Yes, it is a sand filter (sorry!).

We backwashed yesterday, and when I checked it this afternoon, it was back up to 20. Surely I'm not supposed to have to backwash every few days? Could there be another reason the pressure would get high, other than something gunking up the filter? If the water is having to go uphill to return to the pool, would that make the pressure get higher? But then again, if it is 12 after I backwash, and it is 20 the next day, then something must be gunking it up, right?

Judy