Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 183

Metal sheath
(stainless steel, etc.)

Insulating tube

Magnesia

Protection tube

Thermocouple

Temperature
measurement junction

Temperature
measurement junction

Terminal box

(a) (b)

FIGURE 4.33 Construction method of thermocouple: ( a ) structure of temperature
measurement junction of sheath-type thermocouple; ( b ) structure of thermocouple
with protected tube.

as an example. The materials of the thermocouple in this case are tabulated in
Table 4.1. As is clear form curve T in Figure 4.32, for example, this thermo-
couple can be used for low-temperature measurement, and a thermocouple of
this kind is mainly used in thermal therapy. This thermocouple is composed of
copper and a copper and nickel alloy as tabulated in Table 4.1.
There are two types of thermocouples: a sheath type and a protected-tube
type, as shown in Figure 4.33. The sheath type stores the thermocouple wire
in a metal sheath (metal pipe) which is filled with inorganic insulation mate-
rial (MgO or Al 2 O 3 ) to keep mutual insulation and an airtight condition and
to prevent corrosion and degradation of the wire. The outside dimension of
this thermocouple is of the order of 0.2-8 mm. The thermocouple of the
protected-tube type consists of a protected tube, a terminal box, and glass that
insulates the thermocouple wire. As such, protected tubes, a metallic type using
stainless steel, and a nonmetallic type using alumina magnetism have been
produced. The outside dimension is of the order of 3-30 mm. The sheath type
has interesting features: It has a rapid response with respect to temperature
change because the sheath diameter is relatively narrow and the inside of
the sheath is filled with inorganic insulation material when compared to the
protected-tube type. Further, the sheath type is excellent even for heat resist-
ance and vibration resistance characteristics, and it is flexible when using a
narrow-diameter sheath. This is why the sheath type is used mainly in thermal
therapy.

Circuit Structure In the construction of actual thermocouples, a lead wire in
the interval between the temperature-measuring junction P and the reference
junction Q is required, as shown in Figure 4.34.
On the other hand, as mentioned above, the thermoelectromotive force of
the thermocouple is determined by the temperature difference between the
measuring junction P and the reference junction Q . Therefore, if the spacing
from the thermocouple terminal (compensation junction) to the reference