Level sensors are widely used across different industrial sectors. Their application includes determining level of substances that flow including powders, liquids, slurries and granular materials. Due to gravity, these substances flow into their respective containers. The measuring substance can be in the natural form like a river or lake or within a container.

Level measurement can either be continuous or point value. The continuous level sensors measure level of the given medium in a specified range. On the other hand, point level sensors only determine whether or not, the substance is above or below the sensing point. The main purpose for using point level sensors is to detect extremely high or low levels.

Numerous physical variables and applications affect the selection of level monitoring techniques. When selecting level sensors for industrial or commercial purposes, look for the following specifications;

• Pressure or vacuum
• Temperature
• Chemistry
• Vibration
• Physical phase (slurry, liquid or solid)
• Acoustical or electrical noise
• Dielectric constant and density of the medium

The application of level sensors is also an important consideration. Ease of calibration and programming, appearance, cost, accuracy and response rate are other specifications to look for. The mounting of level sensor instruments, its physical size, monitoring or controlling of continuous or discrete levels are other important considerations.

The measuring range, process operating conditions, communication interface and instrument style are some specifications that can help you choose the right level sensors that suit your application needs.

Level sensors are widely used for determining powder or liquid levels. They are also used to find out interfaces between liquids. Two basic types of level-measurements are available – multi-point or point and continuous.

The multi-point or point level sensors mark a particular level and are mostly used as a switch or a high level alarm. On the other hand, continuous level sensors are used for measuring dry or liquid material levels within a specific range. They also offer outputs that constantly indicate the level.

Both continuous and point level sensors can be incorporated into a single device to provide a stepped version of constant level or add a low alarm. Output options include analogue current signals, modulated frequency or frequency, analogue current signal, visual or alarm indicator.

Level sensor technology

Different types of level sensors are manufactured. Options include radio frequency (RF) admittance or capacitive, air bubbler, resistance or electrical conductivity, magnetic or mechanical floats, differential pressure and resistance or electrical conductivity. These sensors can also use microwave or radar, ultrasonic or sonic meters, pressure membrane and radio frequencies.

Sensors that use tuning forks make use of different technologies to vibrate a probe. They continuously monitor the absence or presence of that vibration. Level sensors are basically designed for applications that need a controller, a locally-programmable unit or a simple gauge.

Ultrasonic level transducers transmit and receive waves from different types of sensing devices.

These sensors read the echo to find out specific variables like proximity, distance, level or speed in different applications such as non-destructive evaluation, security applications, web break detection, welding and cutting. Ultrasonic transducers operate at a specific resonant frequency with different beam patterns, construction options and power levels.

Angle beam ultrasonic transducers, delay line ultrasonic transducers and electromagnetic acoustic transducers (EMAT) are the different available types. When it comes to selecting ultrasonic level transducers, you need to make sure that you choose one that suits your application needs.

Some features of ultrasonic transducers

Ultrasonic transducers are available in different types to suit different applications. General purpose ultrasonic transducers including air transducers do not usually come with any special features. However, more specialised ones like contact transducers need to be directly placed on the surface that is to be measured.

Dual element transducers have two different elements which allow the receiver and transmitter to operate independently. As the elements are angled towards each other, they create a reflective receive/transmit pathway.

A range of configurations are available for connecting the ultrasonic transducer to one or more transducers in parallel or in series. While most transducers offer analogue output voltage, they can have provision for current loop output.

With a huge number of different ways to monitor levels needed by industry, a range of different level measurement devices and sensors are available to suit different applications. Thus, selecting the right level measurement device for your needs is easy, if you know what you are looking for. The level-measurement instruments available today can include configuration and diagnostic system as well as just monitoring the level of a liquid. They can also process and communicate the data they obtain over a network for control and monitoring applications.

The PID or proportional integral derivative sensor is one model of sensor that is commonly used in liquid-level measurement. Different methods for liquid level measurement include.

• Ultrasonic
• Radar
• Conductance (conductivity)
• Hydrostatic tank/head gauging
• RF capacitance

Prior to selecting a device for your application, you must initially understand the working of each device as well as the theory behind it.

Things to know about interface level measurement devices

Interface level measurement devices include meters and sensors that can measure and detect levels between different media like water/oil and solid/liquid. There are some interface level measurement devices that can also measure the interface between a settled bed of solids and a liquid.

Sometimes, interface level measurement equipment is also referred to as water oil analysers, oil water analysers, oil level measurement equipment or liquid solid analysers, depending on the application. A vibrating probe extending through a wall of a vessel or tank to detect the level of the interface materials is one type of interface level measurement device.

Hydrostatic ‘screw-in’ level transmitters, hydrostatic ‘submersible’ level transmitters, radar level transmitters and TDR level transmitters are some of the options you can choose from.

The basic operating principle for all vibrating probe level switches is almost the same. These level switches operate by using two piezoelectric elements constructed on a vibration kit inside the tube. The piezoelectric element receives pulse signals from the PCB (printed circuit board) which causes the tube to vibrate. The vibration is transmitted to the PCB in order to get an electrical output signal.

If the vibration of the tube is stopped by the rising material, the PCB cannot receive any vibration signals from the piezoelectric element. Thus, the relay gets activated. Most vibrating probe level switches are well equipped to prevent malfunctioning that is caused due to a power shortage.

The vibrating probe level switches offer a trouble-free and reliable level sensing for bulk solids. With simple calibration, mounting and high stability working principle, it can survive high lateral loads and cannot be affected by static electricity and material build-up.

The following are some of the key benefits of vibrating probe level switches:

• No maintenance required
• Maximum versatility
• Absence of mechanical moving parts that wear out
• Easy to install
• No adjustments needed
• Unaffected by environmental changes, agitation and dust clouds.

Such benefits have added to the wide-scale use of vibrating probe level switches. Given below are some features of vibrating probe level switches which have enhanced the performance of the device.

• Durable and sturdy design
• No calibration needed
• Low / high fail safe modes provide reliability and safety
• No fear of accumulated materials on the sensor
• Adjustable sensitivity can be set depending on different density of materials.

Applications include the detection of powdered materials, dry milk, powdered coffee, powdered coal, chocolate, salt, sugar, dry bulk, glass material, wheat and cement. Vibrating probe level switches are also helpful in detecting the sludge level in waste water.

Level transducers or level transmitters are used for measuring the level of a bulk solid material or liquid within a particular space. They are also used to measure the electrical output that is proportional to the input level. Level transmitters are specially designed to measure these levels based on continuous or point level readings.

Point level transmitters offer an output after reaching a particular level of measurement. Basically, this output is in the form of an electrical charge or an audible alarm that activates the switch. Numerous point level sensors can be integrated together to provide a stepped version of a continuous level.

On the other hand, continuous level transmitters determine the level within a particular range and offer a continuous reading of the level. Most level transmitters can operate through an extensive range of pressures, temperatures, process conditions and vapour gas mixtures.

Advantages of using vibrating probes for level measurement

• Needs no maintenance
• No mechanical moving parts to wear and tear
• Easy to install
• No adjustments required
• Unaffected by agitation
• Unaffected by environmental changes
• Offers maximum versatility.

Level transmitters use different technologies for measuring or sensing levels. Some of these include air bubblers that utilise a constant volumetric rate of gas flow. This is generally fed through a bubble tube that is submerged in a liquid. RF admittance or capacitive transmitters utilise a radio frequency technique depending on the inconsistent dielectric constants.

Tuning fork or vibrating level transmitters use different technologies (usually piezoelectric) to vibrate the probe and constantly monitor the absence or presence of that vibration.