On March 28, 1979, at the Three Mile Island nuclear power plant in the USA, part of the core melted in reactor No. 2. The TMI-2 reactor was destroyed. The cause of the accident was a minor malfunction in the secondary refrigeration circuit that allowed the temperature of the primary refrigerant to increase. This caused the reactor to automatically shut down. This situation developed because the level controls shut off the reactor coolant when they detected the presence of cooling water near the top of the tank. The water at the top wasn't because the tank was completely full, it was because there was so little water in the tank that it boiled and swelled to the top of the tank.
The incident is an example that signifies the importance of fluid level sensors and their proper functioning. They are important not only in nuclear power plants but in many applications. Every car, truck and motorcycle is equipped with a fuel level sensor to measure the amount of gasoline remaining in the fuel tank. In addition, there are sensors for measuring the level of engine oil, brake/power steering fluid, cooling water, windshield washer fluid, etc. Industrial applications include liquid level detection in water treatment tanks, transport and storage tanks, in the petrochemical industry for liquids such as gasoline, etc. Liquid level measurement is important in domestic applications for devices such as automated coffee machines, water dispensers, juicers, water evaporators, steamers, refrigerators and freezers, boilers, heating systems, dishwashers, washing, steam irons, etc.
In short, level sensors are one of the very important sensors and play a very important role in a variety of consumer/industrial applications. As with other types of sensors, level sensors are available or can be designed using various sensing principles. Selection of an appropriate type of sensor to suit the application requirements is very important.
Fig. 1: An image showing the level sensor
WHAT IS A LEVEL SENSOR
A wide range of sensors are available on the market and are commonly classified based on the specific application of the sensor. The sensor used to measure humidity is called a humidity sensor, the one used to measure pressure is called a pressure sensor, the sensor used to measure displacement is called a position sensor, and so on, although they all may be using the similar detection principle. Similarly, the sensor used for measuring fluid levels is called a level sensor.
Quite obvious from the name, level sensors are used to measure the level of free-flowing substances. Such substances include liquids such as water, oil, sludge, etc., as well as solids in granular/powder form (flowable solids). These substances tend to deposit in container tanks due to gravity and maintain their level at rest. Level sensors measure your level relative to a predefined reference.
Classification
CLASSIFICATION BASED ON DETECTION POINTS
Depending on the number of locations where the presence of a fluid (or fluidic solids) will be detected, level sensors can be classified into three categories:
1. Single Point Level Sensors – These sensors are used where the fluid level is to be detected at a single location only.
Fig. 2: Diagram showing single point level sensors
two. Multipoint Level Sensors – These sensors are used where the fluid level must be detected at multiple locations in a single location.
Fig. 3: Diagram showing multipoint level sensors
3. Continuous Level Sensors – These sensors are used where the fluid level at all locations is detected
Fig. 4: Diagram showing Continuous Level Sensors
CLASSIFICATION BASED ON SENSING PRINCIPLES
A wide variety of detection principles are used for measuring liquids, fluidic solids, sludges, etc.
· Floating Level Sensors
In these level sensors, a float moves with the surface of the liquid. The float is connected to a core via a spring. A magnetic reed switch is mounted on the hermetically sealed core and the core moves within a rod with the movement of float. The rod is surrounded by powerful magnets. As the float rises or falls with the liquid level, the reed switch is activated due to the magnetic field generated by the magnets.
Fig. 5: Figure explaining the principle of floating level sensors
Fig. 6: Image showing different parts of the floating level sensors
These sensors are also designed by keeping the rod and core (with magnetic reed switch) stationary and making the magnets part of the movable float. For multipoint level sensors, multiple magnets/multiple reed switches are used (depending on the design).
The principle of sensors (floats that move with the liquid level) can be coupled to dial gauges. Using buoyancy, they can form visual liquid level indicators.
Resistive, Capacitive
· Resistive level sensors
Variable resistors are widely used in fuel level detection. A wiper, connected to a lever arm with a float, moves across a continuous resistive track.
Fig. 7: Diagram showing resistive level sensors used in fuel level detection
The sensor works according to the potentiometric measurement principle. Current is made to flow through the resistance. The voltage drops linearly across this resistance. The slider through this resistance is connected to a float. The voltage output is obtained between the slider and one end of the resistor. Thus, as fluid levels vary, the slider moves and the output voltage varies.
A variant of this type uses the conductivity of the liquid under measurement. Current pulses are sent through a sensing electrode (electrically isolated from the tank or external pipe). When the sensor electrode is immersed in a conductive liquid, an electrical connection is created. The electrical potential is proportional to the liquid level and is measured through a counter electrode or the tank wall. It is used for continuous filling level measurement and is suitable for all conductive liquids.
· Capacitive level sensors
Since capacitance depends on the area of overlap between the plates, the distance between the plates, and the dielectric material between the plates, any of the three can be varied to design a useful capacitive sensor.
One of the simplest capacitive fluid level sensors is shown in the figure. It is made up of two concentric tubes immersed in the fluid whose level is to be measured. As the area of overlap between the plates and the distance between the plates are fixed, the capacitance becomes a function of the dielectric between the plates, that is, of the fluid between the two concentric tubes. As the fluid level changes, the capacitance also changes. This capacitance becomes a function of the fluid level.
Fig. 8: Image showing typical capacitive fluid level sensors with concentric tubes
Another variant of this sensor is the one that uses parallel plates instead of concentric tubes. Also in this case, the change in fluid level will alter the effective dielectric constant and therefore the capacitance between the plates.
