ratus such as the guarded hot plate or the heat flow meter apparatus, if
results are to be representative of end-use Performance. Insulation ma-
terial formed into flat sheets often has different internal geometry from
that of the same material formed into cylindrical shapes. Furthermore,
properties often depend significantly upon the direction of heat flow in
relation to inherent characteristics such as fibre planes or elongated cells:
thus flat specimen one-dimensional heat flow measurements may not
necessarily be representative of the two-dimensional radial heat flow en-
countered in pipe insulation.
Another consideration is that commercial insulations for pipes are often
made with the inside diameter slightly larger than the outside diameter
of the Pipe, otherwise manufacturing tolerantes may result in an imperfect
fit on the Pipe, thus creating an air gap of variable thickness. In those
cases where end-use Performance data rather than material properties are
to be determined, the insulation is mounted on the test pipe in the same
loose manner so that the effect of the air gap will be included in the
measurements. This would not be the case if properties were determined
in a flat plate apparatus where good plate contact is required.
Still another consideration is that natura1 convection currents around insu-
lation installed on a pipe will Cause non-uniform surface temperatures.
Such conditions will not be duplicated in a flat plate apparatus with uniform
plate temperatures.
NOTE 1 Comparison tests on apparently similar material using both pipe appa-
ratus and flat plate apparatus have shown varying degrees of agreement of
measured thermal transmission properties. lt appears that better agreement is of-
ten obtained for heavier density products which tend to be more uniform, homo-
geneous and sometimes more isotropic. For those materials which have
repeatedly shown acceptable agreement in such comparisons, the use of data
from flat plate apparatus to characterize pipe insulation may be justified. As a
general rule, when such agreement has not been shown, the pipe test apparatus
shall be used to obtain thermal transmission data for pipe insulations.

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SIST EN ISO 8497:1997
Thermal insulation - Determination of steady-state
thermal transmission properties of thermal insulation
for circular pipes
of steady-state thermal resistance and related prop-
1 Scope
- Heat flow meter appara tus.
This International Standard specifies a method for the
ISO 8302:1991, Thermal insulation - Determination
determination of steady-state thermal transmission
of steady-sta te thermal resistance and rela ted prop-
properties of thermal insulations for circular pipes
erties - Guarded hot plate apparatus.
generally operating at temperatures above ambient.
lt specifies apparatus Performance requirements, but
it does not specify apparatus design.
3 Definitions
The type of specimen, temperatures and test condi-
NOTE 2 The geometry of pipe insulation requires special
tions to which this International Standard applies are
terms not applicable to flat specimens. The word “linear”
specified in clauses 5 and 6.
is used to denote properties based upon a unit length (in the
pipe axis direction) of a specified insulation size. These lin-
ear properties, identified by the subscript “1 ”, are con-
2 Normative references
venient since the total heat loss tan then be calculated
knowing the pipe length and the applicable temperature.
The following Standards contain provisions which,
through reference in this text, constitute provisions
“Linear” does not denote heat flow in the axial direction. In
of this International Standard. At the time of publica- this International Standard, the direction of heat flow is
predominantly radial.
tion, the editions indicated were valid. All Standards
are subject to revision, and Parties to agreements
For the purposes of this International Standard, the
based on this International Standard are encouraged
following definitions apply. The definitions and sym-
to investigate the possibility of applying the most re-
bols given in the following clauses are based upon
cent editions of the Standards indicated below.
those in ISO 7345 except for the linear thermal
Members of IEC and ISO maintain registers of cur-
transference (3.1).
rently valid International Standards.
3.1 linear thermal transference, K,: Linear density
ISO 7345:1987, Thermal insulation - Physical quan-
of heat flow rate divided by the temperature differ-
tities and de finitions.
ence between the pipe surface and the ambient air in
ISO 8301: 1991, Thermal insulation - Determination
the steady-state condition. lt relates to a specific in-

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SIST EN ISO 8497:1997
(6 ISO
ISO 8497: 1994(E)
sulation size and is a measure of the heat transferred
3.6 thermal resistivity, r: Reciprocal of the thermal
through the insulation to the ambient atmosphere.
conductivity, ;1, for a homogeneous material in the
steady-state condition.
