Level measurement principles

Two main types of level measurement technics are used on industries, depending on the process requirements: continuous and point-level measurement. The difference between point and continuous level measurement is that on point level measurement, the detection is made by level switches, that will only detect the level presence on a particular point or position, when the sensor detects the level presence, a binary switch output is triggered.

On continuous level measurement, a level transmitter constantly measures the level on an extension, in a 0 to 100% range, delivering an analogue output proportional to the measured level.

Normally, level switches are used as safety devices against overfilling and dry-running protection for pumps in combination with a continuous level transmitter, however, some simple level control applications can be done with only level switches too.

Figure 1 – Continuous and point level measurement

Figure 2 – Free space radar (left) and guided wave radar (right)

Radar level transmitters are continuous level meters for liquids and granular solids that uses electromagnetic waves as a measuring method. Radar level meters are suitable for wide variety of applications across all industries. Some examples are buffer tanks, rivers, sea levels, aggressive media, chemical waste, wine storage, water and wastewater, lime milk, bitumen, cement, and flow measurement on open channels.

Two level evaluation methods can be found for this type of instrument: Time-of-Flight – ToF and Frequency modulated continuous waves – FMCW. In the Time-of-Flight method, an electromagnetic pulse is emitted by the instrument transducer, the pulse travels through the air, reflects on the medium surface and returns to the instrument, the level can be evaluated by measuring the time the pulse takes to return to the instrument.

In the FMCW method, a continuously modulated frequency of electromagnetic waves is emitted by the instrument, travelling through the air, and reflecting on the medium surface, changes in the level will cause a frequency shift. The difference frequency between the emitted and the reflected signal is used to determine the level.

Another type of radar commonly found on industries are guided-wave radars, it uses the time-of-flight as measuring method, but this time the pulses travels to the liquid surface using a metallic probe as guide. This makes the signal to be concentrated in a smaller area, being particularly interesting for installations in small spaces and with liquids with low dielectric constant, such as oils.

To the radar level transmitters

Radar level transmitters advantages:

• Radar technologies can be used in solids and liquids
• The measurement is independent of process conditions such as density, temperature, pressure and viscosity
• Non-contact measurement, except for guided wave radars
• High accuracy and reliability when comparing to other level measurement technologies
• Can support high temperature and pressure limits

Radar level transmitters limitations:

• A minimum dielectric constant of the medium is required for a reliable measurement
• Heavy turbulence on the medium surface can affect the measurement
• Internal structures on the tank such as agitators, level switches and heating coils can cause interferences
• Guided wave radar is not recommended for sticky media
• Expensive compared to other technologies

Ultrasonic level transmitters are continuous level meters for liquids and solids that uses ultrasonic sound waves, or in other words, high-frequency mechanical waves as measuring method. Ultrasonic level meters can be used on different applications, such as level measurement on underground water tanks, oil tanks, ponds, lakes, granular solids on silos and stockpiles, and flow measurement on open channels.

Ultrasonic level transmitters use the Time-of-Flight principle as evaluation method, like the radars, but this time, instead of using electromagnetic waves, it uses sound waves, that are mechanical waves. The instrument emits a pulse that travels thought the air, reflecting on the medium surface and returning to the instrument. The level can be determined by measuring time it takes the emitted soundwave to return to the instrument.

Discover ultrasonic level transmitters

Figure 3 - Ultrasonic level meter

Ultrasonic level meters advantages:

• Non-contact measurement
• Measurement not dependent on the liquid dielectric constant, good alternative for oils
• Not influenced by density changes

Ultrasonic level meters limitations:

• Not recommended for applications with foam and turbulent liquid surface
• Dust, vapors, condensation, and ambient temperature changes can affect the measurement
• The irregular shape inside the vessel may cause signal loss
• Not applicable on tanks with vacuum presence
• Temperature and pressure limitations compared to other technologies

Figure 4 – Direct mounting (left), submersible probe (right)

Direct-mounted pressure transmitters

A gauge pressure transmitter can be mounted in the bottom or laterally on non-pressurized tanks to measure the liquid level. The level is directly proportional to the pressure exerted by the liquid column above the sensor, as a rule of thumb, 100 mbar is equivalent to 1 meter of water column. It is important to consider that different liquids have different density values, therefore, the relation between pressure and level can be different.

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Submersible probes

On situations where it is not possible to mount a pressure transmitter in the tank or in an underground reservoir, a submersible pressure transmitter, also known as hydrostatic pressure transmitters, can be used following the same measuring principle. The pressure transmitter is now placed internally in the tank, submerged in the liquid.

To the hydrostatic pressure transmitters

On pressurized tanks, to measure the liquid level with a pressure transmitter, it is necessary to compensate the tank head pressure, in order that it does not influence in the hydrostatic pressure measurement. This can be done with a differential pressure transmitter.

The differential pressure transmitter is constructed in way that it can measure the pressure difference between two points. One measurement point is taken on the tank bottom, sensing the hydrostatic pressure summed with the head pressure, the other measurement point is taken on the tank top, sensing only the head pressure. The instrument can then compensate the head pressure, giving as result only the hydrostatic pressure. Another solution for this scenario is to use two separate gauge pressure transmitters, but then the compensation calculation will have to be done externally in the control system.

