Today, we would like to talk briefly about coupling gels and why we use them at SoundWater.
Coupling gel is the goop/gel/grease that is added between the ultrasonic transducers on your flowmeter, and your pipe. The purpose is to help get more ultrasound out of the transducer and into the pipe by filling all air voids between the transducer and pipe. Ultrasound will not transfer through air.
Pipes have a curved surface and bend away from the ultrasonic transducer creating a small gap. In addition to the curve, pipe surfaces often have scratches, roughness and imperfections which are areas where a very small air gap may exist. These voids are typically small since you press the flowmeters ultrasonic transducers against the pipe, but they will still reflect ultrasound.
The solution is to apply a malleable, flowable material between the transducer and pipe which fills these small air gaps and helps the ultrasound get into the pipe and fluid - and back out - so that the flowmeter may make a flow measurement. Because the flowmeter must be able to penetrate the pipe and fluid with ultrasound to make a measurement, the more we can help this process the better. This is especially true when we are dealing with things like corrosion where the only way to penetrate it is to push more ultrasound into the pipe.
At SoundWater, we have three types of coupling materials, each with their own benefits and applications:
hard silicone rubber (Fig. 1)
silicone based gel (Fig. 2)
water based gel (Fig. 3)
The water based gel ships with all of our Orcas Portable ultrasonic flowmeters. This gel, which is actually aloe vera gel, is water based. It cleans up easily and is thick enough to cling to surfaces like the flowmeter transducer faces, and/or a vertical pipe surface without dripping. It is convenient and not messy. One downside is that it will dry out in a matter of hours (6-12 hours) depending upon the environment. The water based gel may not be the best choice for long term flow studies, for example if you are data logging.
Long term or permanent flow monitoring applications need a gel that will last for many years, which we like to use a silicone grease. Our Cypress Ultrasonic Flowmeter, designed for permanent, long term flow monitoring applications, always ships with a silicone grease. It lasts a very long time, even in harsh outside environments. Silicone grease is a little messy and not particularly pleasant to work with. But, for permanent applications, you apply it one time only.
The best of both worlds is our special transducer option "-T31S," which is a greaseless transducer design. This means that no coupling gel is required! Instead, we bond a hard silicon rubber to the face of the transducer which acts as the ultrasound couplant. For permanent applications, this works for most pipes and materials because the Cypress flowmeter is fixed rigidly against the pipe. It also has the force to press the silicone rubber into the pipe surface filling all the small air voids and creating good ultrasound transfer. Because it is a solid material, it will never wash away so it is great for long term and outdoors applications.
For portable applications, the -T31S option works great for smooth pipe surfaces, such as plastic pipes, or metal tubes. However, for significantly rough pipe surfaces, the portable flowmeters are not attached as rigidly against the pipe, and so do not have enough force to press the silicone rubber pad into all the micro voids on the pipe surface. In this case, just apply a little of the water based coupling gel onto the coupling pad and this will do the trick for those rough pipes.
Whether gel, grease, or hard rubber, coupling gel is a key tool when using ultrasonic flowmeters. It will ensure that you get a more accurate reading every time.
I hope this was helpful. Feel free to contact us with any questions.
Best Regards & Happy Flowmetering!
Jeff Peery
SoundWater Technologies, CEO
