These Dual TDS Meters for Vectamaxx Commercial RO give you a real time comparison of your TDS or total dissolved solids going into the system as well as the TDS of the water treated. Just turn it on and you can toggle back and forth between the raw water going into the system and compare the quality to the treated water coming out. You can keep a log, entering the TDS readings and that way you can monitor performance of your Vectamaxx RO system. I should mention too that we offer FREE shipping and discount pricing on these systems just click the link up here and it will take you right to our website. Also, these are an option on our great Vectamaxx Commercial Reverse Osmosis systems and really can not be fitted afterwards so when you originally order your Vectamaxx is when you need to order the Dual TDS meter.