Level Measurement

What is a level switch and what are its types?

What is a level switch and why is it used?

A level switch can be described as a device that can sense the level of a liquid in a process tank. A level switch is also used to control valves and pumps so that it can maintain the fluid level at a set value and also that it can prevent the tanks from overfilling. So we can see a level switch in measurement and control applications, level switches can indicate the high and low levels in process tanks. There are many level switches and the most common types of level switches are thermal, inductive, ultrasonic, etc.

What are the types of level switches?


Ultrasonic level switches are used in most industrial processes in order to detect high and low levels. The level measurement is done by transmitting pulses of ultrasonic range, so it operates by transmitting ultrasonic pulses between two crystals. The transmitter and receiver could be in the same probe or on the opposite side of the tank. The sound waves can be easily transmitted across a transducer gap in the presence of a medium and it would be attenuated if the gap is dry. The level switch would be actuated if the sound beam is interrupted. The ultrasonic level switches can be used for both solid and liquid material applications.

What are the advantages of ultrasonic level switches?

  • Installation is simple
  • No moving parts so less maintenance
  • It is not affected by foam
  • It can be used in hazardous areas
  • It is not affected by RF interference, conductivity, droplets, coatings


The inductive level switch can be considered as an electronic circuit and this circuit can determine the inductive reactance changes and in order to achieve this, the conductive fluid must interact with the magnetic field. It is mostly used in harsh environments due to its sealed probe and less moving parts. There won’t be any mechanical damage because there are no moving parts. We can see industrial level switches in many industrial applications. These level switches can be used in conductive liquids and solids, it can also be used on the interface between the conductive and non-conductive liquids.


In this type a heated thermal resistor is used to detect the surface of the liquid, the level measurement is done by using a thermistor and the thermistor does the measurement according to the difference in thermal conductivity of air and liquid. The thermistor would react to heat dispersion so that it can be used for the heat-based or oil-based liquids. The thermal level switch has two temperature sensors, and one of them is attached to a constant heating source and the second one is not in contact with the heating source and it compensates for the process temperature changes. The probe can detect the thermal conductivity of the product. Most of the liquids will create an interface and its thermal conductivity is different and this can be sensed.


The capacitive level switches operate according to the capacitance effect, these level switches are sensors with an electrical contact output. When the process fluid will increase and decrease in a tank, then the capacitance in the probe also increases and decreases. The electronic circuitry would detect the change in capacitance, so when the level reaches the set point the electronic circuitry will signal the alarm. This type of level switches can be used with non-metallic materials such as plastics, wood, ceramics, etc.


These are switches that have electrical contact output at specific liquid level applications. The float will be placed on the process fluid and the float must be lighter and thinner than the specific gravity of the process fluid. So when the process fluid level increases and decreases then the float would move up and down and this movement will cause the opening and closing of the electrical contact. It is an inexpensive way to determine the liquid level at a specific point. If it is used in a process tank then it mostly creates a seal between the process and the switch. It is used for applications such as level control, valve control, oil level control, boilers, etc.


In a nuclear level switch, there would be a radioactive source that would transmit radiation to the process fluid and this would be detected by a detector. The source and the detector are placed across the diameter of the tank and in this way, the transmitting and receiving of the radiation would take place. If the height of the process material is more than the source and detector arrangement then it will attenuate the radiation strength and the detector would detect the decrease in radiation. This low radiation could cause the detector to give a switch contact for the measurement.


These level switches can be used to detect the presence or absence of solids in a process tank. It can be used for bulk solid point level indication, it has a low power synchronous motor which would keep the paddle in motion at low speeds if there are no solids. In case if the tank level rises to the paddle, then torque will be applied to the motor drive and the paddle would stop, the level instrument would detect the torque and actuates the switch or set of the switch. Then these switches are used to sound the alarm or it is used to fill the process tank.


In a displacer level switch, the sensing element is a displacer or a weight, which would be heavier than the liquid, suspended from a spring. When the liquid contacts the displacer, a buoyancy force is created and this would cause a change in displacer weight, in turn, causing the spring to seek a new balance position that moves the attraction sleeve to the field of the magnet. So the magnet will move upwards, inside the pressure tube and thus actuating the switching mechanism. Due to this working principle, it can be used for wide switching differentials and allows the desired level switching point to be adjusted by moving the displacer up or down the suspension cable.


In this type of level switch, it has two small leaf springs, which is sealed into a glass tube. The electrical connections of the switch are provided by the output terminals. The two-wire connectors which are inside the tube would overlap with a small clearance and mostly they are made up of silver or gold for good electrical contact. When the switch comes in contact with a magnetic field and the two blades would pick up a different polarity, they would attract each other and makes electrical contact.


This switch would use visible, infrared, or laser light. These sensors are entirely dependent upon the transmitting, reflecting, or refracting properties of the process material while measuring its level. The optical level switch could be a contacting or non-contacting type. So in this type, the light will be transmitted and reflected and it will be received and according to this, the level measurement takes place. So an optical level sensor would have a light source, and it also has a photoelectric detector. These level switches can provide quick response, it is used in hydraulic reservoirs, sumps, conditioning systems, liquid holding tank, etc.

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