A TDS meter is measuring conductivity (usually reported in microsiemens/cm) and then applying a conversion factor to estimate the total dissolved solids, which is usually reported in ppm (mg/L). Lotus has adopted the industry standard to report water recipes for coffee in ppm as CaCO3 equivalent which involves an additional conversion that pretends each ion is an equivalent weight. As such, the correlation between a TDS meter's reading of water and the actual minerality is a somewhat complex relationship.