The DM-1 is a dual inline TDS meter manufactured by HM Digital. This can be used on marine reverse osmosis water filters. We suggest have one probe on the output from the RO membrane and the second on the output from the DI resin stage. Ideal for all water purification applications, aquaculture and hydroponics; suitable for both domestic or commercial use. This unit will allow you to test your water and know if your RO membrane needs replacing.