A float switch gives a level alarm at one point, simply and with no power needed to sense. A displacer transmitter gives a continuous level or interface signal, and works at high temperature and pressure.

Why does a displacer need the density?

Because the buoyancy on it is the weight of the liquid it displaces. A lighter liquid lifts it less, and the transmitter would read a lower level. It is set up for the density at operating conditions.

Can a displacer measure an interface?

Yes, when it is fully submerged: then the buoyancy changes with the position of the interface. The MLT100 measures interfaces with a specific gravity difference as low as 0.1.

What is an external chamber for?

It holds the displacer outside the vessel, at the same level, and can be isolated with valves — so the transmitter can be serviced or checked without opening or emptying the vessel.

LEVEL MEASUREMENT TECHNOLOGIES

Floats and displacers: level switches and level transmitters that use buoyancy

Floats and displacers measure level with buoyancy — a simple, proven principle that is still the right one for many jobs. A float switch gives a simple, robust level alarm; a displacer transmitter gives the level, or the interface between two liquids, at high temperature and pressure. Our catalogue includes the Delta Mobrey horizontal magnetic float switches and the Mobrey MLT100 displacer level transmitter.

See the floats and displacers

How floats and displacers work

A float is lighter than the liquid and rides on its surface. On a horizontal float switch, the float sits on an arm through the tank wall; when the level reaches it, the float rises and a magnet operates a contact outside the liquid. The Delta Mobrey horizontal magnetic float switches use this magnetic actuation for level alarms and control.

A displacer is heavier than the liquid and does not float. It hangs from a spring or a torque element, and as the liquid rises around it, it is lifted by a force equal to the weight of the liquid it displaces — the principle of Archimedes. Its apparent weight falls in proportion to the level, and the transmitter turns that change into a continuous signal.

The Mobrey MLT100 combines this buoyancy principle with LVDT-based electronics and HART. It works from −60 to +320 °C and from full vacuum to 200 bar, for liquids with a specific gravity from 0.5 to 1.5; it mounts directly on the vessel or in an external chamber, and carries ATEX Ex db approval.

Because buoyancy depends on density, a displacer can also measure an interface: fully submerged, it reads where the heavier liquid ends and the lighter one begins. The MLT100 measures interfaces with a specific gravity difference as low as 0.1.

The float, riding on the surface of the liquid
The contact of the float switch, changing state as the float rises
On the displacer: its weight (down) and the buoyancy of the liquid (up)
The signal of the displacer transmitter rises in a straight line with the level

Where floats and displacers fit — and where they do not

A strong choice for

Simple high and low level alarms in tanks, with float switches
Level and interface in separators, knock-out pots and condensate drums
High temperature and pressure: the MLT100 works up to +320 °C and 200 bar
External chambers that can be isolated for maintenance without opening the vessel
Clean liquids of known, steady density

Think twice when

The liquid is dirty, sticky or crystallising, and would clog the float or coat the displacer
The density of the liquid changes: the displacer reads it as a change of level
The surface is turbulent or foaming and keeps a float moving
The medium is a powder or a bulk solid: there is no buoyancy

How it compares with the other technologies

Typical process temperature of each technology, in °C. The technology of this article is highlighted.

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 (the technology of this article)−60…+320 °C
Conductive and HF level switches−30…+150 °C
Rotating paddle−20…+80 °C

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

All technologies

Installing floats and displacers

A displacer can hang directly in the vessel, in a stilling well, or in an external chamber connected to the vessel at the top and the bottom. A chamber with isolating valves lets you service the transmitter without emptying the vessel.

Then:

In an external chamberThe chamber, connected at the top and the bottom through isolating valves, holds the same level as the vessel.

Interface measurementFully submerged, the displacer spans the interface and measures where the heavier liquid begins.

Side-mounted float switchThrough the wall at the alarm level: the float rises with the liquid and falls back when the level drops.

Give us the densities of the liquids at operating temperature: the displacer is sized and set up for them.
Keep the chamber connections clear of deposits, and drain the chamber when you service it.
Mount a float switch where the surface is calm, away from the inlet.
After installation, check that the float or the displacer moves freely.

Floats and displacers by industry

Oil & GasSeparators, knock-out pots and storage vessels, with displacers for the level and the oil–water interface.

Power & EnergyCondensate drums and tanks, at high temperature and pressure.

ChemicalProcess vessels and storage tanks, with float switches for level alarms.

Water & WastewaterLevel alarms on tanks and reservoirs of clean water, with float switches.

Floats and displacers in our catalogue

The float switches and displacer transmitters in our catalogue, each with its own page and technical data.

All floats and displacers

DELTA MOBREYMobrey MLT100 Displacer Level Transmitter Delta MobreyAdvanced HART displacer level transmitter combining proven buoyancy measurement with robust, low-maintenance electronics. View product

DELTA MOBREYHorizontal Magnetic Float SwitchesRobust horizontal magnetic float switches engineered for dependable level detection in demanding industrial applications. View product

Floats and displacers: frequently asked questions

Ask about a float switch or a displacer

Tell us the liquid and its density, the vessel, whether you need an alarm, the level or an interface, the process temperature and pressure, and the connection — and our engineers will suggest the right instrument, with price and delivery time.