Don't assume that your conductivity meter is working correctly. Probes can get dirty, worn and damaged. Give yourself that feeling of confidence that comes from doing things the right way. We suggest you use 2 calibration standards to cover a range that you normally work within to verify the linearity of the instrument. A different correction factor may be required for high EC values compared to low EC values. If your meter is essentially linear over the entire calibration range you can feel confident in your measurement results. TDS is an old terminology still used today in some water testing applications. It attempts to correlate Total Dissolved Solids (TDS) with conductivity. But because the conductivity depends upon exactly which "solids" are dissolved it can vary widely with different situations. Biopharm conductivity standards are calibrated using NIST traceable potassium chloride conductivity reference standards and are shown as uS/cm at 25 C. We may include a "TDS" cross reference value for those meters that have TDS scales but we want you to understand the limitations of this terminology. Some of our standards cross-reference TDS values for ppm NaCl and also TDS from the "442" natural water curve which are more likely to fit readings used in water testing, hydroponic and aquaculture applications. We suggest you use EC (electrical conductivity) whose readings are usually microSiemens per centimeter or microMho per centimeter (uS/cm or uMho/cm).