The sand in a pool sand filter is specially built to trap particles in the 20 - 100 micron range. As a sand filter collects dirt, its efficiency increases, trapping more dirt. When the pressure gauge shows a reading 8 - 10 lbs. over the clean, start-up reading, it is time to backwash the captured dirt out of the filter. We at Clint's Pool Repairs are experts when it comes to this, try us out!