TDS reading ppm of the filtered water:

In time, when the DI resin gets exhausted, they will start to get smaller in size and change from light green or brown color to dark brown, almost black color. You must test the tap water going in, the water coming out of the membrane, and the DI water. All references to this information is found in the installation manual and our website. You will need to compare the three ppm numbers to each other. The water coming out of the membrane should be about 90% rejection from the tap. If it is not, it sounds like you need to replace your membrane.