Level measurement of liquids and granular solids is one of the main measured variables on process industries, being used for safety, quality, monitoring and controlling purposes, directly affecting the operation and profitability of a plant. Because of the different types of liquids and processes conditions that can exist on different industries, diverse level measurement technologies were developed over the years to overcome the challenges that can be present in industrial processes, providing a high-accuracy, repeatability, and reliable measurements.
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
A wide range of technologies and models of level transmitters are available in the market to cover the most common and challenging industrial processes and applications. Selecting a compatible technology with the application is crucial to ensure a reliable and a long-lasting operation of the device, the selection will depend on different aspects like: type of media, measuring range, installation and other process conditions.
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.
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.
Figure 3 - Ultrasonic level meter
Level measurement with pressure transmitters, also known as hydrostatic pressure measurement, is one of the most economical and yet accurate solutions to measure level of liquids on open and closed tanks, boreholes, ponds, and lakes. Different methods can be used depending on the application and installation conditions.
Figure 4 – Direct mounting (left), submersible probe (right)
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.
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.
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
Figure 6 – Laser level meter
Similarly, to radars and ultrasonic transmitters, laser transmitters also take advantage of the Time-of-Flight measuring principle for measuring level of liquids and solids, however, this time the emitted signal is a laser beam.
The instrument emits laser pulses and measures the time it takes to the laser signal reflect on the medium surface and to return to the instrument; the level can then be calculated. The main advantage compared to radars and ultrasonics level meter is that the laser beam is very narrow, being suitable for installing it on reservoirs that can contain internal structures and on narrow tanks.
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.
Video 1: Measuring principle capacitance
Different technologies for point level detection are present in the market to cover different applications. Each technology is better suitable for one application depending on different criteria, like the type of media, process conditions and installation.
Capacitive level switches uses the same capacitance principle as the capacitive transmitters, however, a covered probe and an uncovered probe will have different capacitance values, measured by the instrument electronics, then, the instrument can determine if there is media presence or not.
Special models of capacitive level switches can have an additional electrode for applications with sticky media, this way, the instrument can ignore any build-up in the probe and detect the real level presence in a reliable way.
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.
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.
Figure 7 - Conductive level switches
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.
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.
Figure 9 - Float level switch functioning
The correct selection of a level meter can be challenging, as each process and application can have specific requirements and conditions, however, by understanding the process conditions, the measurement task and following some guidelines, it is possible to have a better view of what instruments can be suitable for the application.
1) What is the type of measurement needed?
Point level or continuous? Different processes have different demands, in some applications only a point level measurement is needed. In other applications, a continuous level measurement is necessary to monitor all the reservoir content constantly. On more demanding scenarios, it is necessary to use both methods in conjunction, specially on applications where safety is the number one priority.
2) What is the media to be measured?
Liquids or solids? Some technologies are designed to work only with liquids or solids, other can handle both, but is still necessary to select an appropriate version of that technology. It is also important to know some properties of the media, such as viscosity, conductivity, density, and grain size, as this can determine if a technology will work or not.
3) What are the process conditions?
Every instrument has limitations in terms of temperature and pressure, because of that, it is important to know this information to check if the instrument is suitable for those conditions or not. It is also important to know if there will be presence of gases, vapours, foam, turbulence, and dust inside the tank, as this can influence in the level measurement.
4) Where is the level meter is going to be installed?
The installation location plays a crucial role for level sensors, it is important to check where the device can be mounted in the tank: laterally, on bottom or on top. This will also determine which technologies can be applied for this application. A correct mounting will also help to avoid inferences caused by other structures inside the tank, such as agitators, heating coils and even the feeding inlet.
5) What are the ambient conditions?
Ambient conditions such as wind, dust and rain can influence in the functioning of some technologies on open tanks. Level meters are commonly installed on the top of tanks, receiving direct sunlight, that could overheat the electronics, in this scenario, a weather protection cover is recommend. Because of the difficult access on tank roofs, it is also interesting to consider using remote process indicators or a communication interface to have remote access to the instrument.
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