Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 176

different combinations of metals and calibrations. The four most com-
mon calibrations are graded as types J, K, T, and E. There are also high-
temperature calibrations R, S, C, and GB. Each type (or calibration) has a
different temperature range of applicability, although the maximum tem-
perature that may be monitored also varies with the diameter of the wire
used in the thermocouple.
Despite the popularity of thermocouples in many civil engineering appli-
cations, they are not suitable as probes for temperature monitoring in most
microwave-assisted processes. Because of their metallic nature, thermocou-
ples are not immune to electromagnetic interference and may be inaccurate
when utilised in a microwave field. The presence of an electrically large
(>λ/10, where λ is the wavelength of the electromagnetic wave) metallic
temperature probe in a microwave environment may cause localised distor-
tions in the electromagnetic field distribution and induce electrical currents
that can affect the electronics used for temperature measurements. The
presence of an electromagnetic field may also give rise to errors caused by
self-heating, heat conduction, or excessive localised heating, particularly at
the tip of the temperature probe.
5.2.2 Infrared thermal cameras
(radiation thermometry)
Infrared thermography converts heat at any temperature into a thermal
image using specialised scanning equipment [3]. Radiation is the mode of
heat transfer that easily allows the recording of a visual image of the ther-
mal conditions prevailing. Thermography senses the emission of the thermal
radiation from the material surface being observed. Radiation can result
from either active or passive sources [4]. A surface of temperature T emits
infrared radiation of intensity I = A T 4 , where A is a constant. Radiometric
measurements of I serve as an accurate indication of T . In thermal cameras,
the infrared radiation emitted from the measuring object is detected and
converted to an electric signal by the two-dimensional uncooled focal plane
array detector, and the amplitude analog temperature signal is converted
into a digital signal. The digital signal is displayed as a thermal image in
colour or black and white. Infrared imaging is widely used for nondestruc-
tive testing of industrial products, in aircraft surveillance, and for medical
diagnostics. It has also been used for quick characterisation of microwave
components [5]. Numerous infrared thermal cameras are commercially
available (Figure 5.4).
In addition to applications in temperature monitoring to avoid overheat-
ing and underheating, infrared thermography can serve as an excellent
method for the detection of cracks developing on the surface of the concrete
components processed with microwaves. This can be useful especially in
accelerated microwave curing of concrete, for which development of thermal