Quality Magazine - April 2009 - (Page 10NDT) BACK TO BASICS A Leak Testing Challenge Discover how a calibrated leak tester verified to a calibrated leak could produce useless data. By Brian Bright reference. The data was useless in correlating previous experiManufactured parts that are out of specification are not only ments with each reset. worthless to the customer but an expense to the producer. “Discoveries consist of seeing what everybody has seen Manufacturers want to understand and control the variables and thinking what nobody has thought,” said Albert Szentto consistently produce not only saleable products, but prodGyorgyi. The monumental discovery in this case was that the ucts with a liability-free service life. Logically, the same gains leak data was not a true leak measurement but only a software should be realized in leak testing by obtaining and analyzing prediction based on what a known leak “looks like.” leak test data. However, over the years many compromises The value of the calibrated leak and zero had become undehave been made to obtain leak test data, clouding leak testing fined variables containing unknown and fluctuating quantitoday and potentially rendering the data useless. ties of true leak to atmosphere and the apparent leak due to The process of only a go/no-go result became insufficient and leak data was now required from the process. This was the first time anyone questioned the leak tester beyond asking for the calibration certificate. Typically, leaks are specified as a maximum leak; for example, a maximum leak of 2 cubic centimeters per minute (cc/min). Historically a “go” resulted when the total measured leak was less than the maximum leak rate specification. At this point the data contained two indistinguishable variables: the true leak to atmosphere and the apparent leak due to stabilization. Industry insisted on having leak test data without ever defining a maximum quantity of apparent leak concealed in the leak test data. Many leak test companies responded with pre-packaged leak test units that provided the data industry was demanding without any constraint to the actual measurement. The paradox with many of the leak test units purchased was that they contained similar off-the-shelf pneumatics as leak testers built in-house. The purchased Is the percentage of “Apparent Leak” measured and defined or just zeroed out to leak test units did offer a calibrated package, able to produce the data people want to see? Source: ATEQ produce and collect the leak test data with an interface facilitating leak test setup with the push of a button. After an automatic calibration type of setup routine, the pre-packaged leak test units were validated with the ability to produce leak test data equal to the calibrated orifice. After the capability of the new leak tester was proven with a gage repeatability and reproducibility (R&R) study, the data became trusted as a true leak measurement. The method used to produce the data was never questioned. Many anomalies were noticed when the leak testers ran in regular production. The manufacture of the leak tester provided many plausible justifications to why the unit had to be frequently re-taught to produce data equal to the calibrated leak. Many people embraced the ability to just reset what was not understood. However, a system proven capable would routinely display values for the same calibrated leak that varied by more than 100%. The data continually shifted and when an operator reset it to the calibrated leak, all the data from previous Is the leak test data like a slow watch, correct when it is set, but inaccurate over experiments became re-established to a different unknown time and reset only when it is noticed to be wrong? Source: ATEQ 10 NDT NDT Special Section I April 2009 www.ndtmag.com http://www.ndtmag.com
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