handbook of modern sensors

16.1 Thermoresistive Sensors 461
2. The contacts are the conductive pads or wires which interface between the sensing
element and the external electronic circuit. The contacts should have the lowest
possible thermal conductivity and electrical resistance. Also, they are often used
to support the sensor.
3. A protective envelope is either a sheath or coating which physically separates a
sensing element from the environment. A good envelope must have low thermal
resistance (high thermal conductivity) and high electrical isolation properties. It
must be impermeable to moisture and other factors which may spuriously affect
the sensing element.
A noncontact temperature sensor (Fig. 16.3B) is an optical thermal radiation
sensor whose designs are covered in detail in Chapter 14. Like a contact sensor,
it also contains a sensing element which is responsive to its own temperature. The
difference is in the method of a heat transfer from an object to the element: In a
contact sensor, it is through thermal conduction via a physical contact, whereas in a
noncontact sensor, it is through radiation or optically.
To improve the time response of a thermal radiation sensor, the thickness of
the sensing element is minimized, whereas for better sensitivity, its surface area is
maximized. In addition to a sensing element, the noncontact thermal sensor may have
an optical window and a built-in interface circuit. The interior of the sensor’s housing
is usually filled with dry air or nitrogen.
All temperature sensors can be divided into two classes: the absolute sensors and
the relative sensors. An absolute temperature sensor measures temperature which
is referenced to the absolute zero or any other point on a temperature scale, such
as 0 ◦ C (273.15 ◦ K), 25 ◦ C, and so forth. The examples of the absolute sensors are
thermistors and resistance temperature detectors (RTDs). A relative sensor measures
the temperature difference between two objects where one object is called a reference.
An example of a relative sensor is a thermocouple.
16.1 Thermoresistive Sensors 2
Sir Humphry Davy had noted as early as 1821 that electrical resistances of various
metals depend on temperature [1]. Sir William Siemens, in 1871, first outlined the
use of a platinum resistance thermometer. In 1887, Hugh Callendar published an
article [2] in which he described how to practically use platinum temperature sensors.
The advantages of thermoresistive sensors are in the simplicity of interface circuits,
sensitivity, and long-term stability. All such sensors can be divided into three groups:
RTDs, p-n junction detectors, and thermistors.
16.1.1 Resistance Temperature Detectors
This term is usually pertinent to metal sensors, fabricated either in the form of a
wire or a thin film. The temperature dependence of resistivities of all metals and
most alloys gives the opportunity to use them for temperature sensing (Table A.7).
Although virtually all metals can be employed for sensing, platinum is used almost
2 Also see Section 3.5.2 of Chapter 3.