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 couplant that permits long term probe placement without significant coupling losse
Source: NDT