Both measure the distance to the surface with no contact. Ultrasonic is often the economical choice for water and wastewater in open tanks, wells and channels. Radar is the choice where foam, vapour, high temperature or pressure would weaken a sound pulse — and guided wave radar also works under foam and measures interfaces.

Why does a hydrostatic transmitter need the density?

Because it measures pressure, and the pressure of a liquid column is its height times its density times gravity. One metre of water gives about 98 mbar; one metre of a liquid with a specific gravity of 0.8 gives about 78 mbar. If the density changes, the level reading changes with it.

Can I measure the level in a closed tank under pressure?

Radar can, since it measures a distance. A hydrostatic measurement must subtract the gas pressure above the liquid: measure the bottom against the top with a differential pressure transmitter.

Which switch for a silo of powder?

A vibrating rod or fork, or a rotating paddle. Both detect high and low levels in powders and granules; the choice depends on the bulk density, the build-up and the mounting.

Can you help us choose?

Yes. Send us the medium, the vessel (open or closed, its height, internals), the process temperature and pressure, whether you need the level or a limit, the process connection and the output you need, and our engineers will recommend the right instrument, with price and delivery time.

LEVEL MEASUREMENT

Level measurement: level transmitters and level switches — how they work and which one to choose

Level is measured in every tank, silo, well and vessel of a plant — to know the contents, to control a pump or a valve, or to stop a tank from overflowing and a pump from running dry. Some instruments give the level continuously; others only switch at a point. This page covers seven technologies — hydrostatic transmitters, radar, ultrasonic, vibrating switches, floats and displacers, conductive and HF switches, and the rotating paddle — how each one works, where it fits and where it does not, when you need a continuous measurement and when a switch is enough, and how to choose.

Seven ways to measure level

For each one: the principle in a sketch, where it fits and where it does not, and how many instruments of that technology are in our catalogue.

Hydrostatic level transmitters

A pressure sensor at the bottom of the tank or well measures the pressure of the liquid column above it. With a known density, that pressure is the height of the liquid — about 98 mbar for every metre of water. Submersible probes hang on their cable, which carries a vent tube to the atmosphere as the reference; flanged versions mount near the bottom of the tank.

Where it fits: Water and wastewater, wells, sumps, open tanks and reservoirs — simple and robust, and not affected by foam, vapour or anything above the liquid.
Where it does not: Closed or pressurised tanks, where the gas pressure adds to the reading, and liquids whose density changes.

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Radar level: non-contact and guided wave

The transmitter sends microwave signals towards the surface and measures the time until their echo returns: non-contact radar sends them through the air from an antenna, guided wave radar along a rod or cable that reaches into the medium. The distance to the surface, taken from the height of the tank, gives the level.

Where it fits: Liquids and bulk solids in tall tanks and silos, with vapour, high temperatures and pressures; guided wave radar also under foam and for the interface between two liquids.
Where it does not: Simple jobs where a switch or a hydrostatic probe is enough, and installations where the beam cannot get past internals near the antenna.

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Ultrasonic level transmitters

A sensor mounted above the surface sends short pulses of sound and times their echo from the liquid. The time and the speed of sound give the distance, and from it the level — with no contact. The speed of sound changes with the temperature of the air, so the transmitter corrects for it.

Where it fits: Water and wastewater: wells, sumps, open tanks and open channels — the Delta Mobrey DMSP900SH is made for wet wells and sumps.
Where it does not: Foam, vapour, dust, heavy turbulence, and closed tanks under pressure or vacuum, which weaken the echo or change the speed of sound.

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Vibrating level switches

A fork or a rod is kept vibrating at its resonant frequency by its electronics. When liquid or powder covers it, the vibration changes — it is damped and its frequency drops — and after a short delay the switch changes its output.

Where it fits: Overfill and dry-run protection in liquids, and high and low level in silos of powders and granules — with no moving parts.
Where it does not: Continuous level, and media that leave hard, heavy build-up between the tines.

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Floats and displacers

Both use buoyancy. A float rides on the surface and, at a set level, moves a magnet that switches a contact. A displacer is heavier than the liquid and hangs from a spring or a torque element: as the level rises, the liquid lifts it with a force equal to the weight of the liquid it displaces, and the transmitter turns that change into a level signal.

Where it fits: Float switches for simple level alarms in tanks; displacers for level and interface in separators, knock-out pots and condensate drums, at high temperature and pressure.
Where it does not: Dirty, sticky or crystallising liquids that clog moving parts, and — for displacers — liquids whose density changes.

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Conductive and HF level switches

Two electronic point-level switches with no moving parts. A conductive switch passes a small current between its electrode and the tank wall, or a second electrode, as soon as a conductive liquid touches it. An HF switch creates a high-frequency field at its tip and detects how the medium around the tip changes it.

