Introduction to Industrial Automation

Figure 2.50  Resistive temperature detectors and their incorporation in a Wheatstone bridge 

circuit: (a) wire-wound RTD element, (b) thin-film RTD element, and (c) RTD element connected 

to a Wheatstone bridge.

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  Introduction to Industrial Automation

The semiconductor sensor of temperature is an integrated circuit fabricated in a similar way as 

all others. These sensors are classified into different types like voltage, current, digital, diode, and 

resistance silicon. The most common semiconductor temperature sensor is based on the funda-

mental temperature and current characteristics of the transistor. If two identical transistors are 

operated at different, but constant collector current densities, then the difference in their base-

emitter voltages is proportional to the absolute temperature of the transistors.

Pressure and temperature are the two most commonly measured or simply detected quantities 

in industrial processes. As happens for temperature, there are many ways of sensing fluid pressure. 

Pressure sensors and switches consist of a mechanical part sensitive to pressure and an electrical 

part producing the output signal, analog (measurement) sensors, or digital (switching contact) 

switches. In general, there are different sensing elements, each of which prescribes the design of 

the mechanical part. The goal of all the pressure sensing elements is to produce a movement as 

a result of the presence of fluid pressure, while the most common of them base their operation 

on a diaphragm and piston configuration, as presented in Figure 2.52. The air or liquid pressure 

at the inlet port acts on a movable surface (e.g., a flexible membrane or piston base surface). The 

force applied to the movable surface depends on the area of the membrane or piston base and the 

pressure of the compressed air or liquid. Since the area is constant, the produced force is directly 

proportional to the pressure. Therefore, the resulting movement is proportional to the pressure, 

and is converted either to an analog voltage, current signal, or to a contact switching signal, when 

movement is greater than a predefined limit. In the second case, which concerns a pressure switch, 

RTD bulb

Stainless steel sheath

Fixing nut

Electronics and

terminal box

Gland

Sealant 

Sensor

probe

Sensor

body

 

Lead wires

Cable

socket