The Bubble Point test measures the pressure region at which diffusive flow transitions into bulk flow, by looking for a deviation from the stable background flow versus flow through open pores. However, a marginal leak in the upstream side of the system under test, or a minor damage to the filter, can add a small amount of gas flow. This could be interpreted by the test instrument as part of the background flow, and therefore, the leak would not be detected. The module factor has been designed into the software of the Palltronic® Flowstar IV software to detect these types of flow contributions.
As stated in USD 2594, Instructions for use for the Palltronic® Flowstar IV integrity test instrument, the Module Factor (MF) defines the sensitivity for the Leak Test phase which is executed at the beginning of the Bubble Point test sequence.
The Bubble Point integrity test using the Flowstar IV integrity test instrument features this stabilization/leak test before proceeding to the actual step-wise increase of pressure for the “Bubble Point” test. This initial leak test seeks to establish that filter system under test shows an expected and typical background flow.
For this test, the upstream side of the filter is pressurized to 80% of the minimum programmed BP value, followed by a measurement of the gas flow occurring at this gas pressure, which is compared against a limit value represented by the programmed filter type related ‘Module Factor’. If the gas flow measured is below the limit value, the BP test proceeds.
The lower the “Module factor” is set, the more sensitive it will be to detect a deviation from the typical background flow for the filter under test during this step in the test sequence.
The expected, typical and thus acceptable background flow caused by gas diffusion is dependent on the filter area. Pall recommends that the “Module Factor” should be set to the number of 10” modules being tested. It should be set to 1.0 for 10” cartridges, 0.5 for 5” cartridges (e.g. AB05 or SLK7002), 0.2 for smaller (e.g. AB02 or SLK7001) filter and 0.1 for flat membranes (e.g. 142 mm discs) or mini capsules (KA02 or similar). The default value is 1 if nothing is entered.
This will ensure that the test sequence is aborted when a filter clearly displays a gas flow above the expected typical background value.
For software versions > 2.0b, the user can set the Module Factor to as low as 0.01 for small area filters such as the Acro 25/50 and KM5 devices. See Table I below for the recommended input values for the module factor.