Types of Level Measurement in Process Industries – Principles, Methods & Transmitters

Classification of Level Measurement Methods

Describe the various types of level measurements.

Level measurement of fluid is categorized into two groups

1. Direct Level Measurement

2. Indirect level measurement

Let us discuss direct-level measurement in detail.

Direct Level Measurement Methods

It is also known as the Mechanical method.

This method is considered a local indication method.

Direct-level measurement is a simple & straightforward method.

This method is most economical for fluid level measurement within the tank.

This method uses a distance as the height from the datum line.

This direct-level measurement has a major disadvantage a measured signal cannot be transmitted for remote indication, & control.

Again this direct level measurement is sub-categorized into 

Dip Sticks and Lead Lines

 Flexible tubes, & cables are linked with dead weights at one end called chains or lead lines and are used to measure fluid level or depth of water in seas or in oceans.

Metallic tape with plump bob such as dead weight is stacked efficiently for extensive level measurement in oil bunkers and large storage tanks such as petroleum, or molasses storage tanks.

The accuracy of this method is about 0.1% to 0.2% with a range of about 20 to 25 feet in height.

Though this dipstick and lead line level measurements are peerless in accuracy, & reliability.

This method has some limitations

This requires an operator to interrupt his work & perform this measurement without continuous representation of the process measurement.

This method is not able to successfully measurement of levels in high-pressure vessels.

This may restrict the benefit of visual level measurement.

Sight Glass Level Measurement

Sight Glass is another simple method known as level glass. 

This method is quite simple, & direct to use.

The level within the sight glass shows or indicates the exact tank level.

Sight or level Glass makes the operator observe the liquid level in a  tank continuously.

Compared to dipsticks, dip rods, and manual gauging tapes, this method is more convenient and favorable. 

Sight glass is best suited to gauge an open tank.

Generally, a metallic ball in a tube is used to avoid the flow of fluid from the gauge body.

Tubular glass is also available for pressure ranging from 500 PSI to 600 PSI with a length of about 70 inches which is rarely used nowadays. 

Reflex glass or sight glass in a closed tank is used in most pressurized & atmospheric pressures with a varying height of about a few inches or feet.

This method is largely used in high-pressure bodies such as boiler drums, evaporators, condensers, tanks, distillation columns, & so on.

Unlike tube-type gauges, these can be gauged for any length of level measurement

Gauge-type level measurement is simple, & reliable for local indication.

This Gauge type level measurement method allows manual control of the process during a power failure, transmitter failure, or in any service maintenance activity.

These devices are susceptible to breakage and may get stained due to mud content.

Safety policies must be presented during hot, corrosive, or flammable liquids are handled.

Chain or Float Gauge

Float Gauge or Chain type measuring devices provides visual means of level measurement that exhibits simplicity and dependability by the float.

Float-type instruments are available in various forms and work on the principle of buoyant

An element floats on a liquid surface and changes its position with raise in the liquid level.

Many methods are used for a level indication from the position of float by using cable arrangement

Float Level Measurement

The float is linked to a pulley through a chain or flexible cable.

The pulley or rotating member is linked to an indicator with measurement graduation. 

As the float lifts up, the counterweight holds the cable tight and the indicating pointer travels along the circular scale.

Indirect or Inferential Level Measurement Methods

The indirect or inferred method of level measurement is based on the material with physical properties to measure related levels.

Many physical and electrical properties are used & are best suited to generate proportional output signals for remote transmission

This Inferential method uses the latest measurement technology.

Indirect or Inferential Methods are further classified as 

Buoyancy Level Measurement

The force generated by the submerged body is equal to the displaced fluid weight

Hydrostatic Head Level Measurement

It is defined as force or weight obtained by liquid height.

Ultrasonic (Sonar) Level Measurement 

Sonar or Ultrasonic sensors normally detect objects to be measured and reflect 

Signal of high frequency at locations near measured material.

Microwave Level Measurement

This uses microwaves instead of ultrasonic or sonar waves but the operation is the same as ultrasonic.

Conductance Level Measurement

The material required to be measured at desired points conducts electricity between two fixed probe locations or vessel boundaries.

Capacitance Level Measurement

The material required to be measured as a variable dielectric at desired points between two fixed capacitor plates. 

Normally, two materials with the desired measurement produce dielectric.

The net change in dielectric value is defined as the rate of one material increases while the other decreases.

Radiation Level Measurement

The material receives some amount of radiated energy.

In the capacitance method, the vapor space above the material has absorbing characteristics.

The change in absorption between two materials is good for measurement related which is quite accurate to the material to be measured.

Weight-Based Level Measurement

For constant density, the force due to weight is related to the level.

Some parameters such as concentration change & temperature change may make it difficult to measure.

Resistance-Based Level Measurement

Material pressure to be measured crushes two narrowly separated conductors together to reduce total circuit resistance proportional to the level

Micro-Impulse Level Measurement

An electrical pulse transmitted travels & reaches back in frequency with direct proportion to the liquid level.

Types of Level Transmitters Used in Process Industries

Level measurement transmitters are classified into seven types. 

Each transmitter works with different principles and makes it useful for various process types.

Capacitance Level Transmitters

These transmitters employ liquid as a dielectric medium between two or more electrodes. 

The energy of the capacitor is directly proportional to the level, the energy increases for more liquid, and decreases for less liquid. 

By determining the change in the capacitance value, the capacitance level transmitter calculates the current fill level of the tank.

Hydrostatic (Pressure) Level Transmitters

These Hydrostatic Level Transmitters are acclaimed as pressure level transmitters.

These transmitters determine fluid level by measuring the pressure of the resting body of fluid.

The force of liquid is the directly proportional volume of fluid if the force of liquid is high then the volume of fluid is also high & Vice Versa.

Magnetic Level Transmitters

These Magnetic Level Transmitters use a magnetic object slanged in a buoyant float.

This narrow auxiliary column limits lateral movements of the float. 

The float movement is measured by different magnetic devices placing the float on top of the liquid.

This measurement provides a precise and stable level to be transmitted. 

This method is appropriate for continuous level measurement owing to float tendency to rise or sink with respect to liquid level.

Radar Level Transmitters

This Radar fill level transmitter works on the radar principle using radio wave emissions. 

Generally, these transmitters are fixed at the top of a liquid-filled tank.

The transmitter transmits a radar signal into the liquid to receive a reflection of the transmitted signal. 

The transmitter now scans the current fill level of the tank based on the time taken by the transmitted signal to return.

Ultrasonic Level Transmitter

In this type, an ultrasonic Level Transmitter is installed near or at the top of a liquid container. 

The transducer transmits an ultrasonic pulse. 

This transmitted pulse hits the surface of the liquid & gets reflected back. 

The sensor determines the level based on the time interval between the signal transmitted and received back.

Guided Microwave (Guided Wave Radar) Level Transmitters

These transmitters work by detecting a microwave pulse through a transducer cable or transducer rod. 

The signal propagates & hits the liquid surface & travels or gets reflected back to the transmitter housing. 

An integrated electronics within the transmitter housing determines the tank level considering the time taken by the signal to travel to the object & come back. 

These level transmitters are found in all areas of process industries.

Liquid Level Transmitters for Interface Measurement

These transmitters are engineered to detect liquid levels. 

These Liquid level transmitters sense interfaces between two various liquids such as oil and water. 

Liquid level transmitters are employed for sensing liquid-level in large storage tanks, & transport tanks.

This pressure transmitter measures the liquid level by determining the pressure head of the liquid.