I added DE to my sand filter---enough to raise pressure 1 psi (from 10 psi to 11 psi). This took 7 cups DE---I have a larger sand filter. For the rest of that day, the pressure crept up slowly, as the pool got noticeably clearer---nice! By the end of the day the pressure got up to 18 psi, and I anticipated backwashing the next day. But when the pump started the next morning (timer shuts it off at night), the pressure was back to ~12 psi, without a backwash! Is this normal? I wonder why the pressure dropped. Pump speed and valve settings did not change.

Big improvement in pool clarity, but I'm curious about the pressure.

Also, I assume all the DE is flushed out whenever I do a backwash?

Thanks for any info--

Dave

When you shut the pump off the bed of DE and debris over the sand could have shifted. That would have caused the pressure to be lower upon the restart. I'd go ahead and backwash and add it again.

I've added DE to a 24" sand filter for the last couple of years, a few hours before backwashing. As you say it clarifies the water very well. I only put one or two large cup fulls in and within a few hours the pressure is right up in the red.

I've always backwashed straight away so can only assume that there has been a little backflow in shutting the pump off enough to disturb the DE layer and create a path through.

Having opened and checked my sand filter after backwashing all traces of DE are have been gone.

Hope that helps a little.

Thanks much--makes sense. I'll backwash in the morning, add more DE & see what happens.