Here's a practical demonstration: Mix started 5 days ago. Peat moss was wet down and sat overnight. Then sand and perlite added. Sand and Perlite was previously rinsed with tap water until the water ran clear. 2 parts peat moss, 1 each of sand & perlite.
RO water from vending machine at 4 ppm
mixture put into a 2 x 2 x 3 inch pot. 2 ounces of water flushed through measures @
second flush of 2 oz measured at 57ppm, no picture of runoff taken (appeared the same as the first to the naked eye). Total water used 4 oz.
3rd flush (2 oz water each flush) 17 ppm total water used 6 oz
4 more flushes later 5ppm at this point I would have considered the water clear by sight only. Total water used 14 oz.
Normally I use tap water to wet and mix the peat moss and to flush until clear by sight and then two flushes of RO water and let to sit overnight before seeds or plants go in. In this case RO water was used throughout.