Figure 5 – Level measurement with Differential pressure transmitter

Level measurement with pressure transmitters advantages:

• Simple and economical solution for level measurement
• Well known measuring method
• Reliable and accurate measurement

Level measurement with differential pressure transmitters limitations:

• Only applicable for liquids
• Temperature changes affects the liquid density, influencing in the measurement accuracy
• Direct contact with the fluid, aggressive media may require additional accessories, such as sealing pots
• Not recommendable for high viscous fluids
• Agitators can cause pressure pulses affecting the measurement

Figure 6 – Laser level meter

Laser level transmitters advantages:

• Narrow beam angle
• The laser emitter can be mounted anywhere on the tank lid, including along the tank wall
• Very high update rates are achievable
• Laser technology can be used for both liquids and solids
• Measurement is not affected by material density or dielectric constant

Laser level transmitters limitations:

• Suspended dust can interfere with the laser beam
• Not recommended on applications where foam can occur
• Not suitable for transparent liquids

Capacitive level transmitters are used for continuous level measurement of liquids, being a simple, reliable, and inexpensive instruments compared to other technologies, capacitive level meters can be a good solution for level of measurement of water, oils, and acids.

Capacitive level transmitters, as the name suggests, works based on a capacitance principle. The instrument probe and the tank walls form a capacitor, the variation on level changes the capacitance of this formed capacitor. The instrument electronics can then relate the capacitance value to the liquid level. In the scenario where the tank is made of a non-conductive material, like plastic, it is needed to use a grounding probe, some models of capacitive level transmitters can have a built-in grounding probe for that purpose.

Discover capacitive level transmitters

Video 1: Measuring principle capacitance

Capacitive level transmitters advantages:

• Suitable for conductive and non-conductive liquids
• Liquid interface measurement
• Cost-effective technology
• Special models can be used for sticky media

Capacitive level transmitters limitations:

• Intrusive technology
• Installation on plastic tanks requires a grounding probe
• Changes on the medium dielectric constant can affect the measurement

Capacitive level switches advantages:

• Capacitive level switches are available for solids and liquids
• Cost-effective technology
• Special models can be used for sticky media

Capacitive level switches limitations:

• Intrusive technology
• Installation on plastic tanks requires a grounding probe
• For solids, a minimum dielectric constant is necessary

Video 2: The vibronic measuring principle

Vibrating level switches, also known as vibronic level switches, are instruments used for point level detection of liquids and granular solids, being one of the most used and reliable solution for point level measurement on different industries, such as food & beverage, pharmaceutical, chemical, and petrochemical.

The vibronic level switches work based on the resonance frequency of a tuning fork, a drive excites the tuning fork to its resonance frequency, as soon the fork gets covered by liquid, the liquid damps the oscillation, causing a shift in the resonant frequency. The device electronics can detect the change in frequency; thus, the liquid presence is detected.

On vibronic level switches for granular solids, the functioning is very similar, however, this time what is being monitored is the amplitude of the oscillations, and not the frequency anymore. The presence of bulk material will change the oscillations amplitude, being detected by the instrument electronics.

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Vibrating level switches advantages:

• The vibronic level switches are compact and easy to install
• No maintenance is required
• Not affected by the fluid properties such as conductivity, density, pressure, and temperature
• Suitable for applications with fluid turbulence, foam, or bubbling liquids
• Coated options for chemical compatibility

Vibrating level switches advantages:

• Intrusive method
• Not recommended for sticky media with heavy build-up

Conductive level switches are used for point level detection of liquids, being a cost-effective method, especially for simple applications, like two-point level control of water cisterns and reservoirs, pump dry-running protection.

In short words, a conductive level switch consists of one ground probe and one or more detection probes. An alternating voltage is applied to the ground probe and the to the detection probes. As soon as the liquid covers both, ground, and detection prob, an electrical circuit is formed. The instrument electronics detects an electrical current circulation on this circuit; thus, the level is detected.

On metallic tanks, the tank itself can be used as the ground probe, on this case, a single-probe level switch can be used.

The alternate voltage prevents corrosion and electrolytic destruction of the probes. The applied voltage is totally unharmful and there is no risk of electrical shock if the probes are touched.

To the conductive level switches

Figure 7 - Conductive level switches

Conductive level switches advantages:

• Simple and cost-effective solution for point detection
• The probe can be cut to the desired length, depending on the model
• Multiple detection points, depending on the model

Conductive level switches limitations:

• Only works with conductive liquids
• Temperature and pressure limitations

Figure 8 - Rotary paddle level switch

Rotary paddle level switches are level switches designed for point level detection of granular solids, such as cereals, sugar, chalk, powders, wood chips and others. Depending on the medium density, different paddles designs can be selected.

The rotary paddle level switch consists of an electronic motor that makes the paddle rotates, as soon as the paddle starts to get covered by solids, it stops the motor, and the instrument electronics can determine the level presence.

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Rotary paddle level switches advantages

• Simple design and functioning
• Suitable for different grain sizes and densities
• Independent of the material electrical properties

Rotary paddle level switches limitations

• Only applicable for solids
• Moving parts can wear over time
• Intrusive method

Float level switches are one of the simplest methods for measuring point level of liquids, being a very cost-effective solution for simple applications, like for example, pump control on water cisterns. Float level switches can have different designs and working principles, depending on the manufacturer and the model, but they all share a common item: a floating body.

In one of its simplest versions, a floating body made of plastic material has a free-moving metallic ball inside that act closing and opening electrical contacts. When there is no liquid, the float points downwards, when there is liquid, the float will point upwards. The metallic ball follows this movement, switching the electrical contacts.

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Figure 9 - Float level switch functioning

Float level switches advantages:

• Cost-effective solution
• Easy installation and commissioning
• Basic models do not require a power supply

Float level switches limitations:

• Intrusive method
• Only suitable for liquids
• Normally constructed with simple materials that can wear over time or may react with some liquids
• Not recommended for sticky media with heavy build-up
• Temperature and pressure limitations

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