Fig. 9: Image showing typical capacitive fluid level sensors with parallel plates
Pressure, Hall Effect, Ultrasonic
· Pressure-based level sensors
Pressure is defined as the force per unit area. The pressure at any depth, in a static fluid, is equal to the weight of the liquid acting on a unit area at that depth plus the pressure acting on the surface of the liquid. Level measurement based on pressure measurement is also known as hydrostatic tank measurement.
It is based on the principle that the difference between two pressures is equal to the height of the liquid multiplied by the specific gravity. Thus, the force on the bottom of the fluid container depends only on the height of the liquid level and therefore, with the measured hydrostatic pressure and knowledge of the specific gravity of the fluid, the level measurement is carried out.
Figure: 10
As these are used for level measurement of corrosive liquids/water etc., the chemical compatibility of the sensing element must be checked. Furthermore, sensors must be calibrated separately for different liquids as the specific gravities are different.
· Hall-based level sensors
Hall-based level sensors have been designed in various configurations. A rotary lever sensor is shown in the figure below.
Fig. 11: Diagram showing the rotary level sensor based on the Hall effect
A linear Hall sensor is placed at the center of the diametrically magnetized magnetic ring, surrounded by the soft iron magnet to guide the flow. The Hall sensor measures only the vertical component of the magnetic field. Thus, as the ring moves with the lever, the component of the magnetic field measured by the Hall Sensor varies. Thus, the output of the Hall sensor becomes a function of the fluid level.
Fig. 12: Graph representing the Hall sensor output as a function of fluid level
Hall sensors can be used in vertical float systems. Depending on the need for continuous or discrete level measurement, a series of Hall sensors can be placed at the desired points. The magnets become part of the floats. Consequently, with the movement of the float, the output of the Hall sensors will vary.
Fig. 13: Image showing Hall sensors used in vertical float system
Hall-based sensors offer good reliability, small dimensions, wide operating voltages and are available at relatively low costs. All these features make them a very attractive option among a variety of other sensors.
· Ultrasonic Level Sensors
Ultrasonic level instruments operate on the basic principle of time of flight, using sound waves to determine liquid/solid/paste level.
Ultrasonic level sensors are made up of two elements; a high efficiency transducer and an associated electronic transceiver. The complete return time between the transmitted ultrasonic pulse and the reflected echo is measured to determine the fluid level.
The frequency range for ultrasonic methods is in the range of 15 to 200 kHz. Lower frequency instruments are used for more difficult applications; such as longer distances and solids level measurements and those with higher frequency are used for shorter liquid level measurements.
They can be used as single point level sensor or continuous level sensors
Fig. 14: Image showing ultrasonic level sensors based on the time-of-flight principle using sound waves
Also read the article about Ultrasonic Sensors
Optical Radar Level
· Radar Level Sensors
Radar level sensors are fundamentally very similar to ultrasonic levels; the only difference between the two is the use of frequencies. Radar level measurement is also based on the principle of measuring the time elapsed between the transmission of a microwave pulse and the reception of the reflected echo.
Range resolution and frequency are two crucial parameters that must be considered when selecting these sensors. The accuracy of such sensors depends on the application, the antenna and its installation, and also the quality of the signal processing software.
· Optical sensors
It is a contact type sensor and uses the principle of optical reflection. This sensor houses an infrared LED and an IR photodetector. The light emitted by the LED is directed towards a prism; the prism forms the tip of the sensor. As long as the prism is out of contact with the liquid, the emitted light will be reflected back to the receiver. However, when the prism is immersed in the prism, the light is refracted into the liquid and therefore very little or no light reaches the receiver. Thus, based on the amount of reflected light, the presence or absence of a liquid is detected.
· ExoSensors
The ExOsense sensor 
(from Gems Sensors and Controls) uses proprietary transducer technology employing piezoelectric material. When the piezoelectric material is excited, it creates an acoustic signal due to the natural resonance of the material. ExOsense sensors generate this acoustic signal, direct it through the bottle wall and detect the reflection pulse.
(from Gems Sensors and Controls) uses proprietary transducer technology employing piezoelectric material. When the piezoelectric material is excited, it creates an acoustic signal due to the natural resonance of the material. ExOsense sensors generate this acoustic signal, direct it through the bottle wall and detect the reflection pulse. The amount of energy reflected is determined by the “acoustic impedance mismatch” of the materials in use. For example, if sound passes through two materials with similar acoustic impedances, very little energy will be reflected. If sound passes through two materials with different impedance values, most of the acoustic energy will be reflected. Acoustic impedance mismatch provides the basis for liquid level detection.
Sensor Selection
LEVEL SELECTION SENSORS
There is a wide variety of commercial solutions available for position detection and level detection. From the available options, designers can select the best possible technologies to meet their business and engineering objectives. But this also creates an abundance problem.
The problem of abundance, i.e. the availability of many options, often confuses designers rather than facilitating them. Level sensing, a form of position sensing, can be done using many different technologies – inductive, capacitive, mechanical, magneto-resistive, Hall effect, optical, etc. More than one solution may be viable for a specific application and this is where confusion arises.
· Number of questions to ask when selecting a sensor
· Required level detection points
· Measuring range
· Is the material being measured electrically conductive?
· Should the sensor be placed inside the material or can it be external?
· Is the material solid or liquid?
· Type of measurement required – contact or non-contact?
· Acceptable accuracy, precision and resolution
· Operating temperature range
· Output type – analog, digital, etc.
All of this needs to be determined to select the appropriate detection technology. Of course, answering these questions is not a simple task. But this is where the system designer's skill set is put to the test.