Quickly and Accurately Measure Media Expansion during Backwash. The US EPA recommends 20% to 25% filter bed expansion to adequately clean the filter media (anthracite, sand). Failure to expand the bed sufficiently will result in `mud balls` that degrade the filter`s ability to trap dangerous microorganisms such as Cryptosporidium oocysts and Gyardia cysts. Until now, the only options for measuring filter bed expansion were home-made type devices or expensive fixed mount microprocessor based echo-time measuring equipment. With the portable Media Expansion Measurement System, Raven offers an affordable solution for water plants of all sizes. 1 year warranty. Made in USA.