Introduction to Industrial Automation

2.2.6   Flow  Switches

In many industrial applications, it is necessary to detect if there is liquid flow inside a pipe. For 

example, the cooling system of the bearings of a heavy-duty machine requires flow monitoring for 

ensuring water circulation. When the cooling fails due to malfunction of the water pumps, the 

switching signal of the water flow detector causes the machine to be switched off, first protect-

ing the machine, and second, saving the water pumps from possible catastrophic dry running. 

Furthermore, it enables a simple light or visualized indication of the fault location. The flow 

switches can detect a minimum of flow rate, while remaining unresponsive below their cutoff 

value. Although the following description of two flow switch types will refer to liquid flow detec-

tion, in general, flow monitors can be used in both liquids and gases.

Paddle-type liquid flow switches are purely a mechanical construction, while the only electrical 

part relies on its electric contacts. As shown in Figure 2.46, the switch operates with the use of 

a paddle made from stainless steel or brass that is inserted in a pipe. When the liquid flow being 

detected pushes against the paddle, the paddle-arm system swings away. This movement changes 

the position of a stick tangent to the paddle cam, and thus activates the SPDT contact output. 

When the flow is interrupted, the paddle moves back to its initial rest position. Some types of 

flow switches use a permanent magnet for actuating and a magnetic switch as contact output. 

Paddle-type flow switches can be mounted on both horizontal and vertical pipe lines. The same 

flow switch of some manufacturers can accept paddles of various sizes according to the pipe size.

The thermal dispersion flow switch operates based on the principle of thermal conductivity. The 

term “calorimetric” flow switch is also used as a detector of the thermal energy that is removed 

(a)

(b)

Photodiode

Phototransistor

Prism type

cover

Switch case

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electronics

Ph

i

Prism type

cover

and

electronics