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Understanding the principle of vacuum decay leak testing

28-08-2020

Leak testing is a familiar process that is widely used in industries such as automotive, aerospace, food and pharmaceutical to test the integrity of the manufactured components and products. The leak test is aimed to detect defects in non-porous or rigid parts or packaging. It is essential procedure during manufacturing process as defects and faulty parts is not fit to use for assembly of final products. Similarly, leaks or compromised seal integrity in food and pharmaceutical packages may accelerate the decomposition of the content. This significantly increases the likelihood that the products arrive at the consumers’ possession in compromised state.

The vacuum decay leak testing systems are ideal for a food manufacturer as they can be easily operated and can be fitted in production environment. This sophisticated leak detector is primarily used to test seal integrity on a  packaging of a wide range of food products such as milk powder, sandwich boxes, biscuits, snack foods, fresh salad pack, animal foods, sausage skins, and beverage containers. The only test requirement is that the package should have dry contents and headspace to achieve accurate and reliable measurement.

Test standards can be developed based on the measurement results to indicate the degree of leak. This non-destructive testing system is technically feasible as alternative to the widely popular water bath leak detector. The vacuum decay leak tester offers fast and non destructive testing in less than 20 seconds. Therefore, it offers significant increase in efficiency and cost savings as products can be retained after testing concluded and results can be stored for quality control and traceability.

Vacuum Decay Leak Testing Principle

The test begins with placing package in a closely fitting evacuation test chamber, which is equipped with an external vacuum source. The test chamber is placed under vacuum whose level is predetermined to simulate the real-time condition experienced by the packages during production and transportation process. A highly sensitive differential pressure sensor is used to monitor the decay in vacuum level. Once it reaches the desired vacuum level, the chamber is allowed to decay to the atmospheric pressure. The pressure decay curve is generated from this measurement, as shown in the figure below. 

The pressure profile can be used to identify leakage. The time taken to create the vacuum can be used to identify large or coarse leak. Similarly, the rate of vacuum decay towards atmospheric pressure, depicted in stabilize and leak region above, is used to identify small or fine leak. This measurement can be further refined to concentrate on the most useful parts of the curve as shown in the next figure.

The peak vacuum V2 must be reached between T1 and T2. The maximum allowable time to reach vacuum level V2 is used to identify coarse leak. Package with large hole will provide extra air for evacuation which considerably slows the evacuation time. The settle time is important to ensure that a stable leak rate is achieved before fine leak measurement is conducted. Fine leak is identified when the maximum vacuum decay rate during the fine leak time T4 is larger than the allowable decay rate. This can be individual set and differ for different type of packaging. For example, small packs or very small satchets have much lower recommended fine leak rate limit in comparison to large packages such as salad bars.

Most tests can be performed rather quickly, with highly accurate results for a different variety of food packages including flexible packages, cans, large bags, heavy bags and many other products. Non-destructive packaging leak testing systems based on vacuum decay principle enable simple and semi-automated testing as test results can be finalized in 5-20 seconds. The test measurement creates a reliable and accurate quantitative result with a pass or fail display. Results can also be logged and downloaded for traceability.

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