An inline ultrasonic flow meter (DUK series) is published at a measuring range of 0.01-15 to 2.5-630 l/min, with brass or stainless steel 1.4408 wetted parts, a 16 bar maximum operating pressure, a 90 deg C fluid temperature ceiling, and a stated accuracy of +-0.7% of reading plus 0.7% of full scale [S2]. Output options on the same product include switching, frequency, analog, IO-Link (U-PACE electronics), and an integrated PT1000 with dosing and totalizer functions [S2].
This guide covers clamp-on, inline (wetted), and insertion-style ultrasonic flowmeters used in clean-liquid custody, HVAC, water treatment, and chemical dosing. The DUK series sits in the lower-mid accuracy band typical of transit-time industrial units, with cCSAus ORDLOC and IECEE CB-Scheme approvals (certificates CA38941CSA / 80190658) supporting cross-region plant deployment [S2]. Chinese OEM catalogues (Teren, Astmeters) split ultrasonic flowmeters into handheld, wall-mount, and modular sub-families, and also offer a dedicated [ultrasonic water meter](/encyclopedia/ultrasonic-flow-meter.html) line for utility billing.
Operating principle and what an ultrasonic flow meter actually measures
Transit-time ultrasonic flowmeters measure the difference in acoustic propagation time between pulses traveling with and against the flow, then convert that delta into a volumetric flow rate using the pipe geometry and sound speed. The technology is inherently non-intrusive when clamp-on, requires no electrical conductivity from the fluid, and is bidirectional by construction [S4]. The free-dictionary engineering definition describes a flowmeter simply as an instrument that measures the rate of flow of a liquid or gas within a pipe or tube, and the ultrasonic variant applies that role using acoustic rather than mechanical or electromagnetic means [S4].
For an ultrasonic flow meter the dominant error sources are flow profile (laminar vs turbulent), gas or solids entrainment, pipe wall condition, and signal-to-noise at low flows. Inline wetted units like the DUK eliminate clamp-on uncertainty by calibrating against a known bore, which is why the published +-0.7% of reading + 0.7% of full scale figure can be held at a fluid temperature up to 90 deg C and pressures up to 16 bar [S2]. Above 90 deg C, the electronics and the integrated PT1000 temperature compensation path are the limiting envelope, not the transducer physics.
Three physical variants: clamp-on, inline, insertion
Clamp-on units mount externally on existing pipework, require no pipe cut, and are the default for retrofit on steel, stainless, or PVC lines in water and HVAC service. Inline wetted units such as the DUK thread into the line at G 1/2 to G 3 (or 1/2 in. to 3 in. NPT) female connections and are preferred where the spec calls for the rated +-0.7% of reading accuracy and repeatability under changing fluid temperature [S2]. Insertion probes sit between flanges or in hot-tap fittings and are typically used on larger diameter lines where a full-bore wetted meter is uneconomic.
For non-utility process use, an inline unit with IO-Link (U-PACE electronics) gives a single twisted-pair digital output carrying flow, temperature from the integrated PT1000, part and grand totalizer, and dosing state, replacing separate 4-20 mA plus pulse plus temperature channels [S2]. When a buyer compares an electromagnetic flowmeter against an ultrasonic one, the gating question is fluid conductivity: magnetic units need a conductive liquid and a full-pipe bore, while ultrasonic units do not, but cannot tolerate large gas pockets or heavy solids.
Selection criteria you can put on a datasheet
Specifying an ultrasonic flow meter should start with five numeric gates: minimum and maximum flow in l/min or m3/h, line size in DN or inches, fluid conductivity, maximum operating temperature, and required accuracy class. The published DUK envelope is 0.01-15 to 2.5-630 l/min, G 1/2 to G 3 female thread, brass or stainless steel 1.4408 wetted parts, 16 bar p max, 90 deg C t max, and +-0.7% of reading + 0.7% of full scale [S2]. Outside any of those five numbers, the buyer is on a different product, not the same one with longer cables.
Output stack is the second gate. Modern inline ultrasonic units are expected to provide at least one of IO-Link, HART, Modbus RTU over RS485, or 4-20 mA plus pulse; the DUK publishes switching, frequency, analog, and IO-Link on a single rotatable TFT display module [S2]. A third gate is approvals: cCSAus ORDLOC and IECEE CB-Scheme are useful for North American hazardous-area and global CB-bypass acceptance respectively, while ATEX or IECEx certification must be added for Zone 1 group IIA-IIB or IIC hydrogen service, and is not implied by the DUK ORDLOC or CB listing [S2].
