Leak Testing Essentials

Leak testing succeeds—or fails—long before the instrument is switched on. Excerpted from the recently released ASNT Level III Study Guide: Leak Testing (LT), second edition, this tutorial walks through the practical decisions that drive accurate, efficient results: designing (or evaluating) the system so the test is even possible, planning the sequence to avoid false outcomes, and defining “leaktight” in measurable, defensible terms rather than the unhelpful promise of “no leak.” From there, it connects method selection to real-world trade-offs in sensitivity, cost, and test time—highlighting where helium mass spectrometry can simplify workflows and where lower-cost approaches like bubble, pressure-change, or specialized techniques make sense. The result is a technician-focused roadmap for planning the test, controlling the variables that distort readings, and choosing equipment and techniques that match the application.

Planning and Performing Leak Tests

Physical Considerations

Leak tests, as with all types of NDT methods, may be expensive and time-consuming operations. They require skilled technicians and need to be performed in an efficient, competent, and cost-effective manner. In many applications, the technician should be brought in during the design phase so that adequate provision can be made for performing a leak test. Without proper planning, it may not be possible to perform the desired test or maintain costs at a reasonable level. The tubulation for evacuating a chamber should be large enough that long pumpdown times of several hours are not encountered, or that the system response time constant is more than a few minutes’ duration. The shorter these times are, the better, because they directly contribute to the time and cost of performing a test.

Some test specifications require the leak standard be at the most remote point in the system from the leak test instrument. During design is the time to provide plumbing for such arrangements; otherwise, this condition cannot be met. In other cases, the equipment may be complex and the most effective procedure is to leak test each module before the final assembly. Then, at final assembly, the only untested boundaries are the main couplings between modules. This greatly reduces the amount of disassembly and reassembly that could be encountered. Such provisions for testing can only be arranged during the design stage of a product.