K, = -
. . .
To - Ta WTO - Tz) 1
r . . .
= # In(D,lD,) = T
3.2 linear thermal resistance, R,: Temperature dif-
ference between the pipe surface and the insulation
3.7 areal thermal resistance, R: Temperature dif-
outer surface divided by the linear density of heat flow
ference between the pipe surface and the insulation
rate in the steady-state condition. lt relates to a spe-
outer surface divided by the areal density of heat flow
cific insulation size and is the reciprocal of the pipe
rate in the steady-state condition. lt is the reciprocal
linear thermal conductance, A,.
of the areal thermal conductance, A.
To - Tz 1
=-=- To - Tz 1
. . .
RI R . . .
Al A
where the surface of area A must be specified (usually
the pipe surface, sometimes the insulation outer sur-
3.3 linear thermal conductance, A,: Reciprocal of
face, or other as Chosen; see note 6 in 3.8).
the linear thermal resistance, R,, from the pipe surface
to the insulation outer surface. lt relates to a specific
NOTE 5 The more common “areal” properties, based
insulation size.
upon unit area, are often confusing when applied to pipe
insulation since the area must be Chosen arbitrarily and may
1 w'
range from that of the pipe surface to that of the insulation
n, =- . . .
outer surface. If these areal properties are computed, the
RI To - Tz
area and its location used in the computation must be re-
3.4 surface coefficient of heat transfer, 4: Areal
density of heat flow rate at the surface in the steady-
3.8 areal thermal conductance, A: Reciprocal of
state condition divided by the temperature differente
the areal thermal resistance, R.
between the surface and the surrounding ambient air.
For pipe insulation geometry the following relation
. . .
applies. A n rF m (8)
10 - 12
. . .
= xD2L(T2 - Ta)
where the location of the surface of area A must be
specified (usually the pipe surface, sometimes the in-
sulation outer surface, or other as Chosen).
NOTE 6 The value of A, the areal thermal conductance,
3.5 thermal conductivity, ;1: Defined by the follow-
is arbitrary since it depends upon an arbitrary choice of the
ing relation specifically applicable to the pipe insu-
area, A. For a homogeneous material for which the thermal
lation geometry. lt applies to homogeneous material
conductivity is defined as in 3.5, the areal conductance, A,
in the steady-state condition and is the reciprocal of
is given by
the thermal resistivity, r.
n= . . .
0 Wz&)) 1
A 1 n(o,/QJ
A. . . l
= 27tL(To - Tz) = -F
If the area is specially Chosen to be the “log mean area ”,
NOTES equal to KL(D~ - Do)/In(D,/D,) then A = 21/(0, - D,). Since
(Dz - D,)/Z is equal to the insulation thickness measured
3 In ISO 7345, the thermal conductivity is also defined by
from the pipe surface, this is analogous to the relation be-
the more general relation 4 = - ;1 grad T.
tween conductance and conductivity for flat slab geometry.
Similar relations exist for the areal thermal resistance, R,
Since the pipe surface temperature, T,, is used, the
defined in 3.7. Since these areal coefficients are arbitrary
therm al conductivity will inclu de the effect of any gap that
and since the area used is often not stated, thus leading to
exists between the insulation and the pipe (sec 6.1).
possible confusion, it is recommended that they be used
only if specified.

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SIST EN ISO 8497:1997
ISO 8497: 1994(E)
4 Symbols and units
5 Requirements
5.1 Test specimens
For the purposes of this International Standard, the
Spetimens may be rigid,
semirigid or flexible
following Symbols and units apply. (See clause 3.)
(blanket-type), or may be loose-fill, suitably contained.
