Maintenance Free filter.  This filter is used on all Arctic Spas from 1998 through 2008 except for the Marmot model.  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.