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1、 Bubble point test: when the filter membrane is completely wetted by the wetting liquid, the liquid is retained in the filter membrane under the action of surface tension. If you want to squeeze the liquid out of the membrane hole, you need additional pressure to extrude the liquid in the membrane hole. The minimum gas pressure required is the bubble point pressure of the filter membrane. Bubble point detection is one of the most widely used integrity testing methods.
Calculation formula of bubble point value:
P=4×K×ϒ×Cosθ/D
Where, P is the pressure difference between the two sides of the membrane, K is the membrane pore shape coefficient, ϒ is the surface tension of the medium, θ is the wetting contact angle, and D is the pore diameter.
2、 Diffusion flow test: when the filter membrane is completely wetted by the test medium, and the pressure difference between the two sides of the filter membrane is less than the bubble point pressure value (usually 80% of the bubble point value of the filter membrane), the macro gas is blocked by the surface tension of the pores, but under the effect of pressure gradient, the gas at the high pressure side diffuses through the liquid in the micropore and releases at the low pressure side. The gas flow at the low pressure side is called diffusion flow. Diffusion flow can be used to evaluate the effectiveness of membrane pore retention, which is an important method to evaluate the integrity of filter element.
Diffusion flow calculation formula:
DF=K×A×(P1 – P2)×ρ/H
Where: K is the diffusion constant, a is the effective porosity of the microporous membrane, h is the thickness of the wetted membrane, ρ is the solubility of gas in liquid, P1 is the high pressure side pressure, P2 is the low pressure side pressure.
Test method of diffusion flow:
1. Pressure attenuation: high pressure gas is applied upstream of the fully wetted filter element. After the air pressure is stabilized, the gas source is cut off. According to the ideal gas state equation PV = NRT, the gas diffusion flow in this pressure range can be calculated.
2. Flow test: apply high-pressure gas upstream of the filter element to keep the pressure constant, and directly test the flow rate of diffusion flow downstream of the filter element.
3. Water invasion method: water intrusion method is based on the surface tension and capillary phenomenon of water on the surface of hydrophobic filter membrane. The minimum pressure required for water pressure to enter the largest pore in the hydrophobic membrane is called water intrusion pressure. When the pressure is less than the water invasion pressure, water will not enter the membrane pore.
The functional relationship between the pressure difference of water pressed into the membrane pore and the capillary is as follows:
△P=-4×K×ϒ×Cosθ/D
Where, △ P is the pressure difference between the two sides of the membrane, K is the membrane pore shape coefficient, ϒ the surface tension of water, θ contact angle, and D is the pore diameter.
Water immersion method is a highly sensitive method for testing the integrity of hydrophobic filter membrane according to water flow rate without alcohol. Since the membrane is not wetted and almost dry, it can be put into use immediately after the integrity test, with minimal or no drying.
