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Advanced Ultrasonic Couplant for Long Term Probe Placement

June 11, 2019

Applications exist where ultrasonic probes are required to be in place for long periods of time, sometimes in relatively extreme conditions. Typically, ultrasonic corrosion monitoring and ultrasonic flow metering place probes on a surface and take readings on a continuous or periodic basis. Water soluble gels eventually dry and the coupling effect is reduced and then lost.

Innovation Polymers has developed a new ultrasonic coupling that permits long term probe placement without significant coupling losses. The material is a high viscosity liquid polymer with virtually no evaporation at normal working temperatures. The tacky texture makes it an effective pressure sensitive adhesive (i.e. it forms a bond when pressure is applied).

In addition to its longterm coupling capabilities, the new coupling is capable of supporting SH shear waves (i.e. it is a non-Newtonian viscous fluid). Because it is not water soluble, the coupling was selected for application in the North Sea at 300m below the surface. The North Sea application included successful performance demonstration at temperatures from 5°C to 50°C.

To demonstrate the longterm stability of the coupling, a test was configured that coupled a 5MHz compression mode probe and a 5MHz SH shear wave probe to a 9mm thick plate of aluminum.

longterm probe configuration

For the compression mode probe, the A-scan was gated to display two thickness multiples, whereas for the SH shear wave probe, only a single backwall was gated. Over a four-week period, the only  significant shift was seen to occur after Day 1 where the thickness of the coupling had gradually reduced to a steady state. Subsequent measurements indicated less than 1 dB variation from the average peak value for both compression and shear modes.

At the time of the initial introduction of AquaBond, the test period was 30 days with the only significant change being at the transition from Day 1 as the thickness of the very viscous coupling thinned to a steady state. The test continued and 120 days after the test started, the results showed no signs of instability or change. Both the compression mode and shear mode signals remained constant.

24 day test of SH shear mode at 5 MHz
24 day test of L mode at 5 MHz
Attenuator settings over 120 days
Attenuator settings used to obtain 80% signals from first back wall echo

Some suggested applications of AquaBond:

  • Clamp-on ultrasonic flow-meters
  • Longterm probe-to-wedge coupling (i.e. automated systems)
  • Underwater shear-wave stress testing
  • Longterm corrosion monitoring
  • Outdoor applications where weathering could remove water soluble gels

Note: Because the liquid is not water soluble, its removal is made possible with a cloth dampened with naphtha or mineral spirits.

Because the liquid polymer is highly viscous, its application is best made by spreading it onto the probe surface with a knife like blade. Pre-heating to about 40°C helps reduce viscosity for application.

Property Value Unit
Specific Gravity
0.91 g/cc
*190 000
cps (centipoises)
Clear amber liquid
Flash Point
150 (COC method)
Glass Transition Temperature

* approximate value at 55°C. Note: for comparison, thick molasses is about 10 000 cps and peanut butter is about 150 000 cps.

AquaBond is available from Innovation Polymers in 2 or 8 oz jars.

About Innovation Polymers:

Innovation Polymers is a leader in acoustic capable polymer solutions for the non-destructive testing industry, providing high-quality products and services to clients across a range of industries. With a commitment to innovation and customer satisfaction, the company has the capability to develop a wide range of unique sound coupling polymeric media and has earned a reputation as a trusted partner in the industry.