I have a very old (early 1970's) Baker HRV 24 sand filter and most of the information has been worn from the data plate. All I could find is 15-20 GPM and 3.14SF. I can't find any information on the size in gallons of the filter, and don't know how much sand to put in. If it even needs to have new sand. I believe it was changed about 8 years ago. Any words of advice?