Level Measurement

What is a level sensor and what are their types?

Level sensors are devices that are used to measure the substances, such as water, oil, slurries, dust particles, solids, etc. The medium which is to be measured could be stored in a vessel, bins, silos, etc. The liquid level is the position of a liquid surface above or below a reference point. These sensors are also capable to detect the powder levels and also the interface between the liquids. Level sensors are required for many industrial applications and they are inevitable in fluid systems. The liquid which is to be measured could be in a tank or it would be in natural forms like a lake or river. The variation in the fluid level could be sensed by the level sensor and it would transmit this info to the operator.

Types of level sensor
Level sensor types

Classification of a level sensor according to the sensing point

Level sensors are classified according to the number of locations where the fluid measurement is to be done.

Single point level sensor

This type of sensor is used for single location measurement of the fluid.

Multi-point level sensor

This type of sensor is used in multiple locations to do the level measurement of the fluid.

Continuous level sensor

This type of level sensor is used where the level measurement at all locations must be done.

What are the different types of level measurement?

Direct level sensor

This type of level sensor would detect the surface or interface of the liquids and the variation in the density of the measured substance won’t affect the measurement.

Types of the direct level sensor

  • Floats
  • Displacer switch
  • Sight glass
  • Capacitive level sensor
  • Conductivity level sensor
  • Vibration level sensor
  • Potentiometric level sensor

Indirect level sensors

In this type of level measurement, the measurement of the liquid level is done by measuring certain parameters such as pressure, temperature, weight, etc. This parameter would be calculated to determine the fluid level.

Types of indirect level sensors

  • Ultrasonic level sensor
  • Hydrostatic level sensor
  • Magnetic level sensor
  • Radar level sensor
  • Radiometric level sensor
  • Thermal dispersion level sensor

Direct level sensing

1. Float

In this type of level measurement, a buoyant object will be placed between the fluid which is to be measured and also the headspace vapor in the tank. The float would sink to the bottom of the fluid and then it would float on the top of the fluid which is to be measured. The float level sensor is composed of a magnet and also a magnetic reed switch, the float would vary its position according to the change in the liquid. The reed switch would open or close and it is dependent if it is in the air or in liquid. 

Working of float level sensor

Advantages of float level sensor

  • It is accurate
  • It can be used with different fluid densities

Disadvantages of float level sensor

  • It can’t be used with dirty fluids
  • The measurement is possible if there is a large amount of liquid
  • Big size

Applications of float level sensor

  • Tank level measurement
  • It can be used to measure the level of water, oil, hydraulic fluids, and chemicals

2. Displacer switch

In this type of level sensor, a displacer is used and spring would be connected to the displacer. So the displacer would be immersed in the fluid which is to be measured and this would change the buoyancy force. Due to this the force that acts on the spring would vary and thus the spring would compress. This is similar to the float switch and it has a magnetic sleeve and this would be attached to the spring so when the spring moves the magnet would move along it and thus switching would be done.

Displacer level measurement

Advantages of displacer switch

  • It can be used for high temperature and pressure
  • Less movement of the displacer

Disadvantages of displacer switch

  • Fluid density change would affect the measurement
  • It is mostly used with small tanks

Applications of displacer switch

  • Heavy oils
  • Liquids with solids
  • Paints
  • Dirty liquids

3. Sight glass

In this type of level measurement, a glass gauge is used and it will be placed close to the pressurized tank. This glass tube will be connected to the tank at the bottom with proper fittings so that the pressure could be retained. The liquid in the vessel could be bypassed through a transparent tube. We can directly observe the liquid level through the sight glass, the liquid level in the glass gauge would be the same as the tank.

Sight glass level measurement

Advantages of sight glass level measurement

  • Cheaper
  • Direct reading

Disadvantages of sight glass

  • It can only be used with clean liquids
  • Only non-corrosive liquids can be measured
  • It can’t be used with dangerous process fluids

Applications of sight glass

  • Food and beverage
  • Lubricating oil

4. Capacitive level sensor

This type of level measurement is a continuous level measurement and a capacitance probe is used for this measurement. The probe will be placed inside the tank, the metal wall of the tank, and the probe would act as two capacitor plates. The media which is to be measured is dielectric, so the capacitance value would change by varying the dielectric. So when the fluid level in the tank changes then the dielectric constant would change and thus the capacitance would vary.

