I’ve got a Hayward sand filter with no air relief valve. I’ve read that a normal pool filter pressure reading is between 10-20, but mine is at 40 after backwashing, rinsing, and cleaning out the pump basket. When I turn off the pump, it only goes down to 30. Pool is filtering normally and there are no bubbles coming out of the return jets. Sand was changed last year. Is 40 a normal filter pressure for a sand filter with no air relief valve or is it possible that my gauge is defective?