Transducers for Corrosion Thickness Gauges (Dual Element)

What dual element transducers are

Dual element transducers (also called “twin crystal”) are the category of ultrasonic probe designed specifically to measure remaining thickness on materials affected by corrosion. They differ from conventional probes in having inside them two piezoelectric crystals separated by an acoustic barrier: one crystal transmits the ultrasonic signal, the other receives the echo returning from the material.

This physical arrangement eliminates ring-down — the tail of noise produced by the transmitting crystal immediately after the emission pulse — and makes it possible to measure accurately even very small thicknesses and irregular, corroded or painted surfaces. For this reason dual element transducers are the de facto standard for corrosion inspection on piping, tanks, pipelines, ship hulls and pressure vessels.

The crystals are also mounted at a slight angle towards each other, optimising the focusing of the ultrasonic beam in the zone of interest and improving the response near the contact surface. This design feature also makes dual element probes ideal for measuring thin materials, or for inspecting components where the surface echo might otherwise mask the back-wall signal.

The Dakota range: frequencies, diameters and configurations

Dakota NDT produces a complete range of dual element transducers covering every need typical of predictive maintenance, quality inspection in production and periodic checks to sector standards. The family comprises over 90 models, distinguished by the combination of frequency, diameter, damping and connector configuration.

Frequencies available

• 1 MHz — for highly attenuating materials (grey cast iron, ductile iron, attenuating alloy steels) and for large thicknesses (over 200 mm)
• 2.25 MHz — a versatile low frequency, excellent on cast irons and attenuating alloys at medium thicknesses
• 5 MHz — the universal frequency, the standard choice for carbon steels and common stainless steels. It accounts for about 70% of the models in the catalogue
• 7.5 MHz — high resolution, for thin thicknesses or for detecting small pitting
• 10 MHz — the highest resolution available in dual element, for precision measurements on thin, homogeneous materials

Contact surface diameters

• 3/16″ (4.76 mm) — compact probes for restricted access
• 1/4″ (6.35 mm) — for measurements on small diameter tubes and curved components
• 1/2″ (12.7 mm) — the reference configuration for most corrosion applications
• 1″ (25.4 mm) — for large thicknesses and strongly attenuating materials

Types of damping

• Standard damping — the basic configuration for corrosion measurements on most surfaces
• Highly damped — high damping, used in the ThruPaint versions to measure through layers of paint, coating or light insulation without having to remove the covering
• Exxon Spec / High Resolution — special configurations with damping optimised for applications to an industrial specification (refinery tanks, oil and gas pipelines)

Connector configurations (potting)

• Potted Top — connector on top, ideal for vertical measurements on walls
• Potted Side — side connector, convenient for restricted horizontal access, under covers, in trenches
• Angled — connector at a preset angle, for inspections on curved surfaces or internal corners

Representative models from the catalogue

Below is a selection of representative models covering most industrial applications. For the complete catalogue of the 91 Dakota dual element codes available for prompt delivery, contact our technical office.

Compatible Dakota thickness gauges

Dual element transducers are designed to be used with every Dakota thickness gauge in the corrosion line:

• CX series — entry and mid-level gauges (CX2, CX4, CX6-DL, CX8-DL)
• ZX series — basic gauges with data logger (ZX-Series, ZX6-DL)
• CMX series — professional gauges (CMX-Series, CMX10-DL)
• MX series — rugged gauges for corrosion inspection (MX1-DL, MX2-DL)
• Sonic Testers for motorsport (R9, RX8-DL) for cylinder wall measurement

The FX70-DL/FX71-DL and FX81-DL flaw detectors, in thickness mode, also accept dual element probes for corrosion measurements integrated into the flaw detection process.

How to choose the right dual element transducer

Selecting the correct dual element transducer depends on five main factors: the type of material to be inspected, the expected thickness range, the presence of coatings or paint, the temperature of the surface and the geometry of the component. A wrong choice can lead to unstable readings, an inability to detect the back-wall, or damage to the probe.

• Carbon steels and standard stainless steels: 5 MHz, 1/2″ diameter, standard damping
• Grey cast iron or attenuating materials: 1 or 2.25 MHz, diameters of 1/2″ or larger
• Small diameter tubes: 5 MHz, 1/4″ diameter for better surface contact
• Measuring through paint or coating: the ThruPaint version with high damping
• Heavily corroded surfaces: 5 MHz high resolution, or 7.5 MHz on thin thicknesses
• High temperature (up to 343 °C): T-042-2700 series with dedicated couplant

For a general treatment, see our technical guide How to choose the right ultrasonic transducer for each application.

Typical applications

• Predictive maintenance of piping, tanks and pressure lines in refineries and petrochemical plants
• Pipeline inspection in oil and gas, searching for internal pitting and general loss of thickness
• Periodic inspection of ship hulls, offshore installations and marine infrastructure
• Corrosion checks on pressure vessels to the ASME, EN and API standards
• Thickness mapping on metal structures, bridges and penstocks
• Quality control in production of tubes, sheet and mechanical components
• High temperature measurements on plant in service, with no shutdown

Complementary accessories

For a complete measuring system, besides the transducer we recommend considering the complementary accessories of the Dakota ultrasonic family: standard and high-temperature couplants, calibration blocks in 4340 steel with a certificate, and replacement probe cables with LEMO 00 or MicroDot connectors.

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Request availability and a quotation: Contact us stating the model of your Dakota thickness gauge and the intended application. We offer immediate delivery times from our warehouses.