I've been using the "trick" to add DE to a Hayward filter for a few years now. I usually add about 2 cups with no notable change in filter pressure. Normal freshly-backwashed pressure is 20psi.
The pool's been open for about one week now. Lots of bather activity yesterday which severely clouded the water. Truthfully, the water was not 100% clear after opening last week. It was properly sanitized with bleach with no noticeable loss of chlorine prior to this weekend - just was not crystal clear yet after opening. I'm a bit of a fanatic with sparking pools.
After swimming yesterday, the pressure was at 34psi so I backwashed and pressure went to 20psi. Added DE. 3 hours later, the pressure was was 24psi! It was time to go to bed, so I set the time to cycle the filter off 6 hours later since, if the pressure continued to rise, I didn't want the pump operating with high pressure. I know using DE can cause rapid rises in pressure if small particles are present but never had a rise this quickly,
When I got up this morning and turned on the pump, I fully expected the pressure to be high and I'd have to backwash. But, the pressure had fallen back to a normal 21psi ! Weird. 12 hours later, the pressure is still reading 21psi.
So, my TFP friends, what can cause high pressure in the filter, but then after the pump was off for about 4 hours, the pressure drops back to normal (without backwashing). Something up with the filter? The filter is 7 years old. Never had issues. Have never changed the sand.
Some other stats: CYA 55, Cl 6, Ph 7.2, TA: 90. At the time pool had in it 1QT of Metal Magic and 4oz of Leslie's UltraBrite. I use both products about once a month but used both after opening.