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Contactless measurement of charge temperature in
heating furnaces using digital photopyrometry
Kamila Halaczkiewicz, Marian Kieloch, Agnieszka
Klos
Introduction
Determination of the form of temperature character
istics
Temperature is a basic parameter of engineering
processes, during which heat exchange occurs
between the environment the material under
consideration. The exact knowledge and control of
the value of metal surface temperature during the
heating-up process provides the capability to
carry out the process properly. The surface
temperature of charge being heated has a direct
effect on heat consumption, steel loss for scale,
CO2 emission, and scale adhesion to the
substrate. The correct progress of the
technological process is made possible by
selecting the appropriate heating technology. The
selection of the appropriate heating technology
allows the proper progress and quality of the
technological process being conducted. The final
result of a correctly completed process is a
charge with permissible temperature differences
on its cross-section. It can be stated with
certainty that the outcome of heating furnace
operation is determined by the heating
technology, and for a given technology - by the
capacity. The technology is understood as the
variation of heated charge surface temperature in
time. In actual heating processes, it is the
furnace chamber temperature that is regarded as
the temperature of the charge being heated. The
furnace chamber temperature is measured using a
thermocouple. It is a basic parameter that
controls the operation of the furnace. It defines
also the value of the flux of heat taken up by
the charge. For this parameter is considered to
be credible, there should be a precise
relationship between oven temperature and the
actual surface temperature of heating charge.
However, the furnace temperature also depends on
many factors, including gas temperature, heat
transfer conditions, chamber geometry, etc.
Therefore, the temperature of the furnace
chamber, although it is a fundamental parameter
determining the heating processes is not
sufficiently objective for the proper control of
the process. This is the reason for unceasing
theoretical analyses and laboratory tests on the
possibility of taking a direct measurement of
charge temperature during the charge heating
process. Using contact methods for the direct
measurement of charge temperature during the
heating process seems to be infeasible. This is
so because of the need for a measuring device to
come into direct contact with the material, whose
temperature is to be measured. It is therefore of
purpose to search for the solution among the
contactless methods. Often, a thermovision camera
is used for this purpose. The principle of
thermovision relies on the recording of the
thermal radiation of material being examined in
the infrared radiation range. Electromagnetic
radiation of wavelengths between visible light
and radio waves is recorded. The advantage of
thermovision is that it enables recording to be
done at a temperature as low as several
kelvins. A major limitation on the use of
thermovision is the high price of the device (a
thermovision camera). Thermovision measurements
may also be burdened with a measurement error
resulting from an incorrectly selected
emissivity. A method that could become an
alternative to thermovision in the future is
digital photopyrometry. Laboratory tests are
being currently conducted into the possibility of
using digital photopyrometry for measuring the
temperature of heated steel charge. The digital
photopyrometry is also a contactless, partially
automated method which is aided by a suitable
computer program. However, its principle of
operation relies on the recording of the image of
material being heated with a still digital
camera. Recording of the image is done within the
wavelength range of visible light.
The effect of temperature on the greyscale level
of the digital photograph The effect of
temperature on the greyscale level of the digital
photo is illustrated in Figure 3. This dependence
was determined for three exposure time settings,
namely S1/60s, S1/125s and S1/250s.
Fig. 3. Effect of temperature on the greyscale
level of a digital photo for carbon steel ISO
80, A4 It has been established that the effect
of temperature on the greyscale level can be
described with the following equation
where a, b, c, d constants, r greyscale
level, t temperature, ?C.
On the basis of measurement results, e.g. for
S1/60s, the following relationship is obtained
The calculation of the value of temperature from
the developed relationship is difficult.
Therefore, relationship (1) is represented in the
following form
The effect of the greyscale level of the object
examined on the value of temperature is
represented graphically in Fig. 4.
Fig. 4. Effect of the exposure time on the
behaviour of the relationship of t f(r) ISO
80, A 4 The values of the constants in
relationship (3) have been established by using
the results of measurements carried out for
developing the correlation between the
temperature value and the greyscale level. Thus,
relationship (3) for S1/60s is obtained in the
form, as below
In the identical manner, the relationship under
consideration can be developed for other
conditions of conducting measurements.
Measurement results and their analysis
Comparison of temperature measurement results
obtained by three methods Based on the
measurements carried out, comparison was made of
temperature values obtained using the
thermovision camera and the still digital camera
with the temperature values obtained with a
thermocouple. The measurement results obtained by
the three different methods are shown in Fig. 7.
Testing methodology
The temperature values obtained using the digital
camera much less differ from the reference
temperatures compared with the temperature values
measured with the thermovision camera. In the
case of the still digital camera, a larger error
is observable in lower temperature ranges. By
contrast, for the thermovision camera, the error
increases with the increase in the temperature of
the sample. This error is caused by increasing
emissivity of the object with the increase in