They may be homogeneous or nonhomogeneous,
isotropic or anisotropic, may include slits, joints or
metallic elements and may include jackets or other
coverings. Spetimens shall be uniform in size and
Symbol Unit
shape throughout their length (except for any inten-
@ W
heat flow rate
tional irregularities which occur weil within the me-
linear density of heat flow rate
tered test section) and shall be designed for use on
(heat flow rate per axial length)
pipes of the same size as the test apparatus available.
areal density of heat flow rate
Generally, specimens will have a circular outside
(heat flow rate per area of a sur-
face) W/m2
shape concentric with the bore; other outside shapes
temperature of pipe surface K are allowed but only thermal transference may then
be determined.
temperature of insulation outside
temperature of ambient air or gas K
5.2 Operating temperature
outer diameter of circular pipe m
The pipe may be operated at temperatures up to the
outer diameter of circular insu-
lation maximum Service temperature of the specimen or of
the materials used in constructing the apparatus. The
length of test section (in the axial
direction) m
lower limit of the pipe temperature is determined by
m2 the restriction that it be sufficiently greater than the
area of specified surface
temperature of the specimen outer surface to provide
linear thermal conductance
wm 0
the precision of measurement desired. Normally, the
linear thermal resistance
apparatus is operated in still air maintained at an am-
linear thermal transference
bient temperature of 15 “C to 35 “C, but this may be
thermal conductivity
extended to other temperatures, other gases and
thermal resistivity
m K)IW
other velocities. The outer specimen surface tem-
surface coefficient of heat transfer
perature may also be fixed by the use of a heated or
of insulation outer surface
cooled outer sheath or blanket or by the use of an
areal thermal conductance
additional layer of insulation. If a cold outer sheath or
areal thermal resistance R
jacket is used, Operation at low temperatures is
thickness of end cap beyond test possible provided that the pipe is maintained at a
pipe (in axial direction) s m
higher temperature.
factor for Nukiyama calculation
5.3 Pipe size and shape
The test pipe shall have a circular Cross-section.
5.4 Orientation
7 The subscript “1” is used to denote linear properties (per
it axial length).
The test pipe normally has a horizontal orientation.
Other orientations may be used but require special
8 The subscript “cyl” is added to the Symbols listed when
considerations because of possible convection effects
it is important to indicate that the properties were derived
both within and around the test pipe and the speci-
from measurements on a pipe apparatus.
9 When both “1” and “cyl” subscripts are used together,
they are written “1, cyl ”.
5.5 Types of apparatus
IO In ISO 7345, the linear density of heat flow rate and the
Two distinctly different types of pipe apparatus are
areal density of heat flow rate are given the Symbols qi and
covered: the guarded end and the calibrated or calcu-
q respectively. The more descriptive ratio Symbols given
here are used throughout this International Standard. lated end types, which differ in the treatment of axial

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SIST EN ISO 8497:1997
ISO 8497: 1994(E)
heat transfer at the ends of the test section. Speci-
6.2 Application to other sizes
mens incorporating elements of high axial conduc-
tance, such as metallic jackets, shall be tested only It is impractical to provide test apparatus to match the
on the guarded end type of apparatus. size of all pipe insulations manufactured. Thus it is
necessary to calculate the properties for other sizes
from data measured on a limited number of sizes of
5.6 Relevant properties
similar insulation. Procedures may differ depending
on whether the specimen material and the test con-
The linear thermal transference (defined in 3.1) tan
ditions are ideal or non-ideal.
be calculated for all specimens and is the
most useful in quantifying pipe insulation perform-
Where end-use Performance is measured including
ante. Knowing its value and the mean temperatures
any air gap and/or imperfect fit, it is not permissible
of the pipe and of the surrounding ambient air, the
to calculate the properties for other sizes.
heat loss for a given length of insulated pipe tan be
directly calculated provided that the conditions in use
6.2.1 Ideal materials and conditions
are comparable with those of testing.
