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Tablets & Capsules October 2014 31 method monitors the time required to pull a vacuum; when this exceeds the specified threshold, it indicates the presence of a large hole. Vacuum-decay leak testing works best on packaging with a consistent free volume of air and is often used on rigid containers. It is less apt for blister packages because it requires expensive tooling and a secondary method of de - tection for large holes, as described above. These limitations make testing multiple formats and cavity sizes impractical. Force decay.The force-decay method uses load cells to measure the force generated as a test pack is subjected to vacuum. The method typically involves three phases: evacuation, stabilization, and testing. Mechanical force is generated as the vacuum is applied (evacuation) and the cavity swells. This force and the vacuum are held at equi- librium (stabilized) so that testing can begin. A cavity with a small hole in it will swell, but gradually deflate during the test period (Figure 1). Under vacuum, packag- ing with a large hole relative to its free volume will gener- ate little force and not swell at all. Bomb test.With this test, a blister package is bom- barded ("bombed") with a test gas, commonly helium. The package is opened and analyzed qualitatively or quantita- tively for the test gas. Alternatively, after flushing, the blis- ter is transferred to a vacuum chamber. A vacuum is applied and a mass spectrometer is used to detect any test gas. Sniffer test. Each blister pocket is charged with he - lium using a needle. A "sniffer" probe is then used to de - tect the presence of helium around the pocket, which will leak from any hole in the blister. Helium gas testing is the most sensitive leak test me - thod, able to detect holes as small as 5 microns. However, such sensitivity is rarely needed and the test's numerous dis- advantages—its inability to identify large holes, time-con- suming nature, operating challenges, complexity, and asso- ciated costs—make it impractical in most cases. Non-destructive alternatives Non-destructive leak-testing allows every product that passes inspection to be returned to production and ulti- mately sold. Furthermore, the test methods are cleaner, faster, more objective, usually more easily validated, and often more sensitive over a wider range of blister materi- als. The most common non-destructive leak-testing methods for blisters are vacuum decay, force decay, laser testing, and vision scanning. Vacuum decay.The vacuum-decay method measures changes in pressure within a vacuum chamber containing the blister pack to be tested. A good pack causes little to no pressure change within the chamber, while a pack with a small hole in it causes an increase in chamber pres- sure as the air inside the pack escapes into the chamber. Large holes, however, will not be detected with the test described above, since the head-space within the pack immediately equalizes with the surrounding envi- ronment as the vacuum is applied. Thus a different vac- uum technique is required for large holes. One such Figure 1 Force-decay test method Vacuum Large hole Good pocket Small hole Time Force Vacuum Table 1 Comparison of leak tests Blue dye 'Bomb' 'Sniffer' Vacuum decay Force decay Laser Vision scan Objective No Yes Yes Yes Yes Yes Yes Repeatable No Yes No Yes Yes Yes Yes Validation No Yes Yes Yes Yes Yes Yes Accuracy No Yes Yes Yes Yes Yes Yes Non-destructive No Yes No Yes Yes Yes Yes Sensitivity Medium High High Medium Medium High Medium Environment Low Medium Low High High High High Operator time/ High Medium High Low Low Low Low involvement Cost Low High High Medium High High Medium Implementation Easy Medium Difficult Medium Medium Medium Medium Flexibility High Medium High Medium Medium High High Comments Easy to implement Unable to detect Detects holes Quick results; Quick results; Requires package- Requires no due to less strict control, which cavity has defect as small as 5 µ limited flexibility requires expensive specific tooling special tooling but should be subject tooling; limited to same requirements flexibility as non-destructive alternatives