ISO 14934-3:2006
(Main)Fire tests — Calibration and use of heat flux meters — Part 3: Secondary calibration method
Fire tests — Calibration and use of heat flux meters — Part 3: Secondary calibration method
ISO 14934-3:2006 specifies a method for the calibration of heat-flux meters for use in fire testing, gives information on the accuracy of the calibration, gives guidance on the calibration and the care of heat-flux meters. ISO 14934-3:2006 applies only to instruments having plane sensing surfaces. ISO 14934-3:2006 does not apply to sensing surfaces in the form of wires, spheres, etc.
Essais au feu — Étalonnage et utilisation des appareils de mesure du flux thermique — Partie 3: Méthode d'étalonnage secondaire
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 14934-3
First edition
2006-09-15
Fire tests — Calibration and use of heat
flux meters —
Part 3:
Secondary calibration method
Essais au feu — Étalonnage et utilisation des appareils de mesure du
flux thermique —
Partie 3: Méthode d'étalonnage secondaire
Reference number
ISO 14934-3:2006(E)
©
ISO 2006
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ISO 14934-3:2006(E)
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ISO 14934-3:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Principle. 1
5 Apparatus . 2
5.1 Radiant source . 2
5.2 Mounting arrangements. 2
5.3 Instrumentation. 2
5.4 Additional equipment . 3
6 Test environment. 3
6.1 Room. 3
6.2 Draught . 3
7 Setting up procedure. 3
7.1 General. 3
7.2 Mounting and alignment of heat flux meters . 3
8 Typical calibration procedures. 4
9 Calculation of results . 4
9.1 General. 4
9.2 Graphical presentation. 5
9.3 Mathematical presentation . 5
9.4 Uncertainty in regression curve. 5
10 Test report . 6
Annex A (informative) Accuracy of calibration . 7
Annex B (informative) Care of heat flux meters . 8
Annex C (informative) Guidance notes . 9
Annex D (informative) Procedure recommended for maintenance of a secondary standard of
irradiance at a test laboratory . 11
Bibliography . 14
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ISO 14934-3:2006(E)
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 14934-3 was prepared by Technical Committee ISO/TC 92, Fire safety, Subcommittee SC 1, Fire
initiation and growth.
ISO 14934 consists of the following parts, under the general title Fire tests — Calibration and use of heat flux
meters:
— Part 1: General principles [Technical Specification]
— Part 2: Primary calibration methods
— Part 3: Secondary calibration method
— Part 4: Guidance on the use of heat-flux meters in fire tests [Technical Specification]
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ISO 14934-3:2006(E)
Introduction
In many fire test methods, the radiation level is specified and, therefore, it is of great importance that the
radiant heat flux is well defined and measured with sufficient accuracy. Radiant heat transfer is also the
dominant mode of heat transfer in most real fires.
In practice, radiant heat flux is usually measured with total heat flux meters of the Schmidt-Boelter
(thermopile) or Gardon (foil) type. Such meters register the combined heat flux by radiation and convection to
a cooled surface. The contribution to the heat transfer by convection depends mainly on the temperature
difference between the surrounding gases and the sensing surface, and on the velocity of the surrounding
gases. It, however, also depends on the size and shape of the heat flux meter, its orientation and on its
temperature level, which is near the cooling water temperature. In many practical situations in fire testing, the
contribution due to convection to the sensing surface of the instrument can amount to 25 % of the radiant heat
flux. Therefore, it is always necessary to determine and control this part.
To determine the fraction of total heat flux due to radiation, a calibration scheme has been developed where
primary calibration is performed on two different types of heat flux meters: (1) a total hemispherical radiometer
sensitive to radiation only, and (2) a total heat flux meter (most frequently used) sensitive to both radiant heat
transfer and to convective heat transfer. A comparison of measurements between the two types of meters in
secondary (or transfer) calibration methods allows a characterization of the influence of convection in the
method. Where possible, in calibrations and in measurements of radiative heat flux, it is advisable that the
uncertainty calculations include the uncertainty associated with removing the convective component. For
secondary calibration methods, a combined use of hemispherical radiometers and total heat flux meters
makes it possible to estimate the convection contribution. The same arrangement can be used in the
calibration of fire-test methods as well.
This part of ISO 14934 describes a method for the calibration of total heat flux meters used in fire testing. A
number of fire tests described in International Standards published by ISO require test specimens to be
exposed to specified levels of irradiance. It is, therefore, necessary for fire test laboratories to be able to
maintain working-standard heat flux meters to measure irradiance.
This part of ISO 14934 describes a method for the calibration of heat flux meters for use as working standards
by comparison with a heat flux meter of known sensitivity, referred to as a secondary standard. The latter will
have been calibrated by reference to a defined primary standard of irradiance from a close-to-black source.
