A TDS meter does not register much of a change between the pre-filtered water and the post-filtered water. This is normal, and the answer has to do with what a TDS meter actually measures.  According to tdsmeter.com, “Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.” This statement is both correct and incorrect.  It is correct in what a TDS measurement consists of, namely the “total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water”. It is incorrect in its conclusion that TDS is related to the purity of the water and the quality of the water purification system.  If the measure of a water purification system was how close it could make water to pure H2O, then maybe it would be correct. But, people need minerals to live. Thus, helpful minerals in your water are actually a good thing.  A quality water purification system should filter out things that are harmful to your body but leave in helpful things such as minerals. Black Berkey elements are designed to leave healthful and beneficial minerals in your water and to extract the unwanted heavy metals such as lead and mercury as well as sedimentary minerals such as iron oxide and aluminum. So, when our customers use a TDS meter, what they find is that the reading both before and after the water has gone through the Black Berkey elements about the same.  And they are correct. The Black Berkey elements do not take out all of the beneficial minerals.  Thus, of the things that a TDS meter will actually detect, the Black Berkey elements will only remove the unwanted heavy metals such as lead and mercury as well as sedimentary minerals such as iron oxide and aluminum.  Therefore, your TDS reading will not change much unless you have a significant amount of heavy metals or sedimentary minerals in your water. A TDS meter does not measure the amount of biological and chemical contaminants. The important thing is to not simply rely on a TDS meter for your measure of whether your water is good. You can have low TDS count water that has harmful bacteria in it. You can have high TDS count water that is perfectly safe (e.g. – pure, unpolluted ocean saltwater…albeit, you wouldn’t drink it, but that’s only because the mineral is salt). Thus, make sure you know what your TDS meter is actually telling you.