Level switches

 

Level measurement

Level switch




Types of level switch

Different types of level switch are used in industries as requirements and functioning in the process control
some mostly uesd types of level switches are listed here
1. Floats type level switch
2. Rotary type level switches
3. Vibrating level switch 
4. Capacitance type level switches
5. Radio frequency type level switches

Point level

It is used to measure whether level of material is belowbor high than a specific point.

   outputs of level switch is used as alarm or tripping purposes of pump, screw conveyor and belt conveyor.

Float switch

Float Switches Applications & Installations

 A simplex (one pump) system will use a single switch wired in series with the motor leads so that the switch directly starts and stops the pump motor.

Three main classification of mounting the float level switch are

1. side-mounted

2. Top-mounted

3. external-mounted 

float switches.

Displacer Switches

 Displacer remains partially or completely submerged. The apparent weight of the displacer is reduced as it becomes covered by more liquid.

 When the weight drops below the spring tension, the switch is actuated.

 Displacer switches are more reliable than regular floats on turbulent, or foamy 

applications.

 These switches are interchangeable between tanks because differences in process density can be accommodated by changing the tension of the support spring.

2. Rotary types level switch

The purpose of the rotary paddle level switch is to detect the presence of solid/powdery material in most kinds of tanks, bins and containers. The unit is usually located through a bin wall at the top, middle or low level of a bin. During normal operation (no material present) a synchronous motor rotates the paddle at RPM. When this paddle rotation is impeded by material, surrounding the paddle, motor will stall and cause the Micro-switch to change state (indicating an alarm or control).

3. Vibrating level switch

 vibrating tuning fork liquid level switch is an instrument for monitoring the height of a liquid and tripping a relay contact switch or transmitting a solid state relay switch output when a maximum, minimum or intermediate level has been reached. The liquid level switch output can then be utilised by other instrumentation to open a valve, illuminate a warning lamp, activate an audible alarm, or switch on a pump.

Vibrating tuning fork liquid level switches are threshold contact trip devices which typically consist of a tuning fork which extends out from the switch housing.

In operation a piezoelectric oscillator is used to vibrate the tuning fork at its natural frequency for air.

A second piezoelectric device is used to pick-up the frequency of the vibrating tuning fork. When the liquid level rises and covers the tuning fork it will vibrate at a lower frequency.

This change in vibration frequency is used to detect when the required liquid level has been reached.

A vibrating tuning fork liquid level switch makes direct contact with the liquid to be measured, and therefore material compatibility has to be a consideration.

Since there are no pressure sensitive components exposed to the inside of the tank it is possible to fit tuning fork level switches to sealed pressurised tanks.

Also, since there is no sensitive components immersed in the fluid it is possible to use them on higher temperature media that would not be tolerated by other types of liquid level switch.

Vibrating tuning fork liquid level switches are generally mounted through the top or side of the tank with the tuning fork protruding horizontally into the tank.


Advantages

  1. Not affected by any surface disturbances such turbulence, foaming, vibration
  2. High temperature fluid compatibility since no sensitive components make contact with the fluid
  3. Small compact size
  4. Sealed pressurised or evacuated vessels will not affect performance

Disadvantages

  1. Thick or sticky media will leave a coating on the probe which will affect vibration frequency
  2. Fouling can interfere with resonant frequency causing false switch activation
  3. Tuning fork protruding into liquid causing obstruction, attracting fouling and hygienic cleanliness issues
  4. High cost



4. Capacitance level switch




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