Introduction to Industrial Automation

2.2.7   Temperature and Pressure Switches

Temperature detection in an industrial process (and by extension, its monitoring and control) 

is a very common problem for an engineer of automatic control systems and automation. The 

production of heat is described in the industry either as a positive (desirable) phenomenon or as a 

negative (undesirable) phenomenon. In the first case, electric energy is converted into heat through 

resistors; for example, in an industry producing plastic objects, where the raw material is heated 

in order to be fluidized, hence it is suitable for processing. In the second case, mechanical friction 

(or the Joule phenomenon) results in rising temperatures; for example, in the case of a heavy-duty 

gearing mechanism or in the interior of a power transformer. In both cases, independently of the 

causation, it is necessary to check the temperature continuously; when it rises over a predefined 

limit, a proper control action needs to be triggered. The temperature detection may be very simple, 

in the form of an ON-OFF thermostatic signal (temperature switches), or a precise measurement 

in the form of an analog variable (temperature sensors).

There are many different types of temperature sensors, all with different characteristics, 

depending upon their actual application and principle of operation. The methods used for temper-

ature measuring can be subdivided into contact and non-contact temperature detection methods. 

In contact temperature detection techniques, it is required that the sensor and the part containing 

the temperature-sensitive element are in physical contact with the object being sensed, while the 

thermal conduction property is used to detect changes in the temperature. In non-contact detec-

tion techniques, heat radiation and convection properties are used to detect changes in tempera-

ture via the energy exchanged between the detected object and the sensor. These two basic types 

of temperature sensors can be further subdivided into many other categories of sensors, according 

to the applied principle of physics. Due to many principles of physics that have been used for 

temperature detection, there is a wide variety of temperature sensors and switches on the market 

today, including thermocouples, resistance temperature detectors, thermistors, infrared, and semi-

conductor sensors or switches. Due to the large number of these sensors, it is not possible to give a 

detailed overview of all of them and thus, subsequently, the most widely used types of temperature 

switches only will be presented, keeping in mind that the use of switching contact outputs in an 

automated system has a similar principal of operation across the board.

Bimetal temperature switch (thermostat): The bimetal temperature switch is a contact type elec-

tromechanical temperature detector whose operation is based on a bimetallic strip described in 

Section 2.2.1. The bimetallic strip of the temperature switch, as shown in Figure 2.48a, consists of 

two different and similar bonded metals, usually made from nickel, copper aluminum, or tungsten. 

Simultaneously, the bimetallic strip constitutes an electric contact in a closed status under a normal 

temperature condition. When the strip is subjected to heat, as shown in Figure 2.48b, a mechanical 

bending movement is produced, opening the electric contact. Therefore, the bimetallic electric contact 

inserted in an automation circuit can interrupt the operation of a machine when the temperature rises.

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  Introduction to Industrial Automation

With additional mechanical or electrical equipment, such as a regulator of the temperature 

interruption set-point or a pre-interruption switching signal, more complex control actions can 

be facilitated. Due to their simple and inexpensive detection mechanism, bimetal temperature 

switches are suitable and preferable for many industrial applications.

Thermocouple: The thermocouple is a type of temperature sensor or switch that is made by 

joining two dissimilar metals or alloys at their one end, as shown in Figure 2.49. The two dis-

jointed ends of these dissimilar metals represent a junction that is kept at a constant temperature 

called a “cold junction”, while the other junction, called a “hot junction”, constitutes the sensing 

or measuring element that is in contact with the object being detected. The operating principle of 

the thermocouple is based on the well-known “Seebeck phenomenon” according to which a small 

voltage (few millivolts) is created between the two terminals of the cold junction when there is a 

temperature difference between the two junctions (cold and hot). If both junctions are at the same 

temperature, the potential voltage across them is zero. Since the resulting voltage is very small, an 

amplification is created by an operational amplifier (A) in order for the thermocouple output to 

produce anything more, such as an On-Off switching circuit.

Various combinations of metals are used in thermocouples according to the desired, detected 

temperature range. For example, the nickel chromium–nickel aluminum combination (Type K) is 

suitable for a −200 °C to 1250 °C temperature range to be detected. Two other types of widely used 

thermocouple materials are the iron-constantan (Type J) and copper-constantan (Type T) that 

have been recognized internationally as standards. One noticeable advantage of thermocouples 

used in temperature switches as sensing elements is their very small size, allowing thermocouples 

to be inserted into very narrow spaces.

Resistance temperature detectors (RTD): A well-known physical law of electricity is the resistance 

variation (to the flow of an electric current) with temperature in metallic materials. Its operation 

is based on this effect, and the resistance temperature detector is constructed from high-purity 

Bimetallic strip

Heat

Base

Wire

Wire

Electric

contact

Electric contact

opens due to heat

(a)

(b)