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Important Considerations for Couplant Selection

Couplant selection is a critical factor to insure reliable, repeatable UT inspection results and to optimize productivity.

Couplant is required due to the acoustic impedance mismatch between air in the interface with the transducer and the test object. If there were no couplant, nearly all of the energy from the transducer would be reflected back into the transducer and very little would be transmitted into the test material.

Couplant selection includes operating range, acoustic properties, viscosity, evaporation rate, environmental restrictions, corrosion inhibition, removal and cost.

  • Performance:  higher performance couplants can provide extended drying time, stronger coupling films and higher acoustic impedance.  Seldom are all three of these technical features available in the same product, therefore, it is helpful to choose the most important one or two.
  • Operating Range: the temperature of the part being inspected and the operating range of the couplant are critical factors in selecting a couplant.  Use of a high-temperature couplant outside the operating range, especially in a port or enclosed environment can result in an auto-ignition (fire without an ignition source).  Use of a couplant below the operating range can result in ice crystal formation on the part with acoustic scattering and noise.
  • Cost: for general inspections, costs can often be reduced using powdered couplants or bulk economy couplants.  Sometimes the lowest cost couplant is economical, in other cases it requires longer inspection times (due to couplant drying) re-inspection (due to signal variation) and preparation (such as mixing powder couplant).  The most economical couplant has the correct operating range, allows for a complete inspection without re-application and has a film strength appropriate for the type of scanning or spot inspection. 

Many UT applications require a specialty UT couplant.  For difficult inspections that require high acoustic impedance, shear wave, reduced surface noise, extreme temperature range or extended service life for sensors, we have the right couplant.

Phased Array Wedge Couplants: 

Forever Wedge is an ideal couplant for use between phased array or angle beam transducers and the wedge.  Forever Wedge couplant won’t leach out, is impervious to water and couplants and when applied correctly, often lasts the life of the wedge or until it is replaced.  Forever Wedge eliminates phased array element drop out due to wedge couplant failure. 

EchoPure is an ideal couplant for phased array scanning due to its strong film strength, slow drying characteristic and very broad temperature range.  Element drop out with phased array transducers can occur from dead elements in the transducer or a failure of the wedge-to-transducer or wedge-to-inspection part coupling.   While minimal PA element drop out at the edges of the array can often be tolerated, failure of transducer coupling where multiple adjacent elements are not coupled into the wedge, will impact inspection quality, reproducibility and reliability.  This drop out is often not detected and the technician continues scanning unaware of increased side lobes (or grating lobes) and altered beam steering characteristics.  EchoPure forms a strong film to prevent isolated air pockets due to localized coupling film separation.

High Temperature Couplant:  EchoTherm Extreme: is the only high temperature couplant with an auto-ignition temperature above 1300°F and an operating range up to 1250°F / 675°C. It is safe for use in all environments for all high temperature inspections including ports and under insulation.

High Impedance Couplant: The advantage of a high impedance couplant is that more of the acoustic energy is transmitted between the transducer (or delay line) and test object and less is reflected and scattered at the surface of the test object.  This is particularly important when:

  • The contact area for the delay line or transducer is small (pencil point probes measuring pit depths).
  • The test object surface is very rough (porous cast iron, scale, rough grinding and fiberglass).
  • Materials such as concrete which are high impedance, scatter acoustic energy and often have a rough surface.

Echo Z+ has an acoustic impedance above 3.5 M Rayls.  Water based couplants typically have an acoustic impedance between 1.5 and 1.8 M Rayls depending on the Glycerin content. 

Cryogenic UT Inspection:  CryoSoniX is the only ultrasonic couplant for use in cryogenic applications such as refrigerated ethylene. With an operating range of -200˚ to 60˚F (-129˚ to 16˚C), CryoSoniX enables repeatable and reliable inspections.

Normal Incidence Shear Wave Couplant: A normal incidence shear wave couplant requires a high elastic modulus to perform acceptably.  Currently there is one commercially available water-soluble couplant that meets this requirement, Echo Shear Wave Couplant.  

Flow Meter and Sensor Couplant:   For permanent or long-term use, a flow meter couplant should resist creep, have good resistance to weather and chemicals as well as a broad temperature range and ultimately be easily removed.  Water-based couplants are suitable only for short-term flow metering applications. Echo 8 ZH is not water-based and is an excellent couplant for flow metering sensors.

At Echo Ultrasonics, we have been helping people select the best couplant for specific applications for over 50 years.  Give us a call or email with your requirements and we will help you select the best couplant for the job.