How much straight pipe does an ultrasonic flowmeter need?

Typically 10 D upstream and 5 D downstream, counted from the nearest bend, valve or reducer — more after disturbances that set the flow swirling, such as two bends in different planes or a partly open valve. Meters with more paths need less, and a flow conditioner can shorten the run further.

Can an ultrasonic sensor measure flow in an open channel?

Yes, indirectly. A non-contact sensor above the water measures the level upstream of a weir or flume, and the transmitter converts it to flow using the known level-to-flow relationship of that structure. That is what the DMSP900FH in our catalogue does, with a measuring range of up to 15 m, IP68 protection and intrinsic safety certification for hazardous areas.

FLOW MEASUREMENT TECHNOLOGIES

Ultrasonic flowmeters: measuring liquids and gases with sound, with nothing in the flow path

An ultrasonic flowmeter sends pulses of sound through the fluid and works out the flow velocity from how long they take to cross the pipe. Nothing protrudes into the pipe and nothing moves, so there is no pressure loss and no wear — and, unlike an electromagnetic meter, it also measures liquids that do not conduct electricity, as well as gases. The range runs from in-line meters for process lines and large water mains, through multi-path meters for natural gas custody transfer, to clamp-on meters mounted on the outside of the pipe.

Ultrasonic flowmeters in our catalogue

How an ultrasonic flowmeter works

Most industrial ultrasonic flowmeters use the transit-time method. Two transducers sit on opposite sides of the pipe, one further downstream than the other, so that the sound path between them crosses the flow at an angle. Each transducer both sends and receives: first a pulse travels from A to B, with the flow, then from B to A, against it.

Sound travelling with the flow arrives a little sooner; sound travelling against it arrives a little later. The difference between the two transit times, Δt, is proportional to the flow velocity along the path — twice the velocity, twice the difference. Because the transmitter uses both transit times, the speed of sound in the fluid cancels out of the calculation: the reading does not depend on it, even when the temperature or composition of the fluid changes it. The differences are tiny, so the electronics have to time them very precisely.

A single path measures the average velocity along one line, and the transmitter corrects it to the average over the whole cross-section using the expected flow profile. Meters with several paths sample more of the cross-section, so they are less affected by a distorted profile and reach higher accuracy — the ALTOSONIC V12 gas meter in our catalogue uses 12 paths. The average velocity multiplied by the cross-section gives the volume flow.

There are other ultrasonic methods too. Clamp-on meters work on the same transit-time principle, but their transducers are fixed to the outside of the pipe and the sound passes through the pipe wall. Doppler meters work differently: they need particles or bubbles in the liquid to reflect the sound, and measure the shift in its frequency — they suit dirty liquids, with lower accuracy. And in open channels, a non-contact ultrasonic sensor above the water measures the level upstream of a weir or flume, and the flow is calculated from that level.

The liquid or gas flowing through the pipe
Transducers A and B: each one sends a pulse and receives the other’s
The sound path, diagonally across the pipe at an angle to the flow
The two received pulses: the one sent with the flow (solid) arrives before the one sent against it (dashed); the gap between them is Δt
The time difference Δt rises in proportion to the flow velocity v

Where an ultrasonic flowmeter fits — and where it does not

A strong choice for

Large water mains and pipelines: with nothing inside the pipe there is no pressure loss, and the OPTISONIC 3400 in our catalogue goes up to DN 4000
Liquids that do not conduct electricity — oils, fuels, solvents, demineralised water — where an electromagnetic flowmeter cannot measure
Gases, including natural gas custody transfer, with multi-path meters such as the ALTOSONIC V12
Clean or lightly contaminated process liquids in oil and gas, power and chemical plants
Existing lines where the pipe cannot be cut or the process stopped: clamp-on versions mount on the outside — also for temporary or check measurements
Open channels, wet wells and sumps, with a non-contact sensor above the water (DMSP900FH)

Think twice when

The liquid carries a lot of gas bubbles or solids: they scatter and absorb the sound, and a transit-time meter can lose its signal
There is little straight pipe after bends, pumps or valves: a distorted profile, or flow that swirls, misleads a meter with few paths
The pipe may run partly full: an in-line meter assumes the whole cross-section is full. In open channels and sewers, flow is measured from the level instead
You are measuring steam, or the process is very hot: the transducers set the limit — −40…+120 °C for the OPTISONIC 3400 in our catalogue, for example, and up to +175 °C for the ALTOSONIC V12
The pipe is small: the sound path is short and the time difference tiny. The in-line ultrasonic meter in our catalogue starts at DN 25; for smaller lines an electromagnetic or Coriolis meter is usually the better fit
You need mass flow: an ultrasonic flowmeter measures volume, so mass needs a known density — or a Coriolis flowmeter

How it compares with the other technologies

Typical accuracy of each flow measurement technology, ± % of the reading, on a logarithmic scale. The further left, the more accurate. The technology of this article is highlighted.

Coriolis±0.1–0.2%
Electromagnetic±0.2–0.5%
Ultrasonic (the technology of this article)±0.5–2%
Differential pressure±0.5–2%
Variable area±1.6–4% *
Vortex±0.75–2%
Thermal mass±1–3%
Positive displacement±0.2–0.5%
Turbine±0.5–1%

Typical ranges for liquids (thermal mass: gases), ± % of the reading; for variable area, the accuracy class (*). The accuracy of a specific instrument depends on the model, the size and the installation — we confirm it for your application.

