A friend of mine asked me that question yesterday. I assumed he meant conductivity in microsiemens (μS). Here is what I told him:
Your DS Meter has been factory calibrated with the appropriate NIST traceable Standard Solution. All Myron L conductivity Standard Solution bottle labels show three values: ppm 442™, ppm sodium chloride, and conductivity in micromhos (metric equivalent is microsiemens).
442: Unless otherwise specified, the calibration used for all DS Meters is the “442” standard. The 442 Standard Solutions™ consist of the following salt ratios: 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride. This salt ratio has conductivity characteristics closely matching natural waters and was developed by the Myron L Company over three decades ago.
Sodium Chloride: For every ppm 442 Standard Solution, there is a ppm sodium chloride (NaCl) solution which will have the same conductivity. The parts per million of the equivalent NaCl solution is on each Standard Solution label. Instruments calibrated to NaCl standards are set using equivalent NaCl values.
Conductivity: All Myron L Company Standard Solutions are traceable to the National Institute of Standards and Technology and are within 1.00% of Potassium Chloride reference solutions. The concentrations of the reference solutions are calculated from data in the International Critical Tables, Vol. 6.