Testing of Gas Filtration Systems is often performed to meet bypass leakage requirements. Typically, a continuous injection of R-11, R-12, HFC43-10 or HFC134a is made for up to five minutes while inlet (upstream) and outlet (downstream) halide concentrations are monitored.

Steady injections of challenge halide, however, result in a "temporary" loading of halide on the charcoal filters being tested. Retesting following correction of bypass leakage defects must be delayed until adsorbed halide is purged (desorbed). This can be as long as 24 hours.

An alternate method for in-place leak testing has been developed. This technique involves injecting pulses of known amount of challenge halide while continuously monitoring upstream and downstream concentrations which are delayed by the charcoal. The advantage of injection of a known amount is that, if the airflow is known, the upstream concentration can be calculated and only the downstream reading is necessary. Pulse mode testing requires the injection of halide in controlled amounts as a series of repeatable small-quantity injections. As a result, less halide is used, retesting is easier and hard-to-test systems can be accurately tested.