Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 185

Thermostable vinyl sheath

FIGURE 4.35 Compensation copper wire with shield.

temperature difference between the measuring junction and the reference ter-
minal. Accordingly, the temperature at the reference terminal should be kept
at 0°, but this is generally difficult. Hence, the temperature of the reference
junction is usually measured by the thermosensor, and the electromotive force
is compensated for in the electronic circuit. For this purpose, there is a com-
pensation circuit at the reference terminal. In this case, it is necessary that the
temperatures of the reference terminal junction and the thermosensor be
equal. The input terminal mount in Figure 4.36 is introduced for this purpose,
so that the reference junction terminal and the thermosensor are submitted
to a homogeneous temperature distribution. The burnout circuit in Figure 4.36
is introduced to not exceed the thermometer scale indication when the
thermocouple sensor wire is suspended. Especially, in thermocouples used to
measure the temperature of a furnace, for example, in a high-temperature
condition, the thermocouple gradually oxidizes and causes a disconnection
problem. When this happens, the input signal before disconnection has been
charged in an input filter condenser and the data (as for normal temperature)
are displayed. As a result, overheating inside the furnace may occur because
the measured temperature is not normal when the thermocouple sensor wire
is suspended. This can cause a serious accident. To prevent breakdown of a
thermocouple wire, a burnout circuit is introduced. In addition, the electro-
motive force of a thermocouple may exhibit a nonlinear characteristic without
being in direct proportion to the temperature. Therefore, a linearization circuit
is introduced to correct the nonlinearity. Finally, as described previously, the
thermoelectromotive force of the thermocouple is very small, and the induc-
tion noise from the outside becomes a problem. As a countermeasure, an iso-
lation amplifier circuit for insulating the input of the thermometry circuit
against an output is introduced.

Thermometer Using Thermistor A temperature sensor called a thermistor
uses the general semiconductor property that electrical resistance decreases