Working of capacitive level sensor

Advantages of capacitance level sensor

  • It can be operated in high temperature and pressure
  • Can be used with solids and slurries
  • It can be used with mixed materials
  • It would give continuous indication

Disadvantages of capacitance level sensor

  • It can’t be used with varying dielectric materials
  • It can’t be used with media that coats the sensing element
  • Measurement is affected by the variations in temperature

Applications of capacitance level sensor

  • Chemical
  • Food and brewery industries
  • Slurries
  • Liquids with solid particles

5. Conductivity level sensor

Conductivity level sensor

In this type of level measurement, the fluid conductivity is utilized to do the measurement. Two conductive probes are used in this one of them is a short probe so when the fluid is not in contact with the probe then the electrical resistance between the probe and the and vessel would be really high. But when the fluid level increases and covers the probe then the circuit will be completed between the vessel and the probe, thus the circuit resistance would be lowered.

Advantages of the conductive level sensor

  • No moving parts
  • Easy to use
  • Low cost

Disadvantages of the conductive level sensor

  • Invasive (contact level measurement)
  • It can only be used with conductive liquids
  • The probe could erode during time

Applications of the conductive level sensor

  • Level measurement of boiler water
  • It can be used with highly corrosive liquids
  • Reagent monitoring

6. Vibration

This type of level sensor uses a tuning fork to do the level measurement, so when the media comes in contact with the blade or fork then a relay contact switch would be tripped. The relay would be switched according to a high, or low level. This device also consists of a piezoelectric element to pick up the frequency of the vibrating fork. So the level measurement is done according to the change in frequency.

Vibration level sensor

Advantages of vibration level sensor

  • It can be used for solid and liquid level control
  • It can be used with media that exhibit sticky properties or foam
  • Compact
  • It is independent of the media characteristics

Disadvantages of vibration level sensor

  • It can’t be used for continuous level measurement

Applications of vibration level switch

  • It is used to control the maximum amount of solids and liquids in many applications such as grains, cement, granules, powder
  • Food and beverage

7. Potentiometric level sensor

This type of level measurement is done by measuring the voltage ratio change between the measurement rod and tank wall which contains the process fluid. The measurement rod or the probe would be immersed in the process media and this measurement tube has low resistance. AC power will be transmitted to the probe from a power source. The voltage ratio between the tank wall and the probe will be taken to determine the level.

Potentiometric level sensor

Advantages of the potentiometric level sensor

  • Level monitoring in metallic pipes
  • It can be used to do the level control of paste or adhesive media
  • It can be used to do the level measurement of small tanks

Disadvantages of the potentiometric level sensor

  • A specified amount of conductivity is required for the media
  • It can’t be used in explosive areas

Applications of the potentiometric level sensor

  • It can be used with sticky or foaming media
  • Level monitoring in the process tank
  • Level detection in cream production

Indirect level sensor

1. Ultrasonic level sensor

This type of level sensor can be used to do the noncontact level measurement. This sensor would transmit high-frequency ultrasonic signal waves and it would be reflected back from the media which is to be measured. So the time taken by the waves to reflect back from the media is calculated and thus the level can be determined.

Working of ultrasonic level sensor

Advantages of the ultrasonic level sensor

  • Non-contact level measurement
  • Compact
  • Measurement is not affected by the type of media which is to be measured

Disadvantages of the ultrasonic level sensor

  • Costly
  • High pressure and temperature could affect the measurement
  • Reflections could happen due to agitation
  • Foam, steam, change in concentration would affect the measurement

Applications of the ultrasonic level sensor

  • Measurement of high viscous fluids
  • Measurement of bulk solids
  • Water treatment applications

2. Hydrostatic level sensor

This type of level sensor is the submersible type and the level measurement is done by measuring the hydrostatic head which is created by the liquid column. In this measurement, a pressure sensing element is utilized and this sensor would be placed under the bottom of the tank. This sensor would determine the tank pressure and this pressure will be proportional to the liquid level.