Comparison: clamp-on vs inline vs insertion on four decision criteria
On installation cost, clamp-on wins because no pipe cut, no flange, no shutdown beyond a planned depressurize. On accuracy, inline wetted units win because the calibration bore is fixed and known, with the DUK published at +-0.7% of reading + 0.7% of full scale [S2]. On suitability for large-bore retrofit (DN200 and above), insertion wins because a single probe scales with line size, where an inline unit would be a heavy, expensive spool piece. On suitability for fluids with entrained gas or high solids, none of the three are first choice; Coriolis or electromagnetic meters move to the front, with electromagnetic flowmeter being the direct rival whenever the fluid is conductive and clean.
Buyers in water utility and HVAC retrofit tend to default to clamp-on because pipe sizes are large and shutdown windows are short. Process and pharmaceutical buyers with G 1/2 to G 3 lines and clean Newtonian media, including demineralised water and water-glycol mixtures, default to inline units because the published accuracy and the IO-Link digital output simplify dosing and validation [S2]. The clamp-on should NOT be picked when the published accuracy window matters at low flow (under 0.3 m/s typical) on small-bore stainless tubing, because transit-time signal amplitude drops and the +-0.7% of reading figure degrades; in that band, the inline wetted design is the safer spec.
Real use cases and the standards that apply
Inline ultrasonic meters are widely used in pharmaceutical, food, and water-treatment skids where media conductivity is low or variable, and dosing functions are required; the DUK series explicitly markets dosing, part and grand totalizer, and bidirectional measurement for these duties [S2]. Wall-mount and handheld ultrasonic flowmeters (DTI-100F, DTI-200F/TDS-200F, DTI-100M modular, and DTI-200P portable series in the Teren catalogue) cover HVAC balancing, leakage surveys, and field spot-checks where portability or wall space matters more than permanent custody-grade accuracy [S1].
Where the meter feeds a billing or custody transfer, the buyer must add ISO 4064 or OIML R49 reference for water meters, and API MPMS or national weights-and-measures rules for hydrocarbon service; the KOBOLD DUK is a process instrument, not a custody transfer meter, and the spec sheet reflects that scope [S2]. For hazardous-area service, ATEX 2014/34/EU and the IEC 60079 series apply to the electronics, while the IECEx scheme covers non-EU markets; CSA ORDLOC and CB-Scheme cover North America and the IECEE member countries respectively, and these approvals are independent of ATEX and IECEx for Zone-rated service [S2]. The Teren ultrasonic water meter line in the same catalogue is built for utility metering rather than process dosing, and should be evaluated under ISO 4064 rather than the DUK's process specification [S1].
Limitations, failure modes, and what to keep an eye on
Ultrasonic meters fail quietly in three modes: signal loss when the pipe is not full, drift when air or solids accumulate on the wetted transducer face, and temperature compensation error when the fluid heat capacity differs from the calibration fluid. Inline units like the DUK mitigate the temperature path with the integrated PT1000 and on-site media parameter programming, but the operator must enter the new media set; the factory calibration to water does not automatically apply to water-glycol mixtures [S2].
Clamp-on units add a fourth failure mode: coupling compound aging, which changes the acoustic impedance between probe and pipe wall and is the most common field service call. Handheld and portable units in the DTI-200P / DTI-100F / TDS-200F class are not specified to the same accuracy band as inline units, and should be treated as survey tools rather than permanent meters [S1]. When an application requires a different sensing principle entirely (conductive dirty liquids, slurries, custody-grade hydrocarbon), the relevant comparison articles cover an electromagnetic flowmeter or Coriolis alternative rather than a retrofit of an ultrasonic unit.
Sourcing, standards, and a shortlist logic
For a 2026 shortlist, the buyer should lock down the four numeric gates first (flow range, line size, fluid conductivity class, max temperature), then the output protocol (IO-Link vs 4-20 mA plus pulse vs HART), then the approval zone (cCSAus, IECEE CB, ATEX, IECEx). The DUK series fits when those gates land inside 0.01-15 to 2.5-630 l/min, G 1/2 to G 3, 16 bar, 90 deg C, and +-0.7% of reading [S2]. For meter pricing context and total-ownership cost on the rival magnetic technology, the magnetic flow meter price 2026 cost drivers article lines up the comparable spend profile. For utility-class cold-chain or outdoor metering on larger lines, the cold chain pallet rack selection article is not a fit, while the AGV robot selection 2026 spec map sits in a different category and is not relevant to this meter decision.
Chinese OEM lines (Dalian Teren DTI-100F, DTI-200F/TDS-200F, DTI-100M, DTI-200P, and Astmeters flowmeters) remain the volume play in handheld and wall-mount categories, with the inline process-grade segment still anchored by European makers [S1][S3].