A TDS meter can identify dangerous impurities, but only if they are electrolytes. But impurities, which are non-electrolytes, will go unnoticed. In other words, using a TDS meter alone, you will not determine the purity and safety of tap water, since: - not all electrolyte substances are dangerous; - not all hazardous substances are electrolytes, which means they remain invisible to the TDS meter. The best way to test the effectiveness of a shower filter is to use a chlorine colorimeter or chlorine test strips.