It is a regulatory requirement that sterilizing grade filters be tested for integrity post use, and its a sound business practice to test them pre-use. There are three standard tests to test filter integrity. Each one is more beneficial than the other given the conditions and specifications of the filter.
The three main tests are:
- A diffusion test which is also known as forward flow
- A bubble point test
- A water intrusion test.
In order to understand the right test for a particular situation one needs to know there are also two main types of filters; hydrophillic (water attractive) and hydrophobic (water repulsive). A diffusion test applies a constant pressure of air on the upstream side of a filter saturated with liquid and measures the volume of air over time that diffuses through the filter. The liquid is usually water and this test is usually used on hydrophillic filters.
The diffusion test is a good choice for large surface area filter systems, as the flow rate is so low on small filter systems it becomes hard to detect. However, the larger the filter system, the higher risk that a passing diffusion rate could mask a small breach in the filter membrane as the larger the system the diffusion rate begins to get closer to the bulk flow rate created by a small, but potential breach in the filter membrane.
Which is why there is the bubble point test. This test applies an increasing pressure of air on the upstream side of the filter while monitoring the flow rate of air that is diffusing through the saturated filter membrane. The “bubble point” is the amount of pressure that when applied to the upstream side of the filter overcomes the water tension of the largest pore in the system and forces the water through the pore and now air is freely flowing through that pore. However, this test also has its limitations as the larger the filter system, the accuracy of the true bubble point is compromised. The method that a filter integrity tester will utilize is detecting the rate of change in the flow through the filter membrane. In a small filter system the diffusion flow will be slow and steady, and then ever so slightly increase as more pressure is applied, until at the bubble point the flow almost instantly and dramatically jumps up exponentially. As opposed to a larger filter system where the exact changing point from diffusion flow to bulk flow is not as definitive due to the larger slow steady flow from a large amount of diffusion. Both the diffusion test and bubble point test can be used on hydrophobic filters, but the wetting agent cannot be water due to the hydrophobic properties of that type of filters. A typical wetting agent in this application is isopropyl alcohol.
The third most common test is called a water intrusion test. This test can only be used on hydrophobic filters as its method is to apply water under pressure to the upstream side of the filter and detect the amount of pressure loss due to the water not being able to penetrate the membrane due to the filters hydrophobic nature. This test is becoming more popular as it removes the need for a solvent based wetting agent such as alcohol to perform integrity tests on hydrophobic filters. The downside of this test is it is extremely sensitive as the measurement of volume loss can be extremely small. So factors like impurities on the surface of the filter or minor fluctuations in temperature can make the test difficult to complete. So in summary the three different test all have pros and cons and should be applied in the situation that best exploits each test positives.
Written By: Arthur Anderson, Managing Consultant
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