For materials that are homogeneous with thermal
The thermal conductivity (see 3.5) is often used in
conductivity either constant or a linear function of
specifications. In theory, it tan be calculated only for
temperature and which are tested under uniform
homogeneous specimens of concentric circular shape
temperature conditions, it is possible to determine the
which fit the test pipe tightly with no air gaps. In
thermal conductivity from a Single test at a specific
practice, it is often necessary to deviate from ideal
mean temperature using the relationship given in
conditions if errors introduced are judged to be ac-
3.5. This thermal conductivity may then be used to
ceptable. The thermal conductivity is useful in deriving
calculate the heat flow rate and other thermal trans-
the linear transference or other properties for insu-
mission properties for other sizes of pipes, other
lation sizes different than that used for the measure-
thicknesses of insulation, and other temperature dif-
ment (see 6.2). The other properties defined in
ferences for the same material operating at the same
clause 3 may be used when specified and appropri-
mean temperature.
6.2.2 Non-ideal materials and conditions
6 General considerations
In practice, many materials are not strictly homo-
geneous because
6.1 Objectives
- their thermal conductivity is a complex function of
Two distinctly different objectives may be addressed
as specified in 6.1 .l and 6.1.2. The specimen prepa-
ration and mounting depend on the choice of objec- - during measurements the outer surface of the
tive made by the User. Procedures aimed at either specimen is not at a uniform temperature due to
objective may be used and shall be fully reported. heat transfer by convection and radiation; and/or
- an air gap tan exist between the apparatus and the
6.1 .l End-use Performance
perfo rmance data are de sired,
If end-use the speci-
A critical evaluation of the practical impact of th
remai n unalte
men shall lred and be moun ted in the
factors shall be made whenever data is to be ex-
same manner as would occur in normal application. In
tended to sizes and conditions other than those of the
this case, the measured properties include the effects
of any joints or slits, and of the resistance of any air
gaps created by a loose fit on the Pipe.
Generally, measurements shall be made for a partic-
ular product or material on a minimum of two pipe
sizes approximating to the range of interest. If the
6.1.2 Material properties
values of thermal conductivity from those measure-
If values of material properties are desired, the speci- ments agree among themselves within acceptable
men shall be Chosen or altered so that all pieces fit limits, then their average may be used for calculation
tightly together without open joints or slits and so that of other thermal transmission properties for other
the specimen fits the test pipe tightly without an air
sizes in the range and for other conditions, for the
products and mean temperature of the tests. If the

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SIST EN ISO 8497:1997
ISO 8497:1994(E)
measured thermal conductivities do not agree within
7 Apparatus
acceptable limits, then suitable trend analyses shall
be employed to determine the appropriate values of
7.1 General requirements
thermal conductivity pertinent to the sizes for which
thermal transmission properties are to be obtained. If
The apparatus shall consist of the heated test pipe
the measured thermal conductivities differ widely,
and instrumentation for controlling and measuring the
then tests on additional sizes should be conducted.
pipe and ambient air temperatures, and the mean
An alternative procedure is to interpolate between
power dissipated in the test section heater. Instru-
values of a measured transmission property (for ex-
mentation for the measurement of the insulation
ample, thermal transference) from measurements
outer surface temperature shall also be included, un-
taken on different pipe sizes but on the same thick-
less only thermal transference is to be determined.
ness of insulation and at the same temperatures.
The pipe shall be uniformly heated by an internal
electric heater such as an electrical resistance winding
on a separate internal Pipe. In large apparatus it tan
be necessary to provide internal circulating fans or to
6.3 Required knowledge
fill the pipe with a heat transfer liquid to achieve uni-
form temperatures. Axial heat flow at the ends of the
Since it is impractical to include all details relating to
test section shall either be minimized by the use of
the wide range of types of apparatus and procedures
separately heated guard sections (see 7.3 and
covered by this International Standard, users shall
figure 1) or by the use of insulated end taps and cor-
have appropriate Prior knowledge and experience in
rections applied to the measured quantity of heat (see
thermal measurements.
7.4 and figure2). An enclosure or room equipped to
control the temperature of the air surrounding the
apparatus shall also be provided.
