Methods of Level Measurement
Methods of Level Measurement: Measuring Principles and Suitable Sensor Solutions
Level measurement is essential in many industrial applications to monitor liquids safely, control processes reliably and protect systems against dry running, overfilling or malfunctions. Depending on the medium, container, installation situation and desired signal, different measuring principles can be used. Established methods of level measurement include hydrostatic, capacitive, optical, ultrasonic and radar methods as well as float-based or conductive solutions.
Which method is suitable depends heavily on the specific application. Should the level be output continuously as a measured value? Or is it sufficient to reliably detect individual point levels such as “empty”, “full” or “maximum permissible level”? Medium properties such as conductivity, density, viscosity, foaming, temperature and chemical resistance also play an important role.
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Continuous Measurement or Point Level Detection?
When selecting a level measurement method, the basic measuring task must first be clarified. In practice, a distinction is mainly made between continuous level measurement and point level measurement.
With continuous level measurement, the level is measured over a defined measuring range and output as an electrical signal. This is useful when it must be known at all times how much medium is in the container. Typical applications include storage tanks, dosing systems, process vessels or mobile systems with continuous level monitoring.
With point level measurement, by contrast, a specific switching point is monitored. The sensor detects, for example, whether a container is empty, whether the level has fallen below a minimum level or whether a maximum level has been reached. Point level sensors are frequently used for overfill protection, empty level indication, pump control or alarm functions.
Depending on the application, the sensors can be individually adapted in terms of material, installation position, switching points, connection and output signal.
Non-Contact Level Measurement: Radar, Ultrasonic and Optical Methods
Non-contact methods detect the level without the sensor being permanently immersed in the medium. This can be advantageous for aggressive, contaminating, sticky or difficult-to-access media. At the same time, the respective environmental conditions must be taken into account, such as container geometry, internal components, vapour, foam, condensation or the condition of the medium surface.
Radar Level Measurement
Radar level measurement works with electromagnetic waves. The sensor transmits radar signals towards the medium, which are reflected by the surface. The distance to the surface is calculated from the signal travel time, and the level is then determined from this distance.
A key advantage of radar level measurement is that it operates without contact and can also be used in many industrial applications under demanding process conditions. Depending on the design, it is suitable for both liquids and bulk solids. Typical areas of application include larger tanks, process vessels or applications in which continuous measurement without direct contact with the medium is required.
For applications where radar level measurement is not required and a robust, media-contacting solution for liquids is being sought, float-based transmitters from Reed can be a suitable alternative. They are particularly suitable when clear, mechanically robust level detection with an electrical output signal is required.
Ultrasonic Level Measurement
Ultrasonic level measurement is also based on non-contact time-of-flight measurement. The sensor emits ultrasonic pulses that are reflected by the surface of the medium. The level is calculated from the time between transmitting and receiving the signal.
Ultrasonic measurement is suitable for many liquid applications, but it is more dependent on environmental conditions than some other methods. Foam, vapour, strong turbulence, condensation or unfavourable container geometries can affect the measurement. The speed of sound, temperature and internal components in the container must also be taken into account during system design.
When non-contact ultrasonic level measurement is not required or is not ideal from a design perspective, Reed sensors provide an alternative for numerous liquid applications. Float switches are often a simple and proven solution, particularly for defined switching points, empty level indication or overfill protection.
Optical Level Measurement
Optical level measurement uses light, often infrared light, to detect contact with a liquid. Depending on the sensor principle, the reflection or refraction of the light changes as soon as the sensor tip is wetted by the liquid. This allows a defined point level to be detected.
Optical sensors are compact and operate without mechanically moving parts. They are often used where a single switching point needs to be detected reliably. However, the optical properties of the medium must be taken into account. Deposits, foaming, turbidity or highly adhesive liquids can affect the function, depending on the design.
For applications involving conductive liquids, a conductive rod electrode from Reed may also be considered instead of an optical solution. It detects point levels through the electrical conductivity of the medium and likewise operates without a float.
Contact-Based and Electrical Level Measurement: Pressure, Capacitance and Conductivity
In addition to non-contact methods, there are measuring methods in which the sensor is directly connected to the medium or the container. These include hydrostatic level measurement, capacitive level measurement and conductive point level measurement. These methods are often used when level measurement needs to be robust, compact or easy to integrate into existing containers.
Hydrostatic Level Measurement and Level Measurement Using Pressure
Hydrostatic level measurement uses the pressure exerted by a liquid column on a sensor. The higher the level, the greater the hydrostatic pressure at the measuring point. The level can be derived from the measured pressure, provided that the density and installation conditions are known.
This form of level measurement using pressure is frequently used for liquids, particularly in tanks, basins or closed systems. It can be a suitable solution when a continuous measured value is required and the conditions are appropriate for pressure measurement.
However, it must be taken into account that hydrostatic measurement can be influenced by factors such as density, temperature, pressure conditions and installation position. If the medium changes or the density varies significantly, the measurement must be designed or compensated accordingly.
Reed does not focus on conventional hydrostatic pressure measurement technology. However, when continuous level measurement of liquids is required and a float-based measuring principle is suitable for the application, Reed transmitters are a robust alternative.
