SM0486
(1) One typical thermocouple application is the cylinder temperature
measurement of an air-cooled engine. The connections for the indicating instrument
are shown in figure 12.
Figure 12.
Thermocouple application
(2) The thermocouple unit represented in figure 12 is made of iron and
constantan, which is an alloy of copper and nickel.
The leads to the engine
cylinder are insulated with asbestos and covered with a cotton braid, which is
impregnated with a fire and moisture resistant lacquer. One of the two junctions
of the unlike-metal leads is formed into a copper ring which serves as a spark plug
gasket.
The other, which is the cold junction, is inside the galvanometer.
The
instrument is calibrated to read temperature in degrees Celsius.
Commonly used
thermocouples are made of iron vs constantan, chromel vs alumel (chromel is an
alloy of nickel and chromium while alumel is an alloy of nickel and aluminum), and
copper vs constantan.
(3) Some thermocouples (figure 13) are constructed by connecting a wire
made of platinum alloy to a wire made of pure platinum.
Such a device may be
called a noble metal thermocouple. A very satisfactory thermocouple of this type
consists of one wire made of 90 percent platinum and 10 percent rhodium and another
Thermocouples often have ranges extending to 3000F.
wire made of pure platinum.
These thermocouples are often used as a standard for calibrating less expensive
thermocouples, or for special installations.
(4) The platinum-platinum rhodium thermocouple, shown in figure 13, is
typical of the thermocouples used for calibration purposes.
Figure 13, A, shows
the outer configuration of the instrument, whereas figure 13, B, is a cutaway used
to show the internal composition and construction of the thermocouple.
The
principle parts of the thermocouple shown in figure 13, B, are the head (5), a
primary protecting tube (11), and the thermocouple element (7).
The thermocouple
wires are 25 1/2 inches long and extend up through an insulator (9), lava-insulator
spacer (8), and fish-spine
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