RESEARCH
Plate thermometers for
measuring heat transfer
Heat transfer is an important process in the spread of fire. A BRANZ research project
has looked at the use of plate thermometers for measuring heat transfer accurately.
By Amanda Robbins, BRANZ Senior Fire Research Engineer
10
Incident Radiative Heat Flux (kW/m2)
R
adiative heat transfer is important in fire spread and heat
damage. Complex processes are involved, so fire experiments
that measure heat transfer are useful. It is fundamental, however,
to determine the limitations and appropriate applications of the
instruments used in these experiments.
Heat transfer can occur by radiation, convection, conduction or a
combination of these, so when setting out to measure heat flux (the rate of
energy transferred to a surface), the type of heat transfer first needs to be
identified along with the range of instruments that might be used. In general,
it is good practice to use as many measurement devices and collect as much
data as possible to produce the most complete picture of the experiment
and to cross-check measurement results. The top two limiting factors for
selection of instruments are cost (to be as low as possible) and operating
conditions (including flames, excessive heat and soot).
9
Plate
Target
8
7
6
5
4
3
2
1
0
100
200
300
400
500
600
Plate thermometers are designed to be relatively inexpensive, robust
and simple pieces of equipment for measuring rate of radiative heat
transfer per unit area. Their construction is described in ISO 834-1:
1999 Fire-resistance tests – elements of building construction – part 1:
general requirements and EN 1363-1:1999 Fire resistance tests. General
requirements. Plate thermometers consist of a metal plate where one side
is exposed to the environment to be measured and the other is insulated.
The temperature of the back of the metal plate is recorded and used to
estimate the radiative heat flux received by the front of the plate, assuming
uniform conditions of the metal plate and the backing insulation.
Investigating the use of plate thermometers
66 BUILD August/September 2009
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Figure 1: An example of the response of a plate thermometer exposed to a body of constant
temperature for a target heat flux of 9 kW/m² and then removed.
Plate thermometers are useful instruments
A project was undertaken by BRANZ to investigate the use of plate
thermometers for the measurement of heat flux during fire experiments.
The aims of this study were to determine the influence of the assumptions
used to calculate the heat flux based on the temperatures recorded using
the plate thermometers and the environmental and fire condition limits of
the use of the instrument in fires.
Three series of tests were performed. The plate thermometers were
initially subjected to a variable heat source that could be held at a constant
temperature, using a cone calorimeter (Figure 1). This allowed calibration
to be performed and the response of the instrument to a known constant
source of heat to be recorded.
For the second series of tests, a gas burner was used as the fire source
(see Figure 2). A burner is more easily controlled compared to other
700
Time (s)
Figure 2: An example of arrays of plate thermometers and a gas burner.
methods of producing a flame. Different heat release rates were used,
some constant and some increasing and then decreasing at different rates.
The plate thermometers were placed at different heights from the floor and
distances from the centre of the flame. Two conditions were considered:
burning in open conditions under a collection hood for the smoke, and
burning in a concrete room of a standard size with the collection hood
located to collect the smoke coming out of the open doorway (consistent
with ISO 9705:1993 Fire tests – full-scale room test for surface products).
The third series of tests involved burning a mattress. This provided
realistic changes and variations in the rates at which heat was released
during the tests. The mattresses also produced more smoke compared to
the gas burner. The two conditions used for the burner set-up were also
used for the mattress tests.
Results compared and analysed
Results calculated from the plate thermometer temperatures were compared
to results from another type of heat flux measurement device (watercooled Gardon gauges that are more expensive and less robust than plate
thermometers). The influence of the parameters used for calculating the
heat fluxes from the plate thermometer temperatures was analysed. For the
range of conditions tested, these comparisons and analyses indicated:
❚❚ the shape of the plate thermometer response when subjected to a
body of constant temperature is an initial rise that asymptotes to an
approximately steady measurement
❚❚ good agreement with Gardon gauge results for steady conditions, after
the plate thermometer initial rise to steady values
❚❚ a slower response time for the plate thermometers compared to the
Gardon gauges
❚❚ the dominant parameter influencing the plate thermometer initial
response is the energy storage term and for the steady period is the
balance of the radiative, convective and conductive heat transfer into
and out of the simplified system
❚❚ the plate thermometer results are influenced by the method used to
attach the thermocouple to the back of the metal plate
❚❚ the plate thermometer results are influenced by the local conditions
that the equipment is subjected to, e.g. whether it is in the hot layer of
a fire room or in the cooler gas at a lower height.
In summary, the current simple approach for analysing plate thermometer
data is appropriate for scenarios where the fire is expected to be
approximately constant or to change smoothly and relatively slowly. It is
recommended that the equipment-specific design and local conditions
during use are included in the analysis of all results. This improved
understanding is important for improving confidence in the results from
experiments designed to test products and improve understanding of
fire behaviour.
Future work
It was determined that this approach is not appropriate for use where the fire
conditions change rapidly. An alternative analysis of plate thermometer data
or measurement approach needs to be developed to provide a more accurate
estimate of the measured heat flux for the more variable conditions.
This research was funded by BRANZ from the Building Research Levy.
The whole report is published as a BRANZ Study Report, available at
http://www.branz.co.nz/cms_show_download.php?id=1065.
BUILD August/September 2009 67