Capacitive Level Measurement
Capacitive level measurement uses changes in electrical capacitance. In simplified terms, the sensor and container or the sensor and medium form an electrical system whose capacitance changes when the level changes or when a medium reaches the sensor area.
Capacitive level measurement can be used for both point level detection and continuous measuring tasks. Depending on the design, it is suitable for liquids, pastes or bulk solids. The electrical properties of the medium, particularly the dielectric constant, are decisive. Adhesion, foaming or material deposits can also affect measurement reliability.
Capacitive sensors are particularly suitable when no mechanical components are desired in the medium. However, for many liquid applications in which a float does not present a mechanical issue, Reed float switches or transmitters can provide a highly reliable alternative that can be adapted to the specific application.
Conductive Level Measurement for Conductive Media
Conductive level measurement is one of the electrical methods of level measurement. It uses the conductivity of the medium to detect a point level. As soon as a conductive liquid touches an electrode, an electrical contact or measuring circuit is closed. A switching signal can then be derived from this.
This principle is suitable for aqueous, conductive media and is frequently used for point level monitoring, dry-run protection or overfill protection. It is not suitable for non-conductive liquids such as certain oils or demineralised water if the conductivity is insufficient.
The conductive rod electrode from Reed is a suitable solution when conductive liquids need to be monitored and a simple, robust point level detection system without a float is required. It operates without moving parts and, depending on the application, can be a useful alternative to float-based switches.
Float-Based Level Measurement with Reed Technology
Float-based sensors are among the proven solutions for level measurement of liquids. They use the buoyancy of a float that moves with the liquid level. In combination with reed contacts or a reed chain, this generates an electrical signal for point levels or continuous level values.
The advantage of this principle lies in its clear, robust and easily understandable operating principle. The sensors can be adapted to the medium, container, temperature, pressure, installation situation and electrical interface. This makes them suitable for numerous industrial applications in which liquids must be monitored reliably.
Transmitters for Continuous Level Measurement
In Reed transmitters, a magnetic float moves along a guide tube. A reed chain is located inside the tube. Depending on the position, the magnet in the float activates individual reed contacts. This generates a measuring signal that represents the current liquid level.
Reed transmitters can be designed with different output signals, for example as a resistance signal, 4–20 mA or 0–10 V. This allows them to be easily integrated into control systems, monitoring systems and system concepts.
Typical areas of application include containers, tanks, dosing systems, mobile systems or customised assemblies in which the liquid level is to be monitored continuously. Depending on the design, various materials, connection types, lengths, resolutions and additional functions are possible.
Float Switches for Point Levels, Overfill Protection and Empty Level Indication
Float switches are used when defined liquid levels need to be detected. The float moves with the medium and activates a reed contact via a magnetic field. This generates a switching signal as soon as a certain liquid level is reached or left.
Reed float switches are suitable, among other things, for overfill protection, empty level indication, dry-run protection, pump control or alarm functions. Depending on the design, one or more switching points can be implemented. The material, installation position, connection, cable routing and switching logic can also be adapted to the respective application.
For liquid applications in particular, float switches offer a proven combination of simple integration, robust operation and high adaptability. For applications involving chemicals, cleaning media or dosing processes, suction lances with integrated level monitoring can also be a suitable solution.
Versions of our float switches
Which Level Measurement Method Is Suitable for Which Application?
The best method of level measurement cannot be determined in general terms. The specific application is always decisive. Non-contact radar level measurement can be suitable for large tanks or difficult process conditions. Ultrasonic measurement can be an appropriate solution when non-contact measurement is required and the environmental conditions are stable. Capacitive or optical methods are often considered for compact point level applications or special medium requirements.
For many liquid applications, however, float-based or conductive sensors are a highly economical and reliable solution. This applies particularly when the sensor can be integrated directly into the container, defined switching points are required or a robust continuous signal is sufficient for system control.
Important selection criteria include:
- Type of medium: liquid, bulk solid, paste or chemical medium
- Conductivity, density, viscosity and foaming
- Temperature, pressure and chemical resistance
- Container shape, installation position and available installation space
- Continuous measurement or point level detection only
- Output signal, for example switching contact, resistance, 4–20 mA or 0–10 V
- Safety function, such as overfill protection or dry-run protection
- Maintenance requirements and cleanability
Careful design is particularly important when sensors are integrated into industrial systems, dosing systems, mobile applications or customised assemblies. Technical consultation is worthwhile here to ensure that the measuring principle, material and electrical interface are optimally matched.
Which Level Measurement Method Is Best Suited to Your Project?
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Reed Electronics AG: Customised Sensor Solutions for Level Measurement of Liquids
Reed Electronics AG develops and manufactures customised sensor solutions for level measurement of liquids. Our focus is on robust, individually designed sensors for continuous measurement, point level detection and level monitoring in industrial applications.
Depending on the task, transmitters, float switches, conductive rod electrodes or suction lances with integrated level monitoring are used. We take into account the medium, container, installation position, material requirements, switching points, output signals and connection concepts.
This results in sensor solutions that are suited not only to the measuring principle but also to the specific application. Whether continuous level measurement, empty level indication, overfill protection or customised integration: Reed supports you in selecting and designing the right solution for your application.
