This article deliberates how it has become possible to test any microfine filter in real time, and in its actual conditions of use. For such testing to be possible it is vital to be able to obtain two particle-counting instruments matched for particle detection over a range of particle sizes. This attribute of matched detection appears to be possible with the acoustic method of detecting particles, and not commonly available with optical particle counters. This article was originally published in the Ultrapure Micro Journal in November 2017.