Why does foam stop an ultrasonic transmitter?

Foam is full of small bubbles that absorb and scatter sound. The pulse loses most of its energy and the echo is too weak, or it comes back from the top of the foam. Under foam, radar — especially guided wave radar — is the better choice.

What is the blind zone?

The distance right below the transducer where it cannot measure, because it is still ringing from its own pulse when an echo from that close would return. Mount the transducer so that the highest level stays below it.

Can the DMSP900SH measure flow too?

The DMSP900SH measures level; its twin, the DMSP900FH, measures open channel flow. Both are non-contact ultrasonic transmitters, two-wire, with HART.

Does temperature affect the reading?

Yes: the speed of sound in air rises by about 0.17 % per degree. Ultrasonic transmitters measure the air temperature and correct for it; a large temperature difference between the transducer and the surface still adds error.

LEVEL MEASUREMENT TECHNOLOGIES

Ultrasonic level transmitters: level from the echo of a sound pulse

An ultrasonic level transmitter sits above the liquid and measures the time a sound pulse takes to reach the surface and come back. It gives continuous level with no contact, and it is a common choice for water and wastewater — wells, sumps, open tanks and open channels. Our catalogue includes the Delta Mobrey DMSP900SH, a two-wire ultrasonic level transmitter for wet wells and sumps.

See the ultrasonic level transmitters

How an ultrasonic level transmitter works

A transducer above the surface sends a short pulse of sound, above the range of hearing, and then listens for its echo from the liquid. The time between the two, multiplied by the speed of sound and halved, is the distance to the surface; the distance taken from the height of the tank is the level.

Sound travels at about 343 m/s in air at 20 °C, and faster as the air warms — by roughly 0.17 % for every degree. That is why ultrasonic transmitters measure the air temperature and correct for it.

Just below the transducer there is a blind zone: an echo from that close would return while the transducer is still ringing from its own pulse, so the highest level must stay below it. The Delta Mobrey DMSP900SH measures up to 15 metres, is two-wire loop powered with HART, and is IP68 and intrinsically safe certified.

The sound pulse leaving the transducer, and its echo returning from the surface
The blind zone just below the transducer, which the level must not enter
The electronics, which time the echo and correct for the temperature of the air
The 4–20 mA signal with HART to the control system

Where an ultrasonic level transmitter fits — and where it does not

A strong choice for

Wet wells, sumps and pumping stations
Open tanks and reservoirs of water and wastewater
Open channels and flumes, where the level gives the flow
Corrosive or dirty liquids, measured without contact
Places that may flood: the DMSP900SH is IP68

Think twice when

Foam covers the surface: it absorbs the pulse and the echo is lost
There is vapour, steam or a gas other than air, or strong temperature layers: they change the speed of sound
The tank is closed under pressure or vacuum, or the process temperature is very high
Dust, heavy turbulence or internals in the beam give weak or false echoes

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 (the technology of this article)−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

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 an ultrasonic level transmitter

Mount the transducer above the highest level plus the blind zone, pointing straight down at the surface, with the beam clear of the walls, ladders, pipes and the inlet stream.

Then:

In a wet wellAbove the pumps, higher than the highest level plus the blind zone, with the beam clear of the pump pipes.

Open channelUpstream of the flume or weir, where the surface is steady: the level there gives the flow.

A clear beamKeep the beam away from pipes, ladders and the wall, which return false echoes.

Keep the transducer perpendicular to the surface: a tilted one sends its echo away.
Shade it from direct sun, which heats the air below it unevenly.
Avoid points where foam collects, such as close to inlets.
For pump control, set the start and stop levels in the controller or the PLC, with enough distance between them.

Ultrasonic level transmitters by industry

Water & WastewaterWet wells, pumping stations, reservoirs and open channels at treatment plants.

ChemicalOpen tanks of corrosive liquids, measured without contact.

Power & EnergyCooling water basins and wastewater sumps.

Food & BeverageProcess water and wastewater tanks.

Ultrasonic level transmitters in our catalogue

The ultrasonic level transmitters in our catalogue, each with its own page and technical data.

DELTA MOBREYDMSP900SH Level and DMSP900FH Flow TransmittersNon-contact ultrasonic transmitter for level, tank content and open channel flow measurement in demanding wet-well applications. View product

Ultrasonic level transmitters: frequently asked questions

Ask about an ultrasonic level transmitter

Tell us the liquid, the tank, well or channel and its depth, the conditions above the surface (foam, vapour, temperature), the mounting and the output you need — and our engineers will suggest the right transmitter, with price and delivery time.