Thanks, so much Janet. It's like the more that I read, the more the matrix becomes legible. I'm pretty sure that I don't have a sand filter. I have a filter with 4 paper-looking coils in there - that's got to be the filter cartridge. The company that opened my pool said that I'll have to take each one of the cartridges out and hose them off after a little while. The gauge reads zero when the pump is off. When we opened the pool, the gauge read 20 and now it's at 25. Sounds like I'll need to clean those coils soon. Any advice on cleaning them?