Maintenance Free filter. This filter is used on all Arctic Spas from 2009 through the current models. 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. Check out our Disposable Filter Program to get your filters sent to you automatically every three months. Used filters can be recycled.