Advanced Leak Test MethodsSource: Advanced Test Concepts (ATC Inc.)
By Hemi Sagi, Director, ATC, Inc.
Leak testing is an engineering challenge. On one hand, engineers must meet stricter leak rate standards. On the other, they have to make the leak testing process less costly and less dependent on operator skill. To meet this dilemma, engineers must understand all aspects of the leak testing process.
What is a product leak? This common term is not always well defined. A product leak is material flow from or into a product (a control volume) during a given time, in excess of allowable limits. Product leaks are caused by open flow paths, such as pinholes, broken seals or material porosity. In most cases, a product leak is a very small flow.
The process of quantifying and eliminating a product leak is called leak testing. In the pharmaceutical, medical and food industries, it is called package or seal integrity testing. Leak testing requires the measurement of very mall flow rates of a gas or liquid. In some cases, the leak flow rate is correlated to a "virtual pinhole," to quantify the size of potential defects.
Many products contain liquids, so using a liquid for leak testing would seem to be a natural choice. However, using common gas, such as air or nitrogen, for leak testing provides a cleaner, more economical and more sensitive test. To understand why gas is more sensitive than liquid, we must examine the relationship between the leak rate from a pinhole, the size of the hole, and properties of the fluid.