Sensor - The Electronic Nose Artificial Olfaction Technology - page 39

Table 2.3 Various types of temperature sensors
Sensory type Limits of

application
(C)

Advantages Disadvantages

Thermocouple Minimum span of 40 C

Temperature versus emf
not exactly linear
Drift over time
Low emf corrupted by noise

Type E: Chromel-
constantan

-100 to 1000 Good

reproducibility
Wide range

Type J: iron-constantan 0 to 750

Type K: chromel-Alumel 0 to 1,250

Type T: copper-constantan -160 to 400

RTD -200 to 650 Good accuracy

Small span possible
Linearity

Self-heating
Less physically rugged
Self-heating error

Thermistor -40 to 150 Good accuracy

Little drift

Highly nonlinear
Only small span
Less physically rugged
Drift

Bimetallic - Low cost

Physically rugged

Local display

Filled system -200 to 800 Simple and low cost

No hazards

Not high temperatures
Sensitive to external
pressure

The freezing points of certain other metals are also used as secondary fixed
points to provide additional reference points during calibration procedures [ 4 ].
Various types of temperature sensors with advantages and disadvantage sum-

marize in Table 2.3 .
Temperature is measured on mainly three scales.

• Degree Fahrenheit (F) (used in U.S. and many other English-speaking

countries)
• Degree

Celsius (C) (used in scientific measurements and industrial

applications)
• Kelvin (K) (extremely low temperature).

2.4.2 Pressure Sensor

Pressure is the force per unit area that a fluid exerts on its surroundings. Mea-
surement of pressure inside a pipeline or a container in an industrial environment
is a challenging task, keeping in mind that pressure may be very high, or very low
(vacuum); the medium may be liquid, or gaseous. Pressure sensors operate on the
basis of the same principle: the detection of a physical force which arises due to
pressure [ 4 , 6 ].