The apparatus shall conform to the principles and
6.4 Detailed instructions
limitations set in this International Standard, but it is
not intended in this method to include detailed re-
Users shall prepare detailed construction and operat-
ing instructions to aid builders and Operators of spe- quirements for the construction or Operation of any
cific apparatus to meet general requirements and particular apparatus. Such detailed instructions shall
objectives. be prepared specifically for each apparatus.
a) b)
a) Test pipe guard
1 Monitoring thermocouples across gap of heater pipe
b) Test pipe centre
2 Guard measuring thermocouple
c) Heater pipe guard
3 Monitoring thermocouples across gap of test pipe
c) Heater pipe centre
4 Centre measuring thermocouple
Figure 1 - Guarded end apparatus

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SIST EN ISO 8497:1997
ISO 8497:1994(E)
homogeneous, are only moderately anisotropic and are of
7.2 Dimensions
a thickness not greater than the pipe diameter. Longer
guard sections may be required when testing thicker speci-
No restriction is placed on the apparatus pipe diam-
mens or with specimens of high axial conductance.
eter, but the length of the test section shall be suffi-
cient to ensure that the total measured heat flow is
A gap, normally not more than 4 mm in width, shall
large enough, compared to end losses and to the ac-
be provided between the guards and the test section,
curacy of the power measurement, to achieve the
and between each guard section if double-guarded, in
desired test accuracy.
both the heater pipe and the test pipe (except for
small bridges if needed for structural support). These
NOTE 11 For a guarded end apparatus (see 7.3) of
gaps may be filled with material the thermal conduc-
88,9 mm outside diameter, a test section length of 0,6 m,
tivity of which is much lower than that of the Pipe.
with a total specimen length of approximately 1 m has
proven satisfactory. Calibrated or calculated end apparatus
Internal barriers shall be installed at each gap to mini-
(see 7.4) of similar diameter usually suit specimen lengths
mize convection and radiation heat transfer between
of 2 m or more. These lengths tan be unsatisfactory for
sections. Thermocouples, connected as differential
some sizes of apparatus and for some test conditions, and
estimates of the required length need to be made from ap- thermopiles, shall be installed in the test pipe on both
propriate error analysis. sides of each gap, and not more than 25 mm from the
gap, for the purpose of monitoring the temperature
As a convenience the apparatus should be constructed to
differential across each gap. Thermocouples shall also
accept an integral number of stan dard lengths of insulation.
be installed on any heater pipes or support members
which provide a highly conductive path from test
7.3 Guarded end apparatus
section to guard sections.
The guarded end apparatus (see figure 1) uses sepa-
7.4 Calibrated or calculated end apparatus
rately heated pipe sections, called “guards ”, located
at each end of the metered test section which are
The calibrated or calculated end apparatus (see
maintained at the test section temperature to elimi-
figure2) uses insulated taps at each end of the test
nate axial heat flow in the apparatus, and to aid in
section to minimize axial heat flow. Corrections for
achieving uniform temperatures so that all heat flow
the end cap loss shall be determined either by direct
in the specimen test section will be in the radial di-
calibration under the test conditions (the calibrated
rection. Both test and guard section heaters shall be
end apparatus) or by calculation using material prop-
designed to achieve uniform temperatures over their
erties (the calculated end apparatus). Internal electric
lengths unless it has been shown that the expected
heaters shall be designed to heat the test section
deviation from temperature uniformity does not Cause
uniformly over its length. If supplementary end heat-
unacceptable errors in test results. Auxiliaty heaters
ers are used within the test section length, the power
at the outside ends of Single guards or a second
to such heaters shall be included in the measured test
guard, at each end, shall be used if required. The
section power.
length of each guard section (or the combined length
of double guards) shall be sufficient to limit, at each
7.4.1 Callibrated end taps and calibrator pipe
end of the test section, the combined axial heat flow
in both apparatus and specimen to an acceptably
For the calibrated end apparatus, the end taps shall
small amount compared to the test section measured
have the same Cross-sectional area as the test speci-
heat transfer rate.
men and shall have approximately the same thermal
transfer properties. Esch end cap shall have a cavity
of minimum depth equal to one half the test pipe di-
ameter and of a size to acc

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