The calibration of heat flux meters for use as primary and secondary standards requires considerable
expertise and equipment that is not covered by this part of ISO 14934. For information on the calibration of
primary standards and for a detailed account of the principles of the measurement of thermal radiation,
reference is also made to ISO 14934-2.
Information on the accuracy of calibration, care of heat flux meters and guidance notes for carrying out the
calibration are given in Annexes A to C. Annex D outlines a suitable procedure for the maintenance of a
secondary standard of irradiance at a test laboratory.
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INTERNATIONAL STANDARD ISO 14934-3:2006(E)
Fire tests — Calibration and use of heat flux meters —
Part 3:
Secondary calibration method
1 Scope
This part of ISO 14934 specifies a method for the calibration of heat flux meters for use in fire testing.
This method applies only to instruments having plane sensing surfaces. It does not apply to sensing surfaces
in the form of wires, spheres, etc.
Annex A gives information on the accuracy of the calibration. It is intended that reference be made to the
International Standard describing the test for which the heat flux meter is intended. Annex B gives guidance
on the care of heat flux meters. Annex C gives guidance notes concerning calibration.
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 13943:2000, Fire safety — Vocabulary
ISO 14934-2, Fire tests — Calibration and use of heat flux meters — Part 2: Primary calibration methods
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14934-2 and ISO 13943:2000 apply.
4 Principle
Calibration of heat flux meters (total hemispherical radiometers and total heat flux meters) for use as working
standards is carried out by comparing heat flux meter response at various levels of irradiance with the
response of a secondary-standard heat flux meter of the same type at the same levels of irradiance. The
measurements are made at different levels of irradiance, which is obtained by varying the distance between
the radiant source and the heat flux meter or by varying the temperature of the radiant source. The secondary-
standard heat flux meter shall be calibrated according to one of the primary methods described in
ISO 14934-2.
Working standard heat flux meters may also be calibrated directly in any of the primary calibration furnaces
described in ISO 14934-2. In this case, it is not necessary to follow the calibration procedure described in this
part of ISO 14934.
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ISO 14934-3:2006(E)
5 Apparatus
5.1 Radiant source
The radiant source can be spherical, flat or conical. It shall be an electrically powered heater. The irradiance
from the radiant source shall be maintained at a preset level by controlling the temperature. This can be done
as with the heater described in ISO 5660-1. The temperature shall be kept constant during the calibration. The
source shall be larger than the measuring surface of the heat flux meters under calibration.
The radiant source shall be placed in such a way that the irradiance from the radiant source is given vertically
downward to the heat flux meter to be calibrated, which is placed below the radiant source.
5.2 Mounting arrangements
The mounting apparatus shall be designed to bring the sensing surface of each heat flux meter (working
standard and secondary standard) quickly in turn into a preset position opposite the centre of the radiant
source in such a way that the irradiance to the heat flux meter can be varied. This can be achieved, for
example, by a system where the position of a heat flux meter is fixed and adjusting the output from the radiant
source (for example the conical heater of ISO 5660-1, ISO 13927 and ISO 17554) or a temperature-controlled
flat-faced radiant source with a mounting system of heat flux meter, which fixes the position of a heat flux
meter.
Irradiance to the heat flux meter can be changed by changing the distance between the radiant source and
the heat flux meter or by changing the power (i.e. temperature) of the radiant source.
The movement of, or a change in, radiant source power shall be over a range that provides an appropriate
range of irradiance. The heat flux meter shall not be placed in any convective air flow initiated by the radiant
source. The means for locking the heat flux meter in position shall be rigid and such that the centre of the
sensing surface is on the normal from the centre of the radiator.
The heat flux meter mounting apparatus shall be designed so that meters are not mounted directly over a
substantial mass of material that gets hot when the radiant panel is running, and so that it can be placed in
position after the irradiance of radiant source reaches a preset level.
The mounting apparatus shall incorporate a means to support two (or more) heat flux meters that can differ in
size, and mounting arrangements shall be designed so that the heat flux meters can be positioned with their
sensing surfaces in the same vertical plane parallel to the face of the radiant source. No part of the mounting
apparatus shall project in front of the heat flux meters being tested.
All exposed surfaces of the heat flux meter mounting apparatus shall be coated with a heat-resisting, matt,
black finish.
NOTE Where the secondary-standard and working-standard heat flux meters are not identical in shape, a special
positioning device can be needed.
5.3 Instrumentation
5.3.1 Secondary-standard heat flux meters
Three or more secondary-standard heat flux meters are required for periodic inter-comparisons to maintain a
reliable standard of irradiance. A suitable scheme for these inter-comparisons is outlined in Annex D.
