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 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 seven different products optimized for different applications. A description of each follows, listed according to their Panametrics-NDT part code.

COUPLANT A - PROPYLENE GLYCOL Propylene glycol is a good general purpose couplant that is often used in ambient temperature thickness gaging applications on smooth surfaces. It has good surface wetting properties, it will not corrode or otherwise attack common engineering materials, and it is easily washed off with water. Because it is chemically non-reactive and does not evaporate quickly, propylene glycol is also recommended for use in delay line transducers to couple the delay line to the wearplate. 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 or fungus 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 Silicone oil is preferred by some inspectors who are doing angle beam testing as a couplant between the transducer and the wedge. Because it does not evaporate and is insoluble in water, silicone oil will not dry out or wash away. This gives silicone oil an advantage as a long-lasting couplant in angle beam assemblies, but the difficulty in removing it from test surfaces limits its use as a general purpose couplant.

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. A number of commercial couplants are available for various temperature ranges. 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.
Panametrics-NDT Couplant E is a semi-solid paste that liquefies at high temperature. It is recommended for use between 500° and 1000° F (260° to 540° C).

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, representing 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).

Products used for this application

Epoch 4
The standard-featured EPOCH 4 flaw detector offers great measurement capabilities and software options such as the fully encoded B-scan.

BondMaster
The lightweight Sonic BondMaster 1000 is a multi-mode, fully-loaded composite tester featuring Pitch Catch RF display mode.

Nortec 1700
The Nortec 1700 eddy current gauge measures conductivity on non-ferrous metals as well as measurement of non-conductive coating thickness. A go/no go alarm is available for quick alloy sorting.

2000 Series
These full-featured Nortec flaw detectors offer high display resolution, multiple scanner support, single or dual frequency, digital conductivity, and non-conductive coating measurements.

PipeWIZARD
R/D Tech PipeWIZARD system is the most advanced and powerful mechanized ultrasonic test system available to the pipeline deployment industry.

MS5800
The multitechnology MultiScan MS5800 acquisition unit is an all-in-one solution. Tube inspection using eddy current, or remote field and magnetic flux leakage, or IRIS. Also conventional UT for corrosion mapping, and eddy current array.

TomoScan FOCUS
With more than 350 units sold, the R/D Tech Tomoscan FOCUS acquisition unit is the benchmark for automated UT inspections. It comes in many configurations.

OmniScan MX
The portable, R/D Tech OmniScan MX modular platform lets you easily switch from ultrasound phased array to conventional UT, eddy current, or eddy current array. Battery operated instrument.

OmniScan PA
The rugged and portable R/D Tech OmniScan PA offers high-speed acquisition rate and powerful software features to efficiently perform manual and automated inspections.

5800PR, 5900PR, 5910PR
The Panametrics-NDT computer-controlled 5800PR (35 MHz), 5900PR (200MHz), and 5910PR (400 MHz) are designed to use with computer-based systems for automatic testing.

5072PR, 5073PR, 5077PR
The 5072PR (35 MHz) and 5073PR (75 MHz) manually controlled pulser-receivers are broadband, negative spike excitation pulsers with superior performance. The Panametrics-NDT 5077PR (35 MHz) employs a tunable square wave pulser.

1200 Series
These advanced Sonic units offer additional flaw detection capabilities and various software options. High display resolution and Smartknob™ are some of their innovative features.

EPOCH 4B
Combines low cost and many practical measurement feature. This Panametrics-NDT flaw detector offers some software options.

EPOCH 4PLUS
The advanced EPOCH 4PLUS with color LCD offers more measurement features and software options than any other Panametrics-NDT flaw detector.

35 Series
General purpose, lightweight Panametrics-NDT gauges feature optional live Waveform and Adjust Mode. 35DL and 35DL-HP have alphanumeric data logger.

MG2 Series
Pocket-sized, easy to use Panametrics-NDT corrosion gauges offer many measurement features such as B-pscan and optional A-scan. MG2-XT and MG2-DL offer Thru-Coat and Echo-to-Echo options to measure both coating and thickness.

25DL PLUS
General purpose Panametrics-NDT gauge features large LCD with live Waveform and sophisticated alphanumeric data logger.

37DL PLUS
Advanced Panametrics-NDT corrosion gauge with large A-scan display and sophisticated alphanumeric data logger. Features include Thru-Coat, Echo-to-Echo, B-scan acquisition, Emat, and optional Oxide/Scale measurement.

25HP PLUS
Panametrics-NDT thickness and velocity gauge for thicker materials. Features large LCD with live Waveform and sophisticated alphanumeric data logger.

25MX PLUS
This Panametrics-NDT gauge makes multipoint and single thickness measurements. Up to eight transducers can be connected to the optional MX-8 Multiplexer.

26MG
Basic gage combines simple operation and low cost. This Panametrics-NDT gauge has a variety of practical measurement features.

25 Multi Plus
For multilayer measurements. This Panametrics-NDT gauge measures simultaneously up to 4 layers. Features large LCD with live Waveform and sophisticated alphanumeric data logger.

EPOCH LT
A truly handheld, affordable flaw detector with many standard features and software options. This Panametrics-NDT unit is only 1.5 inch thin (38 mm).

1000 Series
These standard-featured lightweight Sonic® flaw detectors offer high display resolution. Their innovative design features an uncluttered keypad, and Smartknob™, making it easy to operate.

5058PR
This Panametrics-NDT pulser-receiver (10 MHz), with up to -900 V high voltage pulser, is made for ultrasonic test applications requiring a high material penetration capability.