Effective Size : A term used in specifying sand. It is the sieve size in millimeters that permits 10 percent of the filter sand by weight to pass. (SSWID)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}