5.3.2 Recording instrumentation
Instrumentation shall be capable of assimilating the incoming data and producing a record, both permanent
and immediately available to the operator, of the reading of each heat flux meter at intervals not longer than
5 s, having range settings appropriate to the outputs of the heat flux meters. It is recommended to use the
same recording instrumentation for both secondary and working heat flux meters.
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ISO 14934-3:2006(E)
5.4 Additional equipment
5.4.1 Protective clothing
Protective clothing, such as heat-resisting gloves and eye protection, should be worn, as necessary.
5.4.2 Low-pressure water supply
Low-pressure air and/or water supply for the heat flux meters should be supplied as required. An example is
given in Clause C.7.
6 Test environment
6.1 Room
The influence of the surroundings should be stable over time. The radiation from everything except for the
radiant source should be limited to 1 % of the radiant source radiation during the entire procedure.
6.2 Draught
The test apparatus shall be contained in an essentially draught-free environment, where the air flow does not
exceed 0,2 m/s when the apparatus is cold. Particular care shall be taken to avoid draught across the
instruments under test. If necessary, screens shall be provided, but these shall be at least 1,5 m away from
the heat flux meter under test.
7 Setting up procedure
7.1 General
Check that the apparatus is assembled correctly. Ensure that any mounting apparatus moves smoothly in
relation to the face of the radiant source. Lubricate any sliding parts using a heat-resisting grease or graphite,
if necessary.
Laboratories supplying instruments for calibration should be aware that with a new working-standard heat flux
meter or one that has not been previously used, it is advisable to age the sensing surface artificially before a
calibration is carried out to avoid, or reduce, initial drift in sensitivity. It is recommended that this be done by
exposing the sensing surface to radiation for 20 h to 25 h in a series of exposures of several hours' duration,
at an irradiance near the maximum at which it is likely to be used. With some types of heat flux meters, it is
advisable to monitor sensitivity and continue ageing until it has stabilized.
7.2 Mounting and alignment of heat flux meters
7.2.1 With the radiant source off, mount the secondary-standard heat flux meter and the working-standard
heat flux meter, or heat flux meters that are being calibrated, in the mounting apparatus. Ensure that all leads
and tubes to heat flux meters are protected against radiation (wrapping with thin, shiny aluminium foil has
been found suitable; see Clause C.8), and that they do not become entangled in any mechanism when it
moves. Connect the secondary-standard and working-standard heat flux meters independently to the
recording instrumentation using appropriate leads.
If a water-cooled heat flux meter is being tested, connect the appropriate supply and ensure that the flow rate
is in accordance with the manufacturer’s recommendations. The temperature of the cooling water shall be the
ambient room temperature and shall not be below the dew point of the ambient temperature.
7.2.2 Place the mounting apparatus into position for calibration. Adjust the heat flux meters in turn
horizontally, vertically and at an angle, so that when brought into the test position, their sensing surfaces lie in
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ISO 14934-3:2006(E)
the same plane relative to the radiant source, with the centres of the sensing surfaces on the normal from the
centre of the radiant source.
Ensure that the sensing surfaces of the heat flux meters are clean and that the line-of-sight between the
sensing surfaces and the radiator is not obstructed when in the test position. Place small screens in front of
the heat flux meters to shield them from the radiator.
As far as possible, the heat flux meters should be screened from radiation except during actual readings.
8 Typical calibration procedures
8.1 Switch on the recording instrumentation, allowing any necessary time for warming up.
8.2 With the heat flux meter mounting apparatus away from the calibration position, switch on the radiant
source and set the temperature to a preset level.
8.3 After the temperature of the radiant source reaches a preset level, place the radiation shield.
8.4 Place the heat flux meter mounting apparatus in preset position. Set the secondary-standard heat flux
meter at its central position on the mounting apparatus and lock it in this position.
8.5 Remove the radiation shield to expose the meter to the radiant source. Continue the exposure in this
position until the temperature of the radiant source reaches the preset level and the value recorded from the
heat flux meter is essentially constant over a period of 1 min. Note the heat flux meter output reading (F ).
1
Reinsert the radiation shield.
8.6 Move the working-standard heat flux meter to the position occupied by the secondary-standard heat flux
meter.
8.7 Remove the radiation shield to expose the meter to the radiant source. Continue the exposure in this
position until the temperature of the radiant source reaches the preset level and the value recorded from this
heat flux meter is essentially constant over a period of 1 min. Note the heat flux meter output (W). Reinsert the
radiation shield.
8.8 Return the secondary-standard heat flux meter to its central position and repeat 8.3, obtaining another
output reading (F ).
2
8.9 Repeat the measurements, alternating the working-standard and secondary-standard heat flux meters
until two successive measurements from the latter differ by less than 1 % (see Clause C.8).
8.10 Repeat 8.2 to 8.9 with the radiant source set at different power levels until at least ten different levels of
irradiance have been used (see Annex A). Ensure that the irradiance levels cover the required calibration
range of the working-standard heat flux meter and that they are evenly spread over that range.
