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Lead Contents

Level Switches

These devices equip electrodes to detect liquid levels. They have been widely used in water works and sewers for buildings and housing complexes, industrial facilities and equipment, water treatment plants and sewage treatment facilities, and many other applications.

Overview Features
Principles Classifications
Engineering Data Further Information

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Primary Contents

Operating Principle

The operating principle is explained using a case where water is supplied from the water mains.

Office and apartment buildings normally have a ground tank and an elevated tank. Water is supplied from the water mains into the ground tank, pumped up to the elevated tank, then distributed to each floor.

When the water level in the elevated tank is low, water is pumped up from the ground tank to supplement it. When the water level reaches a certain level, the pump stops. (See figure 1.)

Elevated tanks are controlled in this manner to maintain the water level within upper and lower limits as shown below.

Figure 1. Water Supply Control


Pump Control According to Water Level (Two-pole Method)

1. When electrode E1 is not in contact with the conductive liquid as shown in figure 2, the electrical circuit is open, and no current flows between electrodes E1 and E3. Consequently relay X does not operate and the contact remains at the b side.

2. When electrode E1 is in contact with the conductive liquid as shown in figure 3, the circuit closes due to the conductive fluid completing the circuit between E1 and E3. Relay X operates and switches to the a side.
By connecting the relay contacts to a contactor, the pump can be turned ON and OFF.
However in practice, with only two electrodes, ripples on the surface of the liquid cause the relay to switch rapidly. This problem can be solved by forming a self-holding circuit. (The configuration shown in figures 2 and 3 can be used as water level alarms.)

Figure 2. Low Water Level


Figure 3. High Water Level


Liquid Level Control with Self-Holding Circuit (Three-pole Method)

An extra electrode E2 is added, and E1 and E2 are connected via contact a2 as shown in figure 4. When electrode E1 is in contact with the conductive liquid (as in point 2 of previous section), relay X operates and switches to the a side. Even if the liquid level falls below E1, the electrical circuit made through the liquid and the electrodes is retained by E2 and E3, as long as contact a2 is closed.

This kind of circuit made from electrode E2 and a contact is called a self-holding circuit.

When the liquid level falls below E2, the circuit made through the electrode circuit opens, which de-energizes relay X, thus closing the NC contact of X. This enables control of relay X to be switched ON and OFF between E1 and E2.

Figure 5 shows the timing chart of this mechanism.

Operating as simply as it does, possible applications of the Floatless Level Controller other than liquid level control include applications as leakage detection, and object size discrimination.

Figure 4. Self-holding Circuit


Figure 5. Timing Chart


Note: Non-conductive liquids, such as oil, cannot be controlled using this method.