Today, we’ll discuss two factors that will lead to incorrect results and that should, therefore, be avoided: residual liquid within the tested part, and testing a part that is above or below the ambient temperature.
Residual Liquid Within the Tested Part
Washing/cleaning processes and condensation are the most likely culprits, but sealed parts can inadvertently get liquid in them in any number of ways. If there’s an unwanted hole in the part, and it has had any exposure to liquid, there’s a very good chance that standing liquid is present inside the part. When air pressure is applied to the part, droplets of liquid can plug a hole, masking the leak and skewing test results.
Later, when the liquid dries or evaporates, the hole will be cleared again and the part could—and likely will—fail to work as designed. Cincinnati Test Systems (CTS) offers pressure decay, vacuum decay, mass flow, and tracer gas measurement instruments. Dependent upon the wall thickness of your part and part material; these methods can easily detect leaks large enough for liquids to pass through.
Tested Parts Not at Ambient Temperature
Master part testing and calibration testing should always be performed on a workpiece at room temperature. However, there are many reasons a part may be above or below the ambient temperature—it could have recently been cleaned with cold or warm water, it may have come straight from a machining or welding process, it could simply have been stored in a room that is a different temperature than the product test environment.
Testing parts when they are colder or warmer than ambient temperature can potentially result in false rejects or even worse—mask borderline leaks. Testing parts that are lower than ambient temperature (cold) will absorb heat from tool fixtures, ambient air temperature, and compressed air (i.e., heat caused by the friction between the compressed air molecules). Because of this, you’ll see less of a pressure drop when testing a cold part rather than one at room temperature. This, in turn, can lead to a bad part receiving an Accept result.
Testing parts that are above ambient temperature, i.e. tested while cooling, will exhibit greater pressure drops than parts at room temperature. This can lead to incorrect test measurements that show a higher leak rate, which can lead to false Reject results. The physics of leak testing rely heavily on correctly measuring pressure drop. Pressure drop is directly related to the temperature of the part. Even a small temperature fluctuation—residual body heat transferred from someone handling a part for an extended period, for example—can negatively affect a leak test reading. To achieve the most accurate results, it is critical that part temperatures are controlled.
For more information on leak testing dos and don’ts, turn to the leak test experts at Cincinnati Test Systems. We are your #1 source for high precision leak test instruments.
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