Selecting the best leak test method is an economic decision that balances the cost of meeting the leak test specifications within the conditions and requirements of production. There are several major factors that influence the right decision. The most important is the leak specification, but other factors are test pressure, part size/volume, part material of construction, test environment, and production rate.

Pressure Decay and Mass Flow Testing
The pressure change methods use either a pressure transducer, differential pressure transducer or mass flow meter to measure changes in part resulting from air escaping the test volume. Pressure decay and mass flow leak testing, while the most widely used for testing products, are directly dependent on the stability of the test parts' temperature and volume during the testing cycle. These methods are based on the Ideal Gas Law (PV=nRT). Slight variations in part temperature and/or test volume during test may cause unacceptable test inconsistencies depending on the part volume, leak rate, and time cycle. To meet test requirements, the pressure change differential or associated flow between an accept and reject part must be significantly greater than the influence of temperature or volume variations due to high test pressures, materials of construction, and test environment to meet the test requirements. It is usually advantageous to recognize and control these variations whenever possible to achieve a good economic decision.

Tracer Gas Testing
The tracer gas methods monitor the change in concentration of a tracer gas on the lower pressure side of a test part. Typical tracer fluids are air, helium, hydrogen, Freon, or SF6. Some of these methods are manual and depend on human observation like bubble testing. Others are manual but utilize instrumentation like tracer gas specific sniffer systems. Or finally automated systems utilize tracer gas analyzers like helium accumulation sensors, helium mass spectrometers or residual gas analyzers with a vacuum test volume. These methods are not affected by changes in temperature or volume because they are not dependent on the Ideal Gas Law.

Select the test method from the chart that provides the most economical solution to the leak rate requirement. Then evaluate the production process to determine the influences of test pressure, part size, production rate, part material of construction, and temperature on the test method to determine if they can be adequately controlled to achieve satisfactory production test results. If a pressure change method was initially selected and the adverse conditions can not be controlled or overcome, then consider the alternative tracer gas methods that may require additional initial capital expenditures.

Ultimately it is important for long term cost savings to have a robust test system that can overcome reasonable variations in the production process to give reliable test results.

Determining The Right Leak Rate Specification
Just saying that the part must not leak is not a design specification, because everything leaks! Leak testing verifies that a manufactured product will not allow a specified fluid (liquid or gas) to escape or infiltrate a product. The objective is to specify a reasonable and measurable leak rate that defines when a product no longer performs its design function to the satisfaction of the intended end user.

Air or helium can flow through a path that a liquid cannot penetrate because of the fluid viscosity, surface tension, path length, and path diameter. It is important to specify a reasonable leak rate because it has a direct influence on the price and test time of the test system. The accompanying chart estimates the leak rate measurement range for the typical technologies used in production environments.

Automated test methods are usually selected to assure a consistent, operator independent test result. Pressure decay, differential pressure and mass flow are widely used because they provide economical, pass/fail results. When leak rate requirements get more restrictive, tracer gas methods are used (Falcon™, helium accumulation/ hard vacuum mass spectrometer).

For more information, visit CTS—Cincinnati Test Systems at www.cincinnati-test.com.