Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 19

2.5.2 Electronic Temperature Sensors

Thermocouples : Thermocouples are based on the Seed-back effect. When a pair
of dissimilar metals is joined at one end, and there is a temperature difference
between the joined end and the open end, thermal electromagnetic force is gener-
ated. This will create a flow of current through the wires that is proportionate to the
temperature difference. The open ends must be kept at a constant reference tem-
perature. Several standard types of thermocouples are use.

2.5.3 Silicon Sensors

Silicon sensors make use of the bulk electrical resistance properties of semiconductor
materials, rather than the junction of two differently doped areas. Especially at low
temperatures, silicon sensors provide a nearly linear increase in resistance versus
temperature or a positive temperature coefficient.

2.5.4 Resistive Temperature Sensors

Resistive temperature sensors are devices whose resistance changes with the
temperature.
Thermistors : A thermistor is a type of resistor with resistance varying according to
its temperature. They typically consist of a combination of two or three metal
oxides that are sintered in a ceramic base material.

Thermistors can be classified into two types: positive temperature coefficient
(PTC) and negative temperature coefficient (NTC). PTC devices exhibit an increase
in resistance as temperature rises, while NTC devices exhibit a decrease in resis-
tance when temperature increases.
The main disadvantage of the thermistor is its strong nonlinearity. Cheap thermistors
have large spread of parameters (“tolerance”) and calibration is usually necessary.

2.5.5 Resistive Temperature Detectors (RTDs)

Unlike thermistors that use a combination of metal oxides and ceramics, resistive
temperature detectors are made from pure metal (copper, nickel, or platinum are
usually used). RTDs are useful over larger temperature ranges, while thermistors
typically achieve a higher precision within a limited temperature range.
As a RTD is a resistance device, it needs measuring current to generate a useful
signal. Because this current heats the element above the ambient temperature ( P = I 2 R),