Dunk testing, sometimes called bubble testing, is used for applications that do not require high sensitivity. With dunk testing, the part under test is pressurized, submerged in a liquid—typically water—while the operator looks for bubbles. Bubbles form at the source of the leak as a result of air pressure, and the amount of bubbles per minute can signify the size of the leak. Automotive radiators often are checked for leaks this way. If a leak is present, the bubbles indicate where and the leak can be repaired. Leak testing works best when speed is not a factor. On a production line where test time is critical, leak testing is not the best choice. While the initial cost of a dunk tank is low, in production it is expensive primarily because the water becomes contaminated, thus producing a hazardous toxic waste requiring special disposal. Tramp oil on the parts, as well as residual brazing flux are the main problems. Tramp and brazing flux from the parts leech into the water causing water contamination and costly special disposal. In one plant, aluminum condensers—AC radiators—are tested on four lines, six tanks total. Each operator has 19 seconds to determine if the bubbles represent air trapped in the fins or an actual leak. The operator is solely responsible for quality control prior to final mass spec testing. Failure of the operator to find a leak of 3.5 standard cubic centimeters per minute (sccm) or greater can shut the mass spec line down for hours. In addition to disposal, maintenance of the water for pH, bacteria and skimming the surface to control skin rashes is a big factor—possibly $30,000 per year for maintenance of each tank. In the above mentioned condenser plant, each 3- by 3- by 12-feet tank is said to cost $30,000 per year to maintain.

This is an example of a familiar process for low-volume applications and repairs but an inappropriate use in high-volume applications. High-speed leak testing in a production line situation hampers the operator’s ability to accurately identify bubbles. However, dunk testing can be used on fuel tank filler assemblies and fuel tanks themselves.

One advantage of water dunking is temperature stability. The large volume does not change temperature, which affects most of the more sophisticated testers.