Level Measurement Instruments: Technologies, Comparison and Selection Criteria

Accurate tank level measurement is essential for inventory tracking, process control and preventing overfilling or dry running. Level measurement instruments determine the height of a liquid in a tank or vessel, or whether it has reached a given point, and transfer this information to an indicator, switch or control system.

Each technology has different strengths and limitations. This guide covers the most common level technologies, how they compare and the criteria to consider when selecting one.

Point Level vs Continuous Measurement

• Point level detection: Reports whether the liquid has reached a certain level (level switches). Ideal for pump start/stop and alarms.
• Continuous level measurement: Shows the level over the entire range or outputs it as an analogue/digital signal (indicators and transmitters). Required for inventory and proportional control.

Common Level Measurement Technologies

Float Level Devices

The movement of a float on the liquid surface is transferred to a switch or indicator via a mechanical linkage or a magnet. Simple, economical and reliable; used in water, fuel and many chemical tanks. In sticky or very dirty liquids the float movement can be obstructed.

Magnetic Level Indicators

A magnetic float inside a chamber connected parallel to the tank turns two-coloured flaps outside to show the level. The medium does not touch the indicator rail, so the design is sealed and safer than glass gauges. Magnetic switches or transmitters mounted on the chamber can send signals to automation systems. With suitable materials they are used on high-pressure, high-temperature process tanks and boilers.

Capacitive Level Devices

As the level changes, the capacitance between the probe and the tank wall (or a reference electrode) changes and is converted into level information. The probe is usually in contact with the medium, but has no moving parts and can be used with many conductive and non-conductive liquids as well as powders. Calibration matters for media that build up on the probe or have a varying dielectric constant. Common in food, chemical and pharmaceutical plants.

Ultrasonic Level Transmitters

A sensor mounted on the tank roof emits sound pulses and calculates the level from the time the echo takes to return from the liquid surface. It measures continuously without contact and provides signals such as 4-20 mA. Heavy foam absorbs the sound pulse and dense vapour or dust can disturb the measurement, so choose carefully under these conditions. Common in water and waste water, storage tanks and open channels.

Hydrostatic (Pressure-Based) Level Transmitters

The hydrostatic pressure of the liquid column is proportional to level and density; the transmitter measures this pressure to calculate the level. In open tanks a single pressure transmitter or submersible sensor is sufficient; in closed, pressurised tanks a differential pressure transmitter is used to cancel the effect of the gas pressure. Preferred for deep tanks, wells and underground tanks. Density changes cause measurement error.

Technology Comparison

Selection Criteria

• Medium properties: Temperature, pressure, density, viscosity, corrosiveness, conductivity, foam or solids content.
• Tank properties: Size, geometry, material, open/closed, internals (agitators, coils).
• Measurement need: Point or continuous, required accuracy and range.
• Output signal: Relay contact, analogue 4-20 mA or digital communication such as HART or Modbus.
• Installation and maintenance: Top, side or bypass mounting; access for maintenance.
• Cost: Initial investment plus operating and maintenance costs.

Frequently Asked Questions

Conclusion

A level instrument selected according to the medium, the tank design and the required signal improves process safety and reduces operating costs. ORFA can help you choose the right technology; see our level control devices page for all product groups.