It is advisable to start at a low irradiance and increase the power up to the maximum required, and then
decrease the power (covering the same power levels) so that outputs are obtained from increasing irradiance
and decreasing irradiance.
9 Calculation of results
9.1 General
Derive the irradiance corresponding to an output of W from the working-standard heat flux meter from the
arithmetic means of those two consecutive values of F bracketing this reading (see 8.8) and the calibration
data available for the secondary-standard heat flux meter.
Present the results graphically (see 9.2) and mathematically (see 9.3).
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ISO 14934-3:2006(E)
9.2 Graphical presentation
Tabulate the irradiance and the output of the working-standard heat flux meter at each irradiance level used
during the calibration.
Construct a graph of irradiance as ordinate against output voltage from the working-standard heat flux meter
as abscissa.
The graph now represents the calibration of the working-standard heat flux meter with reference to the
secondary-standard heat flux meter used.
9.3 Mathematical presentation
Express the calibration in the form of a regression equation of irradiance on output readings, expressed in
millivolts. For many instruments, a linear regression equation is suitable but some instruments require second-
order regression techniques to express adequately the relationship between output and irradiance. The
evaluation of the uncertainty in the regression curve should be performed according to 9.4.
Before subjecting the data to statistical or other analysis, examine them for outlying observations; readings
occasionally arise that appear to be substantially out of line with the main body of the readings and that can
affect substantially an overall calibration obtained, for example, by regression techniques.
If such readings are noticed at the time of the laboratory work, repeat the readings and discard any that are
not confirmed.
Discard outlying readings where a physical explanation exists for questioning their validity, for example, where
some disturbance is known to have occurred.
NOTE Where no physical reason exists for suspecting the accuracy of an outlier, an objective statistical test can be
required to decide whether to retain or exclude it.
9.4 Uncertainty in regression curve
The number of radiation levels used for calibration influences the uncertainty in the regression of the
2
ˆ
calibration. When a linear regression has been performed, then the standard deviation, s , is calculated
either by means of the computer program used for the regression or from Equation (1):
m
1
22
sˆˆ=−()yy (1)
∑ii
ν
i=1
where
ν is the number of degrees of freedom;
m is the number of radiation levels;
yˆ is the value from the model;
i
y is the measured value for the level x .
i i
The number of degrees of freedom, ν, equals number of radiation levels minus the number of parameters that
are set in the regression, i.e. 2 for linear regression. The uncertainty for the regression curve is then
calculated as the standard deviation times a coverage factor in order to get a 95 % confidence interval. If the
degree of freedom is small, i.e. less than 10, then this shall be taken from the t-distribution.
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ISO 14934-3:2006(E)
10 Test report
The test report should contain the following information:
a) reference to the source of calibration of the secondary-standard heat flux meter and the date of its last
calibration;
b) source temperature to the nearest 50 °C;
c) identification of the calibrated working-standard heat flux meter (e.g. manufacturer, model, serial number);
d) irradiance range employed (use the sentence: “This heat flux meter has been calibrated in the range from
2
x to y kW/m ”);
e) date of calibration;
f) results as detailed in Clause 10;
g) instrument temperature;
h) date and identification number of the report;
i) name and address of calibrating laboratory;
j) name and address of the client, if known;
k) name and type of heat flux meter under calibration;
l) calibration method;
m) identification of the calibration equipment used;
n) traceability of measurements;
o) any deviation from the calibration method;
p) uncertainty of the test results;
q) date and signature;
r) reference to this part of ISO 14934.
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ISO 14934-3:2006(E)
Annex A
(informative)
Accuracy of calibration
The accuracy of calibration of the working-standard heat flux meter depends on the accuracy of calibration of
the secondary-standard heat flux meter and also on the accuracy with which the inter-comparison can be
made. The latter depends both on the accuracy of positioning of the working-standard heat flux meter in
relation to the secondary-standard heat flux meter and on the statistical errors due to averaging, combining or
comparing sets of data that, because of random processes such as physical perturbations and reading errors,
exhibit some variation. Establishing confidence limits for the calibration of secondary-standard heat flux
meters is the subject of several ongoing investigations (HFCAL and FORUM). In the absence of new data, an
uncertainty of better than ± 6 % (95 % confidence limit) is a conservative estimate. The main error due to
imprecise positioning arises from errors in positioning with respect to the distance from the radiant source.
Because of the relatively large angle subtended by the source at the sensing surface for much of the
irradiance range applying, the error in irradiance is much smaller than can be expected from the inverse
square law (with a small source). For instruments of identical size and construction, it should not be difficult to
position the heat flux meters to this accuracy, and ± 0,5 % can be regarded as 95 % confidence limits. These
uncertainties apply to relative errors, but not absolute u
...
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