Working of hydrostatic level sensor

Advantages of the hydrostatic level sensor

  • Reliability
  • Measurement is not affected by the media properties such as foam, vapor, density, etc.
  • Easy calibration

Disadvantages of the hydrostatic level sensor

  • It can’t be used with bulk materials
  • The tank must be emptied to do the sensor maintenance

Applications of the hydrostatic level sensor

  • It is used in storage tanks
  • Processing vessels
  • Measurement of corrosive liquids

3. Magnetic

In this type of level sensor, the sensor would be placed parallel to the tank and this device consist of a column with visual indicators and it would show the level. The device consists of magnetic floats and it would move up and down according to the water level in the tank. This device would indicate the level by using a float that is magnetically coupled to an index or a column of rotating flaps. This magnetic float would also move the visual indicators in the column.

Level indication by magnetic level sensor

Advantages of the magnetic level sensor

  • Visual indication of the level
  • The visual indicators are not composed of process fluid that is being measured so there is no risk if the glass breaks
  • We can measure the level of colorless fluids
  • We can also measure the level of fluids that may foul the sight glass

Disadvantages of the magnetic level sensor

  • The float in the magnetic indicator could get stick
  • The magnetic float is dependent upon the pressure in the vessel and also the specific gravity of the process fluid

Applications of the magnetic level sensor

  • High pressure and high-temperature applications
  • It can be used with corrosive liquids
  • Mining

4. Guided wave radar

Guided wave radar operation

In this type of level measurement, a microwave is transmitted to the process fluid. In the above image, we can see a transmitter that is mounted to the tank, and the transmitter probe would transmit the microwave to the process fluid, a significant proportion of this microwave would be reflected back. The time taken by the wave to reflect back from the fluid is calculated to determine the level.

Advantages of radar level sensor

  • High accuracy
  • Noncontact measurement
  • It can measure level through plastic tanks
  • It can be used with all types of liquids

Disadvantages of radar level sensor

  • This device is sensitive to the build-up in the sensor face
  • Highly expensive
  • Limited detection range

Applications of radar level sensor

  • It can be used for applications where the temperature and pressure would be a problem
  • It can be used in moist, vapor, and dusty environment

5. Radiometric

Radiometric level measurement

This type of level measurement is done by transmitting gamma source and in order to do that cesium or cobalt isotope is used as the source. So from the source, electromagnetic radiation is transmitted and this radiation is emitted in a way that it would pass through the material to reach the transmitter which is placed on the opposite side of the tank. So the intensity of the received signal is used to determine the level of the process fluid.

Advantages of the radiometric level sensor

  • Noncontact measurement
  • Safety and reliability
  • It can be used in extreme conditions
  • Maintenance is not required

Disadvantages of the radiometric level sensor

  • Accuracy could be affected by external radiation
  • Expensive
  • There could be many safety problems due to the radiation

Applications of the radiometric level sensor

  • It can be used for high temperature and pressure applications
  • It can measure toxic media
  • It can be used with high viscous fluids
  • It can be used for applications where installation is difficult

6. Thermal dispersion level sensor

thermal dispersion level sensor

This type of measurement is done by using a thermistor, the thermistor is a device that would decrease its resistance when there is a rise in temperature. The internal temperature of this device is related to the heat dispersion of its surrounding environment. The heat dispersion property of the liquid is really high so when we place the thermistor probe in the process fluid then the resistance of the probe would vary immediately.

Advantages of thermal dispersion level sensor

  • Low maintenance because there are no moving parts
  • Good accuracy
  • High sensitivity
  • It can be used with corrosive and hazardous fluids

Applications of thermal dispersion level sensor

  • Hydroelectric plants
  • Petrochemical
  • HVAC
  • Food and beverage

How to select a level sensor?

  • It should be selected according to the media which is to be measured like clean fluid, slurries, liquids with solid particles, etc.
  • It should be selected according to the temperature and pressure range of applications
  • Point or continuous level measurement
  • Measurement range
  • We must consider if the measured material is electrically conductive
  • Does the liquid surface contain turbulence, foam, or vapor
  • Contact or non-contact level measurement

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