Where it fits: Simple high and low limits in conductive liquids (conductive), and compact, hygienic switches for liquids, viscous media and pastes in food and pharmaceutical processes (HF).
Where it does not: Continuous level; and — for conductive switches — oils, solvents and other liquids that do not conduct electricity.

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Rotating paddle level switches

A small motor turns a paddle slowly inside the silo. When the bulk solid covers the paddle and stops it, the switch signals the level; when the material falls away, the paddle turns again. The same switch can mark a high or a low level.

Where it fits: High and low level in silos and hoppers of powders, granules and dense solids — the Delta Mobrey paddle switch covers a broad range of material densities.
Where it does not: Liquids, and continuous level — a paddle switch only tells you whether the material has reached it.

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At a glance

The typical properties of the seven technologies in one table, and their typical process temperature in one chart.

Level measurement technologies compared: what they give, contact with the medium, media, typical process temperature, output and main limitation
Technology	Gives	Contact with the medium	Media	Typical process temperature	Output	Main limitation
Hydrostatic level transmitters	Continuous level	Sensor in the liquid, at the bottom	Liquids, including wastewater and sludge	−20…+100 °C	4–20 mA, HART	Reads level correctly only at a known, steady density; in a closed tank the gas pressure must be subtracted
Radar level	Continuous level	Non-contact; guided wave: a probe in the medium	Liquids and bulk solids	−40…+200 °C	4–20 mA, HART	Higher cost; build-up on the antenna and obstacles in the tank need care when installing
Ultrasonic level	Continuous level	Non-contact	Liquids, wells, sumps and open channels	−40…+80 °C	4–20 mA, HART	Foam, vapour, dust and turbulence weaken or scatter the echo; a blind zone just below the sensor
Vibrating level switches	Point level (switch)	Fork or rod in the medium	Liquids, powders and granules	−40…+120 °C	Relay or transistor (PNP/NPN)	Point level only; heavy, hard build-up between the tines can hold the switch
Floats and displacers	Point level or continuous	Float or displacer in the liquid	Clean liquids and liquid–liquid interfaces	−60…+320 °C	Contact (float switch); 4–20 mA, HART (displacer)	Moving parts in the liquid; a displacer reads correctly only at a known density
Conductive and HF level switches	Point level (switch)	Probe in the medium	Conductive liquids (conductive); liquids and pastes (HF)	−30…+150 °C	Relay or transistor (PNP/NPN)	Point level only; a conductive switch needs a liquid that conducts electricity
Rotating paddle	Point level (switch)	Paddle in the bulk solid	Bulk solids, from light powders to dense solids	−20…+80 °C	Contact	Point level in solids only; moving parts, and the paddle must suit the bulk density of the material

Typical process temperatures, not a model commitment; several technologies have versions for higher temperatures. We confirm the figures of each instrument for your application.

Typical process temperature by technology

Typical process temperature of each technology, in °C. Versions for higher temperatures exist for several of them.

Hydrostatic level transmitters−20…+100 °C
Radar level−40…+200 °C
Ultrasonic level−40…+80 °C
Vibrating level switches−40…+120 °C
Floats and displacers−60…+320 °C
Conductive and HF level switches−30…+150 °C
Rotating paddle−20…+80 °C

Application guide: which technology for which job

Each level technology exists to solve a particular problem. Start from yours — the medium, the vessel, whether you need the level or a limit — and see which technology we would suggest, which one to avoid, and why.

