Introduction to Industrial Automation

2.2.5   Level  Switches

Level switches or level sensors are devices of various kinds, from fully electronic to purely mechan-

ical types, with a general usage in detecting if the level of a liquid has a definite height. The detec-

tion of the liquid level inside a tank is a common need in any industrial process, large or small. 

For example, from large tanks in a petrochemicals refinery to small tanks in a brewery or water 

treatment plant, it is quite necessary to know if the liquid level has reached a high or low limit 

(level switches) or to measure continuously the value of the liquid level (level transducers). The 

same need exists for various granular materials of any kind inside a silo in many industrial appli-

cations, while in the following, the most representative basic types of level switches or sensors are 

described.

Electronic level switches for conductive liquids: These are electronic devices that can, by using 

immersed electrodes, detect if the level of a conductive liquid is between two certain levels that 

the engineer can define according to the application needs. There are many examples of electri-

cally conductive liquids encountered in industry applications, such as water, fruit juices, milk, 

beer, sewage, acids, alkaline solutions, etc. The main device consists of the power supply circuitry 

(230 V AC, 50 Hz) including the voltage reduction to the required low voltages, the electronic 

circuit, the electrodes’ terminals, and the SPDT contact output, as shown in Figure 2.39. Its 

operation requires the use of three rod electrodes, where one rod (E

3

) represents the earth connec-

tion and the other two electrodes (E

1

, E

2

) represent the upper and lower limits of the liquid level 

respectively. If the liquid tank is metallic, the earth electrode can be connected to any point of the 

metallic construction, and therefore does not necessarily need to be immersed. The voltage applied 

to electrodes is an alternating one to avoid any possible electrolytic effect and very low for safety 

purposes. The electronic circuit can detect if there is a current flow between electrodes through 

the liquid. The existence or absence of a current flow between the electrodes is converted by the 

electronic circuit, using a measuring amplifier and an electronic self-holding unit, to a switching 

contact alteration. Thus, the current detection is equivalent to the detection if the liquid level is 

at the upper or lower level. It should also be noted that the behavior of the SPDT contact output 

is different during the level’s rise versus fall. As shown in Figure 2.40, when the level is rising and 

N

Power supply

230 V AC

R

SPDT switch output

Electronic

circuit

24 V 

8 V 

Electronic level switch

High level

Low level

Earth

E

2

E

3

E

1