Maintenance Free filter.  This filter is used on all many of the Apollo Spas and on some of the Coyote Spas from 2011 and newer.  It is the threaded version meaning you have to thread it in to install it.  The Silver Sentinel Filter utilizes a thick filter medial to capiture the particles inside the filter not just on the outside.  This filter will filter down to 1 micron where other cartridge filters go down to only 70 microns.  This filter is completely maintenance free. Just install it and forget about it.  Every three months just replace it. Used filters can be recycled.  Also replaces Unicel 6CH-940.