All technologies

Installing an ultrasonic flowmeter

An ultrasonic flowmeter measures velocity along one or more lines, so it needs a settled flow profile. A typical requirement is 10 D of straight pipe upstream and 5 D downstream (D = nominal diameter) — more after two bends in different planes, a pump or a partly open valve, which set the flow swirling. Meters with more paths need less, and a flow conditioner can shorten the run; for each model, the manufacturer’s figure is what counts.

With liquids, the pipe must stay full and the sound path must stay clear of gas at the top of the pipe and of sediment at the bottom. Clamp-on meters add one more condition: the sound has to pass through the pipe wall, so the pipe material, wall thickness and inside diameter must be known. In open channels, the level is measured where the water is calm, upstream of the weir or flume:

Straight runsTypically at least 10 D of straight pipe before the meter and 5 D after it, counted from the nearest bend, valve or reducer.

Transducers on the horizontal axisIn a horizontal line carrying liquid, turn the meter so that the transducers sit at the sides: gas bubbles rise to the top and solids settle at the bottom, both away from the sound path.

Clamp-on mountingClamp-on transducers are fixed to the outside of the pipe, with a coupling compound between them and the wall — no cutting and no shutdown. In this arrangement the sound reflects off the opposite wall and returns to the same side.

Open channelsA non-contact sensor above the water measures the level upstream of a weir or flume; the transmitter converts it to flow using the known level-to-flow relationship of that structure.

With liquids, install the meter where the pipe stays full: in a rising line or at a low point, never at the highest point of the line or in a downward line that may run empty.
Fit control valves after the meter, and keep it away from sources of ultrasonic noise, such as partly closed valves and pressure-reducing stations, which can drown out the signal.
For a clamp-on meter, pick a straight length of pipe in good condition: heavy scale, internal corrosion or a thick lining can weaken the signal, and any error in the inside diameter the transmitter assumes goes straight into the reading.
In an open channel, mount the sensor on a rigid bracket, square to the water surface and clear of the walls, where the surface is calm and far enough upstream that it has not yet started to dip towards the weir — and high enough that the highest water level stays outside its blanking distance.

Ultrasonic flowmeters by industry

Oil & GasCustody transfer of natural gas at transmission and distribution stations, with multi-path meters approved for it — the ALTOSONIC V12 carries AGA, ISO 17089, MI-002 and OIML R137 certification and is rated up to 450 bar. In-line meters for hydrocarbons and other process liquids (OPTISONIC 3400).

Water & WastewaterLarge water mains and transfer lines, where pressure loss costs pumping energy; clamp-on meters to check existing meters and for temporary surveys; and open channels, wet wells and sumps at treatment plants and pumping stations, with a non-contact sensor (DMSP900FH).

Power & EnergyCooling water, demineralised water and fuel lines in power plants, where the meter adds no pressure loss and has no parts to wear (OPTISONIC 3400).

ChemicalSolvents and other liquids that do not conduct electricity, which rule out an electromagnetic meter, and process lines where nothing should protrude into the flow (OPTISONIC 3400).

Food & BeverageProcess water and utilities, and lines where nothing should touch the product: a clamp-on meter measures through the pipe wall, with no contact and nothing to clean.

Mining, Minerals & MetalsProcess water, raw water and reclaimed water lines — often with clamp-on meters, which can be moved from line to line for surveys. For ore slurries and tailings, an electromagnetic flowmeter is usually the safer choice.

Ultrasonic flowmeters we supply

Ultrasonic instruments from our catalogue — for process liquids and large pipes, gas custody transfer, and non-contact level and flow measurement in open channels.

All ultrasonic flowmeters

KROHNEALTOSONIC V1212-path ultrasonic gas flowmeter engineered for high-accuracy custody transfer applications. View product

KROHNEOPTISONIC 3400Ultrasonic flowmeter for a broad range of process liquids. View product

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 flowmeters: frequently asked questions

It depends on the type and the installation. Our comparison table gives ±0.5–2% of the reading: the lower end applies to in-line meters such as the OPTISONIC 3400 in our catalogue, specified at ±0.5%, and the upper end to clamp-on meters, whose accuracy depends on how well the pipe is known. Multi-path meters for gas custody transfer are approved for billing — the ALTOSONIC V12 to OIML R137 and MI-002. We confirm the accuracy for your application before you order.

Yes — gas is one of the main uses of ultrasonic flowmeters, from process gases to natural gas custody transfer. It does need meters built for it: sound passes less readily from a transducer into a gas, so the transducers and electronics differ from those for liquids. Like every volumetric meter, it measures actual volume; a flow computer converts that to volume at standard conditions, or to energy, using the pressure, temperature and composition of the gas.

A transit-time meter whose transducers are fixed to the outside of an existing pipe, so it goes in without cutting the pipe or stopping the process. It suits existing installations, very large pipes and temporary or check measurements. On the other hand, its accuracy depends on how well the pipe is known — material, wall thickness, inside diameter, lining and condition — and is usually lower than that of an in-line meter. Our catalogue lists in-line meters; for a clamp-on measurement, tell us about the pipe and the application.

Ask about ultrasonic flowmeters

Tell us the fluid (liquid or gas), the pipe size and material, the flow range, the pressure and the temperature — or, for an open channel, the type of weir or flume — and our engineers will suggest the right meter, with price and delivery time.