Introduction to Industrial Automation

Figure 2.39  Electronic level switch with immersed electrodes for conductive liquids.

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Hardware Components for Automation and Process Control 

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53

reaches the upper electrode (E

1

), then the contact output is activated. If the liquid level starts to 

fall, the output remains activated and will be deactivated only when the liquid level falls just below 

the lower electrode (E

2

). When the electronic level switch is combined with a pump, either filling 

or emptying a tank automatically with its so-called “differential” behavior, the frequent and rapid 

start-stop repeated operation due to ripples on the surface of the liquid is avoided. Because of this 

differential behavior between the two-level limits and their electronic structure, the electronic 

level switches are sometimes called “level controllers”. Since the conductivity of the liquids can 

differ considerably, the response sensitivity of the electronic circuit detecting the current through 

the liquid is usually adjustable.

Capacitive level switches: It is obvious that the electronic level switches described in the previ-

ous section cannot be used for the case of non-conductive liquids or solid materials. The measuring 

system of level switches is capable of detecting liquids, as well as bulk solid materials, and is based 

on the capacitance measuring method. The capacitive level switch is a unified device that is side-

mounted entirely on the tank walls, exactly at the ideal point the level should reach. As shown 

in Figure 2.41, the device body is on the outside of the tank, while the projected set of the two 

capacitive electrodes is on the interior of the tank. The first electrode is the sensitive one, meaning 

that it senses the presence of material, while the second electrode is called insensitive and usually 

is permanently connected to the earth (tank wall). The two electrodes are electrically isolated from 

one another by a suitable polyacetal material and form a capacitor with dielectric material, the 

air. The operation principle of the capacitive level switch is based on the variation of the capaci-

tance due to the presence of liquid or bulk solid material around the electrodes. In general, the 

ELS = electronic level switch

Pump

ELS

Level

rise

Level

fall

Level

height

Level switch contact

Open

Closed

Clos

.

Open

E

1

E

2

E

3