Introduction to Industrial Automation

Figure 2.51  Typical form of an RTD temperature sensor or switch.

Motion

Pressure

inlet

Motion

Pressure

inlet 

Motion

Pressure

inlet

(a)

(b)

(c)

Figure 2.52  Pressure sensing elements for motion production: (a) Bellow, (b) Diaphragm, and 

(c) Piston.

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Hardware Components for Automation and Process Control 

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63

the pressure set-point at which the switch is activated may be constant or adjustable. The above-

described relation between the movement and the pressure is valid when the pressure on the other 

side of the membrane is the atmospheric pressure. In the case that the pressure switch accepts two 

different pressures, it is called a “differential-pressure switch” and the movement is proportional 

to the difference between the two pressures. The term “differential” should not be confused with 

the differential behavior of the pressure switch output during rising and falling of the pressure, as 

shown in Figure 2.53a. The difference between the switch operating point on rising pressure (P

max

) 

and the switch operating point on falling pressure (P

min

) is called the “dead band”, and may be 

adjustable either independently in the setting points or as a fixed range.

A pressure switch with a piston as a sensing element is shown in Figure 2.53b. This is the 

simplest form of a pressure switch that is able to activate the SPDT contact, when a hydraulic or 

pneumatic system has reached a defined pressure value. In this case, the pressure enters through 

the connection inlet port and acts on the piston base. If the force resulting from the inlet pressure 

is greater than the downward force of the pre-loaded spring, then the piston is moving upward. 

The piston rod is also moving upward and the lever changes the state of the electric contacts. The 

trip-setting regulator is a nut that compresses the spring in order to increase the pressure set-point 

value where the switch is activated.

Trip setting

regulator

NO

NC

Switch

case

NO

NC

C

Range

spring 

Piston

Block of

contacts

Cable socket

Pressure inlet

Piston

rod

P

max

P

min

t

NC

contact

t

Open

Closed

P

(a)

(b)