Page 252 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

232 MICROSENSORS

Platinum is the most commonly used metal in resistive temperature sensors because it is
very stable when cycled over a very wide operating temperature range of approximately
—260 to +1700°C, with a typical reproducibility of better than ±0.1 °C. In fact, platinum
resistors are defined under a British Standard BS1904 (1964), made to a nominal resistance
of 100 £2 at room temperature, and referred to as Pt-100 sensors. Platinum temperature
4
sensors are very nearly linear, and «T takes a value of -1-3.9 x 10~ /K and fa takes a value
-7
2
that is four orders of magnitude lower at —5.9 x 10 /K . In contrast, thermistors, that
is, resistors formed from semiconducting materials, such as sulfides, selenides, or oxides
of Ni, Mn, or Cu, and Si have highly nonlinear temperature-dependence. Thermistors are
generally described by the following equation:
(8.3)

where the reference temperature is generally 25 °C rather than 0°C and the material
2
coefficient ß is related to the linear TCR by —B/T . The high negative TCR means
that the resistance of a pellet falls from a few megaohms to a few ohms over a short
temperature range, for example, 100°C or so.

8.2.2 Microthermocouples

Unlike the metal and semiconducting resistors, a thermocouple is a potentiometric temper-
ature sensor in that an open circuit voltage V T appears when two different metals are joined
together with the junction held at a temperature being sensed T s and the other ends held
at a reference temperature T ref (see Figure 8.5).
The basic principle is known as the Seebeck effect in which the metals have a different
thermoelectric power or Seebeck coefficient P; the thermocouple is conveniently a linear
device, with the voltage output (at zero current) being given by

V T = (V B - V A) = = (P B - (8.4)

Thermocouples are also widely used to measure temperature, and their properties are
defined in British and US standards for different compositions of metals and alloys, for

Reference junction
MetalB
-o-

Sensing
Metal A junction

O

Figure 8.5 Basic configuration of a thermocouple temperature sensor (a type of potentiometric
thermal sensor)