Sensor - The Electronic Nose Artificial Olfaction Technology - page 37

• the triple point of equilibrium hydrogen -259.34 C
• the boiling point of oxygen -182.962 C
• the boiling point of water 100.0 C
• the freezing point of zinc 419.58 C
• the freezing point of silver 961.93 C
• the freezing point of gold 1,064.43 C
(all at standard atmospheric pressure)

Resistance Temperature Detector (RTD):
The electrical resistance of many metals changes with temperature. RTD is
resistive temperature sensor, mostly made up of Platinum. It consists of the carrier,
the measuring winding and the connecting wires. A two-wire circuit with Pt 100
RTD's is appropriate for simple measuring applications. Commercially available
control, measuring and evaluating equipment is considered for the Pt 100 con-
nection with a three wire circuit. In laboratories and mainly difficult applications in
industrial measuring systems four-wire circuit with Pt 100 is used. In Fig. 2.5
types of RTD circuit is shown.

Thermocouple:
Thermocouples are small, rugged, relatively inexpensive, and operate over the
widest range of all temperature sensors. They are especially useful for making
measurements at extremely high temperatures (up to 2,300 C) in hostile envi-
ronments. They produce only millivolts of output, however, and require precision
amplification for further processing. They also require cold-junction compensation
(CJC) techniques. Principle of thermocouple is explained in Fig. 2.6 .
The most common metals used are Iron, Platinum, Rhodium, Rhenium,
Tungsten, Copper, Alumel (composed of Nickel and Aluminum), Chromel
(composed of Nickel and Chromium), and Constantan (composed of Copper and
Nickel) as shown in Table 2.2 [ 5 ].

Fig. 2.5 Types of RTD circuit. a Two-wire, b Three-wire, c Four-wire circuit