Lucas was designed for ease of use with r/o water in recirculated hydro systems..no PH or EC meter required
I don`t understand what you mean by "dtw once a week". The only res change you will need to make using Drain to waste (DTW) is when your res is empty. The nutes run to the roots only once and the runoff is discarded, it provides a very stable elemental profile, PH and EC for the life of the res. The first and last drop from a dtw res will be pretty much identical. In a recirculated system the runoff goes back to the reservoir minus whatever the plants took from it. Over time, the elemental profile, PH and EC are modified by the ongoing dilution of old and new. Most growers using Lucas formula use the add-back method to judge when to dump the res.Best practices even with Lucas due to biofilm/effluent is to DTW at least once a week, I've used Lucas personally on 2 week reservoir changes, I wouldn't recommend it, it will stunt your growth, res change every week.