In order to consider the influence of the temperature, each filter integrity test method needs to be treated independently, as they are based on different physical principles.
Forward Flow integrity test limits issued for Pall filters, when wet with standard wetting fluids such as water, 60/40 IPA or similar solutions, apply to a test temperature of 20 °C ± 5 °C.
Any variation in the temperature of any gas volume in the filter test assembly during the measurement phase has an effect on the flow measurement. Most integrity test instruments measure Forward Flow on the upstream side of filter, as a function of gas pressure change1. Variations in temperature during the test lead to expansion or compression of the gas in the test assembly (tubing and housing upstream of the filter). Such variations of the gas volume may lead to inaccurate flow measurements. Therefore, it is recommended to keep the temperature of the filter assembly constant during the test period.
Pall recommends that the temperature of the filter assembly during the test should not vary more than ± 1 °C. Some simple ways to accomplish this are listed below.
Changes of surface tension are of direct relevance for bubble point testing as the surface tension influences the capillary forces holding the wetting liquid in the membrane pores. The measured bubble point of a given filter which is fully wetted will increase and decrease in direct proportion with the surface tension of the wetting liquid.
This means that lower bubble point values will be measured at higher temperature, and higher bubble point values will be measured at lower temperature.
Water Intrusion Test (WIT)
For Pall filters, the water intrusion limit values apply to ambient temperature (20 °C) with a specified range of ± 2 °C. During the test period, the temperature of the filter assembly should not vary more than ± 1 °C.
For the WIT, the temperature of the gas in the filter assembly will have the same effects on the gas volume as indicated above for the Forward Flow test. Water temperature will also have an effect on the measurements.
As the WIT measures evaporative flow, which is typically much lower than diffusion measurements (as measured by the Forward Flow test), any temperature changes will have a greater effect on the WIT measurements compared to the Forward Flow test and may not be identified by environmental or assembly temperature monitoring.
Maintaining Constant Temperature During Integrity Testing
The following is a list of approaches to maintaining a constant temperature during integrity testing:
- Acclimate the filter and fluids to room temperature before starting the test. This is especially important for the WIT: Our guidance is to dispense the water into a container and acclimate for >4 hours.
- Avoid placing the filter assembly under heating or cooling registers.
- Avoid handling the filter during the test.
- In cases where room temperature fluctuates, it may be necessary to insulate tubing and filter assembly.
1 The Palltronic Flowstar line of integrity test instruments measure Forward Flow by direct measurement. The impact of a change in temperature during the measurement (“Test”) phase will result in an unstable flow measurement, which will extend the test time.