Integrity tests, such as the diffusive flow, pressure hold, bubble point, or water intrusion tests, are non-destructive tests, which are correlated to the destructive bacteria challenge test with 107/cm² diminuta. Derived from these challenge tests, specific integrity test limits are established, which are described and documented within the filter manufacturers’ literature. The limits are water-based; i.e., the integrity test correlations are performed using water as a wetting medium. If a different wetting fluid, such as a filter or membrane configuration, is used, the integrity test limits may vary. Integrity test measurements depend on the surface area of the filter, the polymer of the membrane, the wetting fluid, the pore size of the membrane, and the gas used to perform the test.
Wetting fluids may have different surface tensions, which can depress or elevate the bubble point pressure. The use of different test gases may elevate the diffusive gas flow. Therefore, appropriate filter validation has to be established to determine the appropriate integrity test limits for the individual process.
Bubble Point Test:- Microporous membranes will fill their pores with wetting fluids by imbibing that fluid in accordance with the laws of capillary rise. The retained fluid can be forced from the filter pores by air pressure applied from the upstream side. The pressure is increased gradually in increments. At a certain pressure level, liquid will be forced first from the set of largest pores, in keeping with the inverse relationship of the applied air pressure P and the diameter of the pore, d,described in the bubble point equation:

Where,
g is the surface tension of the fluid,
y is the wetting angle,
P is the upstream pressure at which the largest pore will be freed of liquid, and
d is the diameter of the largest pore.
When the wetting fluid is expelled from the largest pore, a bulk gas flow will be detected on the downstream side of the filter system. The bubble point measurement determines the pore size of the filter membrane, i.e., the larger the pore the lower the bubble point pressure. Therefore, filter manufacturers specify the bubble point limits as the minimum allowable bubble point. During an integrity test, the bubble point test has to exceed the set minimum bubble point.

MANUAL BUBBLE POINT TEST SET UP
Diffusion Test:- A completely wetted filter membrane provides a liquid layer across which, when a differential pressure is applied, the diffusive airflow occurs in accordance with Fick’s law of diffusion. This pressure is called test pressure and commonly specified at 80% of the bubble point pressure. In an experimental elucidation of the factors involved in the process, Reti simplified the integrated form of Fick’s law to read as follows:

Where,
N is the permeation rate (moles of gas per unit time),
D is the diffusivity of the gas in the liquid,
H is the solubility coefficient of the gas,
L is the thickness of liquid in the membrane (equal to the membrane thickness if the membrane pores are completely filled with liquid),
P (p₁ –  p₂) is the differential pressure, and
r is the void volume of the membrane, its membrane porosity, commonly around 80%.
The size of pores only enter indirectly into the equation; in their combination, they comprise L, the thickness of the liquid layer, the membrane being some 80% porous. The critical measurement of a flaw is the thickness of the liquid layer. Therefore, a flaw or an oversized pore would be measured by the thinning of the liquid layer due to the elevated test pressure on the upstream side. The pore or defect may not be large enough that the bubble point comes into effect, but the test pressure thins the liquid layer enough to result into an elevated gas flow. Therefore, filter manufacturers specify the diffusive flow integrity test limits as maximum allowable diffusion value. The larger the flaw or a combination of flaw, the higher the diffusive flow.

MANUAL DIFFUSION TEST SET UP
Pressure Hold Test:- The pressure hold test is a variant of the diffusive airflow test. The test set-up is arranged as in the diffusion test except that when the stipulated applied pressure is reached, the pressure source is valved off. The decay of pressure within the holder is then observed as a function of time, using a precision pressure gauge or pressure transducer. The decrease in pressure can come from two sources:
1) the diffusive loss across the wetted filter. Because the upstream side pressure in the holder is constant, it decreases progressively as all the while diffusion takes place through the wetted membrane and
2) the source of pressure decay could be a leak of the filter system set-up.
An important influence on the measurement of the pressure hold test is the upstream air volume within the filter system. This volume has to be determined first to specify the maximum allowable pressure drop value. The larger the upstream volume, the lower will the pressure drop be. The smaller the upstream volume, the larger the pressure drop. This also means an increase in the sensitivity of the test, and also an increase of temperature influences, if changes occur. Filter manufacturers specify maximum allowable pressure drop values.

MANUAL PRESSURE HOLD TEST SET UP
Water Intrusion Test:- The water intrusion test is used for hydrophobic vent and air membrane filters only. The upstream side of the hydrophobic filter cartridge housing is flooded with water. The water will not flow through the hydrophobic membrane. Air or nitrogen gas pressure is then applied to the upstream side of the filter housing above the water level to a defined test pressure. This is done by way of an automatic integrity tester. A period of pressure stabilization takes place over time frame, by the filter manufacturer’s recommendation, during which the cartridge pleats adjust their positions under imposed pressures. After the pressure drop thus occasioned stabilizes, the test time starts, and any further pressure drop in the upstream pressurized gas volume, as measured by the automatic tester, signifies a beginning of water intrusion into the largest (hydrophobic) pores, water being incompressible. The automated integrity tester is sensitive enough to detect the pressure drop. This measured pressure drop is converted into a measured intrusion value, which is compared to a set intrusion limit, which has been correlated to the bacteria challenge test. As with the diffusive flow test, filter manufacturers specify a maximum allowable water intrusion value. Above this value, a hydrophobic membrane filter is classified as non-integral.
FIND MORE AT…
Reference links
Encyclopedia of pharmaceutical technology
http://www.filtration.3mil.co.il/sites/3mfilter/_media/stories2/203_st_pdf_2b303.pdf
https://wine.appstate.edu/sites/wine.appstate.edu/files/Bubble%20Point%20Test.pdf