Assembly - March 2009 - (Page 36) Advertisement How To Testing Select the Right Test Method to Leak Test Your Products electing the best leak test method requires a careful evaluation of the product leak test specifications, the manufacturing conditions and the product composition & function. The most important is the leak specification, but other critical factors include test pressure or vacuum, part size/volume, part material of construction, test environment, production rate and capital budget. Determining the right leak rate to specify can be the toughest decision. S Determining The Right Leak Rate Specification J Automated Testing with Pressure Decay or Mass Flow Instrumentation Pressure Decay and Mass Flow Testing Methods These test methods use a pressure transducer, differential pressure transducer or mass flow meter to measure changes in the part under test. Pressure decay and mass flow leak testing, while the most widely used methods, Typical Manufacturing Production Leak Rate Application Capability By Technology -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 *sccs are directly dependent on the stability of the test parts’ and environmental conditions Pressure Decay Mass Flow during the testing cycle. These methods are Falcon™ Helium based on the Ideal Gas Law (PV=nRT). Slight Dunk / Bubble Test variations in part temperature and/or volume Helium sniffer probe Helium Mass Spec Accumulation during the test may cause unacceptable Helium Vacuum Mass Spectrometer repeatability. To meet tight test requirements, * Standard Cubic Centimeters Per Second the pressure change differential or associated flow between a good and reject part must be significantly greater For more information, visit CTS—Cincinnati Test than the influence of either temperature or part volume variations. Systems at www.cincinnati-test.com. These are usually caused by high test pressures, part materials that stretch, and uncontrolled part or environmental temperature. 36 M A R C H 2 0 0 9 ASSEMBLY • How-To Guide ust 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). http://www.cincinnati-test.com
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