Ultrasonic couplants are used in virtually all contact testing applications to facilitate the transmission of sound energy between the transducer and the test piece. Couplants will typically be moderately viscous, nontoxic liquids, gels, or pastes. Their use is necessary because sound energy at the ultrasonic frequencies typically used for nondestructive testing is not effectively transmitted through air. Aside from attenuation effects, air represents a severe acoustic impedance mismatch with respect to both transducer wearplates and typical test materials. Even an extremely thin air gap between the transducer and the test piece will prevent efficient sound energy transmission and make conventional testing impossible.

A number of common substances such as water, motor oil, grease, and even some commercial products like hair gel can be used as ultrasonic couplants in many applications. However, for best results in high temperature and normal incidence shear wave testing it is necessary to use specially formulated couplants, and testing in the nuclear industry requires couplants whose halogen and sulfer content is limited and documented. The Panametrics-NDT line of ultrasonic couplants includes five different products optimized for different applications. A description of each follows, listed according to their part code.

COUPLANT A - PROPYLENE GLYCOL This product is no longer sold by Olympus NDT, however generic propylene glycol is widely available from chemical suppliers. Propylene glycol is a good general purpose couplant that is often used in ambient temperature thickness gaging applications on smooth surfaces. The maximum recommended temperature for use of propylene glycol is approximately 200° F (90° C).

COUPLANT B - GLYCERIN Glycerin is also a general purpose couplant with both advantages and disadvantages as compared with propylene glycol. Glycerin's advantage is that it is more viscous and has a higher acoustic impedance, making it the preferred couplant for rough surfaces and highly attenuating materials. Glycerin has an acoustic impedance of 2.42 x 10⁵ gm-cm²/sec (versus 1.61 for propylene glycol, approximately 1.5 for motor oil, and 1.48 for water). Glycerin's acoustic impedance closely matches that of plastics, and it is closer to that of metal than other common coupling liquids, providing more efficient sound transmission between the transducer and the test piece. In metal testing, glycerin typically provides a 3 to 6 dB increase in signal strength as compared to propylene glycol.

The disadvantage of using glycerin is that if it is not removed from the test surface, it can promote rusting or corrosion in some metals by absorbing and holding water from the atmosphere. It can also support growth of mold if not removed.

While glycerin is often recommended for testing of castings due to its good acoustic properties, care should be taken to wash it of after use. Note that this corrosion effect does not occur in plastics, fiberglass, or composites, so glycerin may be freely used on those materials. The maximum recommended temperature for glycerin is approximately 200° F (90° C).

COUPLANT C - SILICONE OIL This product is no longer sold by Olympus NDT.

COUPLANT D - GEL Gel type couplants are often recommended for rough surfaces such as sand-cast metals and fiberglass layups, where their high viscosity and relatively high acoustic impedance will maximize sound coupling in a situation where the transducer cannot make smooth contact with the test surface. Gel couplants are also usually employed in weld inspections where it is necessary to slide the transducer across a wide surface area. Here the advantage is simply that gel can be quicky spread across a large area, and that it will maintain good coupling as the transducer is scanned along the weld. Gels are also useful in inspecting overhead surfaces or vertical walls, since they won't drip or run. Most gel couplants can be used on moderately hot surfaces up to approximately 200° F (90° C).

COUPLANT E - ULTRATHERM Ultrasonic testing at high temperatures requires specially formulated couplants that will remain in a stable liquid or paste form without boiling off, burning, or releasing toxic fumes. Panametrics-NDT Couplant E is a semi-solid paste that liquefies at high temperature. It is recommended for use between 500° and 970° F (260° to 520° C).

With all high temperature couplants it is important to be aware of the specified temperature range for their use, and use them only within that range. Poor acoustic performance and/or safety hazards can result from using high temperature couplants beyond their intended range. Note that at very high temperatures even high temperature couplants must be used quickly since they will tend to dry out or solidify and no longer transmit ultrasonic energy. Dried couplant residue should be removed from the test surface and from the transducer before the next measurement.


COUPLANT F - MEDIUM TEMPERATURE COUPLANT This product has been replaced by Couplant G, which offers superior performance.

COUPLANT G - MEDIUM TEMPERATURE COUPLANT Couplant G is a non-toxic, biodegradable gel that is particularly recommended for testing parts that are too hot for ambient temperature couplants, but not hot enough to require the use of Couplant E. It may be used at temperatures from ambient up to approximately 600° F (315° C).

SHEAR WAVE COUPLANT - SWC Normal incidence shear wave transducers require couplants of very high viscosity, since liquids such as oil or glycerin will not support shear waves. Typically, sticky resins have been used for normal incidence shear wave coupling. Panametrics-NDT SWC is a non-toxic, water-soluble organic substance of very high viscosity that is easy to apply and remove. Normally a very small amount should be places on the face of the transducer or the test piece and then pressed into a very thin layer. Note that for optimum coupling, it is important that the couplant layer be thin, a small fraction of the test wavelength. Following testing, residue may be washed off with warm water. SWC may be used at temperatures up to approximately 100° F (38° C).