Sensor - The Electronic Nose Artificial Olfaction Technology - page 38

Fig. 2.6 Principle of thermocouple

Table 2.2 Types of thermocouples

Typical useful
range (C)

ANSI
configuration

Platinum(6 %)/Rhodium-Platinum (30 %)/Rhodium 38-1,800 B

Tungsten (5 %)/Rhenium-Tungsten (26 %)/Rhenium 0-2,300 C

Chromel-Constantan 0-982 E

Iron-Constantan 0-760 J

Chromel-Alumel -184-1,260 K

Platinum (13 %)/Rhodium-Platinum 0-1,593 R

Platinum (10 %)/Rhodium-Platinum 0-1,538 S

Copper-Constantan -184-400 T

Thermistor:
A thermistor is made of a semiconductor material that exhibits a predictable and
repeatable change in resistance as temperature is changed. Unlike a metal, the
molecular structure of a semiconductor is such that increasing its temperature
reduces its resistance. Thermistors are low cost temperature-sensitive resistors and
are constructed of solid semiconductor materials which exhibit a positive or
negative temperature coefficient (PTC or NTC).

Pyrometers:
Pyrometers are used for indirect (non-contact) temperature measurements.
Pyrometers are used at standard measuring temperatures from -150 Fto
6,300 F(-100 to 3,500 C) and up to 9,000 F (5,000 C) in special cases.
Basic principle of the pyrometer is the thermal radiation (infrared to visible
range) of a measured object filtered optically and concentrated on a radiation
receiver. Its electrical reaction consists of a change in the resistance, voltage, or
current of the radiation receiver induced directly or indirectly via a temperature
increase depending on the principle used. The electrical change is amplified,
measured, and processed further.
Pyrometers can be used, if:

• the surface temperature is to be determined,
• the medium to be measured is visually accessible (window, openings, optical
fiber).