this is a 38 or 40 year old stainless steel sand filter.....I don't think they make them anymore because they last so long... It has a seam around the middle that I have had welded twice in all those years.....still works!......don't know if I ever even had a manual....but I want to know how much sand to put in it....it is a cylinder about 24 inches across, 14 inches high and 76 inches in diameter....multiport on the top....should it be 1/2 full? 3/4 full? Any help is appreciated.