Integrity Testing Filters or Bubble Point


To integrity test a filter, the filter must be wetted with a fluid with a know surface tension. After filtering the desired medication or base, the filter must be flushed with either DI water or 60% IPA (isopropyl Alcohol) and balance DI water. We prefer the 60% IPA / 40% water be used. This solution is easier to wet the capsule and the acceptable bubble point values are much lower; see the acceptable bubble point values in the table below. 18 Psi is a much safer pressure to operate at than 50 psi. This is important when the chemical compatibility is unknown and the capsule may not be structurally sound.


Bubble Point Fluid PES - BP Value (PSI) PTFE - BP Value (PSI) Nylon - BP Value (PSI)
Water 50 50 50
60% IPA/ 40% DI H2O 18 18 18
70% IPA/ 30% DI H2O 17.6 17.6 17.6

Note: All values above pertain to Pharmaceutical grade membranes

Bubble Point Theory

All ZenPure standard, biological, and pharmaceutical grade membrane filters are 100% integrity tested during the manufacturing process using a standard Diffusional integrity test. Additionally, the bubble point of each roll of incoming membrane and a subset of the final filter devices is verified by way of a standard bubble point test. The bubble point can be directly correlated to membrane pore size and is used to detect minor defects or out-of-size pores which could make a filter unsuitable for use. The Diffusional integrity test ensures that each filter is integral and free from manufacturing defects that could lead to filter bypass. In combination, these tests ensure the high performance of all ZenPure membrane filters. The test methods described here can be applied to all ZenPure membrane and specialty filters. Consult with a ZenPure applications specialist for additional details based on your application requirements.

Non-Destructive Testing

The use of a Diffusional integrity or bubble point test provides a non-destructive method to verify filter performance. Unlike direct particle and bacterial retention testing, these tests can be performed on each and every filter, with no degradation of the filter’s performance. During product development and validation, a precise correlation between a filter’s bubble point and its ability to perform particle or organism retention (for use in sterilization, for example), is well characterized. This correlation is used to define specified limits (with a built-in safety factor) for each product type using each test method. As integrity testing is a requirement for many applications, an overview of the test methods is contained in this document.


Each filter’s micro-porous membrane contains millions of pores creating a tortuous path used to capture particles while allowing the desired fluid to flow through the filter assembly. A nondestructive bubble point test measures the minimum pressure required to overcome the capillary forces which hold a wetting liquid within the filter’s pore structure. This pressure is known as the bubble point. The capillary forces and, therefore the bubble point, are directly proportional to the diameter of the largest pore contained in the filter. This relationship between pore size and bubble point is described by the bubble point equation (1).

Call Now for a Free Consultation