Application guide for level measurement: for each application, the technology we suggest and why, and the one to avoid and why
Application	We suggest	Why	Avoid	Why not
Foam, vapour or condensation, or a medium that must not be touched	Radar level	Microwaves pass through vapour and are hardly affected by the temperature or pressure of the gas above the liquid; non-contact radar touches nothing, and under heavy foam guided wave radar keeps its pulse on the probe.	Ultrasonic level	Foam absorbs the sound pulse, and vapour and temperature change the speed of sound — the echo is lost or the distance is wrong.
Powders, granules and bulk solids in silos and hoppers	Radar level Vibrating level switches Rotating paddle	Radar gives the continuous level of solids; for high and low limits, a vibrating rod such as the Delta Mobrey VLS, a fork such as the DMS & DMT, or a rotating paddle switch.	Hydrostatic level transmitters Floats and displacers	Both work only in liquids: a hydrostatic sensor weighs a liquid column, and a float or a displacer needs buoyancy.
Wells, sumps and wastewater	Hydrostatic level transmitters Ultrasonic level	A submersible probe such as the Klay AquaCer or HydroCer, made for water, wastewater and sludge, or the Delta Mobrey DSGE for tanks, deep wells and piezometers; or, with no contact at all, an ultrasonic transmitter such as the DMSP900SH, made for wet wells and sumps.	—	Any probe in the liquid must cope with rags, grease and sludge: hang a submersible probe above the bottom sludge, and keep moving parts out of the flow.
Hygienic process: food, dairy, beverages, pharmaceuticals	Hydrostatic level transmitters Conductive and HF level switches Vibrating level switches	A flush diaphragm on a hygienic connection, as on the Klay 8000-SAN; compact switches such as the KMW, with EHEDG-compliant connections; and tuning forks with smooth wetted parts (Ra below 1.5 μm) and hygienic connections, such as the Klay Vibraswitch.	Floats and displacers	Moving parts, hinges and chambers leave crevices where product stays and cleaning cannot reach.
Overfill and dry-run protection: high and low alarms	Vibrating level switches Conductive and HF level switches	A separate switch keeps protecting the tank or the pump even if the continuous measurement or the control system fails, and an electronic switch has no moving parts to stick.	—	Do not rely only on an alarm from the continuous level transmitter: one failure would then take away both the measurement and the protection.
Interface between two liquids: oil and water in separators	Floats and displacers Radar level	A displacer such as the Mobrey MLT100 measures interfaces with a specific gravity difference as low as 0.1; guided wave radar sees both the surface and the interface along its probe.	Ultrasonic level	Sound reflects from the top surface only; an ultrasonic transmitter cannot see the interface below it.
A liquid whose density changes: mixtures, batches, temperature swings	Radar level Ultrasonic level	They measure the distance to the surface, so the density of the liquid does not enter the reading.	Hydrostatic level transmitters Floats and displacers	The pressure of a liquid column and the buoyancy on a displacer both depend on density: a lighter liquid reads as a lower level.
Sticky, viscous or coating media: sludge, syrups, resins, pastes	Radar level Conductive and HF level switches	Non-contact radar does not touch the medium; for limits, HF switches such as the KMW are made for liquids and viscous media, with a longer tip for viscous media and adjustable sensitivity.	Floats and displacers	A coating on a float or a displacer changes its weight and can stop it from moving.
A simple limit in a conductive liquid: water, wastewater and other water-based liquids	Conductive and HF level switches	A conductive switch such as the Klay FLX is the simplest electronic switch: one probe, a relay or PNP output and adjustable sensitivity, with insertion lengths from 20 mm to 2 m.	—	Not for oils, solvents or other liquids that do not conduct electricity; there, use a vibrating or an HF switch.
High temperature or high pressure	Floats and displacers Radar level	The Mobrey MLT100 displacer works from −60 to +320 °C and from full vacuum to 200 bar; radar is hardly affected by the temperature or pressure of the gas above the liquid.	Ultrasonic level	The transducer has a limited temperature range, and the speed of sound changes with the temperature and the pressure of the gas.

Continuous measurement or a point-level switch?

A level transmitter gives the level at every moment, as a 4–20 mA signal or over a fieldbus: for inventory, for controlling a pump or a valve, for trends and alarms in the control system. A level switch only says whether the medium has reached a point — high, low, overfill, dry-run.

Most tanks need both. The transmitter runs the process; separate switches at the critical points protect the tank and the pumps on their own, so that one failure cannot take away both the measurement and the protection.

Continuous level: hydrostatic transmitters, radar, ultrasonic transmitters and displacers.
Point level: vibrating, float, conductive, HF and rotating paddle switches.
A switch is simpler to set up: it needs no range, only the point where it is mounted.
For overfill protection, use a separate switch, not only an alarm on the continuous measurement.

How to choose a level technology

The right technology follows from the application. These are the criteria we look at:

The level or a limit
A value for control and inventory needs a transmitter; an alarm, a pump start or overfill protection can be a switch.
Liquid or solid
Hydrostatic sensors, floats and displacers work only in liquids; radar, vibrating rods and rotating paddles also in powders and granules.
The vessel
Open or closed, under pressure or vacuum, its height, internals such as agitators and ladders, and the nozzles available for mounting.
The medium
Its density and whether it changes, its conductivity, foam, vapour, build-up and viscosity — and whether it may be touched at all.
Temperature and pressure
The process conditions decide the technology and the version — see the typical temperatures in the chart above.
Environment and signal
Hazardous areas, hygiene rules, marine use, and the output the control system expects: 4–20 mA with HART, a relay or a transistor output.

Level measurement: frequently asked questions

Need help choosing a level instrument?

Tell us about the medium, the vessel and the operating conditions, and our engineers will recommend the right technology and the specific instrument.