Couplants

Couplants are materials placed between the ultrasonic transducer and the test surface to facilitate the transmission of ultrasonic energy. By eliminating air gaps and providing acoustic impedance matching, couplants reduce reflection losses and attenuation at interfaces, improving signal strength and image or defect detection.

Common couplants include liquids such as water, water-based gels, mineral oil, and silicone-based gels; semi-solid and solid options like dry pads and silicone sheets are used when liquids are impractical. In medical imaging, diagnostic ultrasound gel is typical, while in underwater or other specialized applications water can serve as the couplant. The choice depends on the inspection environment, surface geometry, and temperature conditions.

Key properties of effective couplants include low ultrasonic attenuation, suitable viscosity to fill crevices and maintain contact, chemical compatibility with the surface and transducer, and an appropriate temperature range. Hydration and wetting are important to prevent air pockets, while excessive viscosity can hinder wetting and reduce coupling efficiency. Some couplants are designed for single use, others are reusable after cleaning.

Applications span non-destructive testing, where couplants enable flaw detection in metals, composites, and welds, to medical ultrasound imaging, where gels improve contact with skin and acoustic transmission. In addition, specific couplants are chosen to minimize residues, facilitate cleaning, and meet safety or biocompatibility requirements.

Selection and handling involve matching acoustic properties with the transducer and test material, ensuring cleanliness to avoid contaminants, and following user guidelines for application, temperature limits, and disposal.