temperature. Comparison of the accuracy of red
out temperatures The direct result of the
measurement is a digital photograph of the object
examined. The results of reading out the surface
temperature by the manual method (t1) and by the
automatic method (t2), and the results of
measurements of the reference temperature (t0)
are illustrated in Fig. 8.
The basis of measurement in digital
photopyrometry is the recording of the image of a
radiating sample with its simultaneous storing in
the cameras memory. The core to this measurement
is the recording of the spectral emission
density, ec?. The digital camera has a CCD
converter, instead of the traditional
photographic plate, and a processor that
processes the data and enables them to be stored
in the cameras memory. Data are recorded in the
RGB (Red, Green, Blue) mode as a 24-bit colour
image. Then, using an appropriate computer
program, the image can be transformed into an
8-bit multi-shade bitmap that can be red out in
the greyscale. 256 greyscale levels (from 0-black
to 255-white) are recorded. This transformation
enables the temperature of any object to be
represented as a function of the absolute
greyscale level. The greyscale level is
determined by the computerized analysis of
digital photographs using suitable graphic
software. The greyscale level determination is a
basis for developing a temperature
characteristics. Such a characteristics will
define the dependence of the surface temperature
of the heat radiation-emitting object being
photographed on the average greyscale level of
the photograph showing the object under
examination. Recently, the temperature read-out
process has been automated by developing a
relevant computer program. This program
automatically transforms the colour image of
material examined into a greyscale image.
Moreover, it enables a quick temperature readout
by indicating the coordinates of the point
(location), at which temperature is being
measured, with the computer mouse. The greyscale
level is then automatically read out and
subsequently converted into the value of
temperature. An example digital photo of an
examined sample is shown in Fig.1.
Fig. 7. Results of temperature measurements done
by three methods.
It can be found from the imaged analysis of the
measurement results that the temperatures red out
from the digital photographs only little deviate
from the model temperatures, which is indicative
of high measuring accuracy of the method used,
which is digital photopyrometry. An attempt to
automate digital photopyrometry by the
development of a suitable computer program will
enable a faster temperature readout and enhance
the accuracy of measurements. Temperature
characteristics for different solid materials To
compare the forms of temperature characteristics,
temperature measurements were taken for different
materials. The results of measurements for
chamotte, steel, graphite and chamotte L6 are
shown in Fig. 9.
Fig. 1. Digital photograph of an examined sample
at a temperature of 1150?C.
Fig. 8. Comparison of the accuracy of red out
temperatures.
Summary
The measuring stand
The accurate measurement of temperature is
necessary for the process to be run correctly.
Therefore, it seems appropriate to search for new
methods and possibilities for measuring this
parameter. Digital photography may become one of
such methods in the future, as indicated by the
results of the laboratory tests presented in this
paper. From the tests carried out, the following
conclusions can be drawn
Tests were carried out on a steel sample. The
specimen was heated up directly in the chamber of
an electric-gas oven. The reference sample
temperature was measured using an NiCr-Ni
thermocouple and red out under stationary heat
flow conditions, with the simultaneous
photographic recording of the image of the
specimen being examined. The distance of the
camera from the test specimen was approx. 1.5m. A
digital reflex camera was used for the tests. The
technical specification of the camera is as
follows ISO 80 S 1/60 s, 1/125 s, 1/250 s
A4. For comparison, the recording of test
specimen temperature was also done using a
thermovision camera. The distance of the
thermovision camera from the test specimen was
approx. 1.5m. The emissivity, on the other hand,
was measured by adjusting the temperature
measured with the camera to the temperature value
indicated by the thermocouple. The measured value
of e was e0.9. The temperature, as measured with
the still camera, was red out by two methods. The
first method was by manual reading out the
average greyscale level of for the assumed
surface. The value of the greyscale level readout
was then converted into the temperature value.
The temperature was also red out using the
authors computational program. Using the still
digital camera, attempts were also made to
measure the temperatures of materials, such as
graphite and chamotte. On this basis, temperature
characteristics were drawn up.
Fig. 9. Temperature characteristics for several
solid materials

• The form of the calibration curve depends on the
  sensitivity of the CCD matrix, the exposure, as
  controlled by the shutter opening time and the
  lens diaphragm, the filter used, and the type of
  material examined.

• It has been established that the dependence of
  the temperature value on the greyscale level of
  the digital photograph is most accurately
  described by the equation

• Comparison of the measurement results obtained
  using the thermovision camera with the results
  obtained with the still digital camera shows that
  digital photopyrometry yields results similar to
  those provided by thermovision in terms of
  accuracy. The use of a still digital camera for
  temperature measurement is easier and requires
  less financial outlays compared to thermovision.

• The analysis of measurement results shows that
  temperature measurement using a still digital
  camera are taken with high accuracy. In addition,
  the use of a suitable computer program enables a
  considerable automation of the process and
  facilitates the readout to be taken, while
  enhancing the accuracy of the temperature
  measurement. By using the program, the punctual
  readout of steel surface temperature is possible.

Fig. 2. Schematic of the measuring stand