I'm a little concerned about the pressure on my Haywood DE filter. I've had the pool for 6 months now and have done several back washes. After backwash and adding new DE the starting pressure comes up to about 10 psi which is the low end of the markers on the gauge. The high marker is at 20 psi. I've been back-washing when the pressure got up around the 20 psi mark. However, the pressure is not longer rising after the last backwash. My last backwash was in April and the pressure is still sitting about 10 psi. Is this normal or does it indicate an issue? I see no bubbles in the pump basket window so I don't think air is getting in. The flow seems good as far as I can tell but I'm not sure how to measure it or what it should measure if I could. Might it be not enough DE?