Up until recently, a universally accepted test method to measure the media pore size has not existed. A newer test procedure called multi-pass testing or Beta ratio testing has changed this. This method yields readily comparable test results and was introduced to give the filter manufacturer and the end user an accurate comparison between filter media.
Multi-pass testing uses a specified contaminate, of known sizes, added regularly in measured quantities to the fluid which is being pumped through the filter. At timed intervals, samples of the fluid are simultaneously taken from both downstream and upstream of the filter. Using particle counters, particles in each sample are measured and counted. Based on the results of these measurements, a Beta ratio is determined by dividing the number of particles of a particular size in the upstream flow by the number of particles of the same size in the downstream flow. In essence, the Beta ratio is an indicator of how well a filter controls a specifically sized particulate. For example, if one out of every two particles in a fluid pass through the filter, the Beta ratio is 2/1 = 2. This shows the number of particles upstream divided by the number of particles downstream. Based on this method, filters with a higher Beta ratio retain more particles, have higher efficiency, and therefore are more effective filters.