Most Ultrasonic NDT systems use Gel Couplants to get a good signal connection between the probe and the spot weld. The problem with Gel Couplants is that they need to be cleaned thoroughly before the metal surface can be painted. There is also a cost for purchasing and maintaining inventory of the gel couplants. Some Ultrasonic NDT systems can work well with water as a couplant.
Couplants are a liquid or gel material that provide a good continuous connection between the Ultrasonic probe and the surface of the spot weld to fill in any air gaps that can interrupt the Ultrasonic signals. The properties of couplants must allow the Ultrasonic signals to travel through the couplant at very near to the same speed as the probe interface. Some Ultrasonic probes will have a column of water and a membrane to hold the water in the probe, the membranes are usually made of a material that has similar Ultrasonic transmission properties as water so there are no reflections of signals from dissimilar materials as the signals pass through them.
Systems that use a gel couplant will specify the specific gel to use and there is a cost to purchase and maintain inventory of the couplant. The biggest issue with using a gel couplant is that it must also be cleaned off the surface of the spot weld and surrounding metal before that part can be painted; which can be time consuming.
Water couplant based Ultrasonic NDT Systems use water as a couplant simplify the process and saves significant time and money. Because the water filled column, the membrane, and water couplant all have similar sound penetration characteristics, the Ultrasonic signals are uninhibited as they make their way to and from the spot weld that is being tested. Gel couplants can usually work with systems that are designed to work with water as a couplant, but water usually works as well as gel couplants with the systems that are made to work with water as a couplant.
Because water is practically free and at most needs a simple cloth to wipe off from the surface of a part, there are now consumables for you to purchase and keep track of and you save the time and money involved with cleaning the part after testing the spot welds.