I have an 18,000 gallon pool with spa and installed a new DE filter (Hayward DE6020) yesterday in place of my broken sand filter. When I first started up the filter before adding DE, the pressure was reading 15 psi and after adding 8 pounds of DE (the filter calls for 7.5 pounds) through the skimmer the pressure still reads 15 psi. Is this normal? I expected a pressure increase when I added the DE. I have been running the filter for 14 hours since adding the DE and the pressure is still at 15 psi. I expected an increase as the filter due to the initial clean of the water. How can I be sure the filter elements got coated with DE and everything is working correctly?