Introduction to Industrial Automation

Figure 2.42  Ultrasonic sensor for non-contact level measurement (a), and ultrasonic switch for 

contact level detection (b).

Frequency

(GHz)

f

max

f

min

f

0

f

1

t

0

t

1

Time

Δt

Δf

f

1

f

0

d

Δf ~ Δt ~ d

Transmitted wave

Reflected wave

Radar level sensor

Dead

zone

Figure 2.43  Radar level sensor operation principle.

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  Introduction to Industrial Automation

Float level switches: The simplest level switches from a construction point of view are the float-based 

level switches. In Figure 2.44 some basic types of float switches are shown. The float switch shown in 

Figure 2.44a requires sidewall mounting, while a similar one shown in Figure 2.44b is suitable for a 

vertical placement. Both types use a sealed-in-glass magnetic switch (called “reed relay”, which is pre-

sented in Section 2.3.4) and a floating part that contains a permanent magnet. When the liquid level is 

low, the permanent magnet is far from the magnetic switch, and thus its SPST contact is open. When 

the liquid level increases, the floating magnet is moved toward the magnetic switch. Once the mag-

netic switch is reached, the floating magnet activates the SPST contact, which subsequently closes. 

Therefore, the floating magnet follows the changes of the liquid level, and at a designated height, the 

contact output changes status and remains there as long as the level height does not change.

Another type of a float level switch is shown in Figure 2.44c where the two metallic plates play 

the role of an electric contact, and the metallic ball consists of the medium closing the contact. 

All the mentioned components are inside a plastic floating box, the orientation of which, up or 

down, depends on the liquid level height and determines the status, closed or open, of the contact.

Float

Reed switch

Liquid up-level

(contact closed)

Liquid down-level

(contact open)

Liquid down-level

(contact open)

Liquid up-level

(contact closed)

Silver alloy-contact float switch

(c)

Metallic

plates

Metallic

ball

Switch

housing

Level switch for

vertical installation

Permanent magnet

Reed

switch

Float

cylinder

Magnet

ring

(a)

(b)

Side-mounted level switch

Figure 2.44  Various types of float-based level switches: Magnet–reed relay float level switch 

for (a) sidewall mounting, (b) vertical placement, and (c) metallic plate–ball float level switch