Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 178

When applying infrared thermography to detect surface defects, the effects
of convection and conduction should be taken into account. Convection is
a significant mode of heat transfer, especially if measurements are made in
the field. This is because fluid flow along the surface can transfer heat. Wind
velocity is therefore a critical parameter that affects the precision of this
method. Other critical parameters include solar radiation, surface emissiv-
ity, and ambient temperature. If not carefully controlled, these factors may
complicate data interpretation [4]. Conduction may result in surface defects
that are small in size compared to their depth and be undetectable with
thermography. This is because energy is gradually diffused into the material
with little thermal contrast observed at the surface [4]. For this reason, ther-
mography is more suited to detecting the presence of larger surface defects.
Thermography cannot provide data on the depth of defects; however, if com-
bined with radar (microwave) techniques, this limitation can be overcome.
5.2.3 Fibre optics
When temperature measurements are performed in the presence of an elec-
tromagnetic field, conventional temperature sensors such as thermocouples
may not be accurate. This is mainly because of the metallic nature of these
sensors. The induced currents and voltages in the metallic conductors and
the local heating of the sensor as a result of electromagnetic induction cause
electromagnetic interference, reducing the precision of the metal-based
sensors [7]. To improve the accuracy of temperature measurement in the
microwave processing of materials, temperature sensors based on optical
fibres have been studied.
Optical fibre sensors are well known to be virtually unaffected by electro-
magnetic fields [7-10]. Because of their glass-based nature, optical fibres do
not interact with the electromagnetic field and hence maintain their accu-
racy in the presence of microwaves. The considerably smaller size of optical
fibres compared to conventional temperature sensors facilitates the wiring
needed for instrumentation inside the microwave heating chamber. This is
especially important because the size of the openings permitted in micro-
wave heating chambers is usually rather small for safety. The small opening
makes it difficult to deploy conventional thermocouples for measurements.
The use of optical fibres as temperature sensors has been investigated by
several researchers in different fields [7,11-24]. The more commonly used
types of optical fibre temperature sensors may be categorised into three
groups. In the first group, a length of the optical fibre's cladding is replaced
with materials that have a temperature-dependent refractive index [7]. The
variations of the refractive index in such sensors leads to variations in the
optical power transmitted along the fibre, which is then correlated to indi-
cate temperature changes. The second group of optical fibre temperature
sensors makes use of the fluorescence lifetime approach. Such sensors have