I have new sand in my sand filter this year and everything has been going well, with an average filter pressure of around 10-12 psi. A couple of days ago I noticed that the pressure had risen to around 16-17 psi in a couple of days time. I figured it was just getting to "backwash" time. Today I noticed that the pressure was back down to about 11 psi. Why the variation? Does warm weather have anything to do with it? frankly, I haven't a clue. Any feedback is much appreciated. Thanks.