microsiemens per centimeter
Electrical conductivityexact by definition
The microsiemens per centimetre is the standard unit of water conductivity worldwide, equal to exactly 1e-4 S/m. The factor is exact, following from the micro prefix and the centimetre. Ultrapure water is 0.055 μS/cm at 25 °C, distilled water 0.5 to 3, typical drinking water 50 to 1500, and seawater about 53 000.
Watch out: Conductivity readings are strongly temperature dependent, rising by roughly 2 percent per Celsius degree for typical natural waters. Nearly every meter reports a value temperature-compensated to 25 °C, and an uncompensated reading taken from a hot loop is not comparable to a compensated one. Separately, the conversion to total dissolved solids is not fixed: TDS in mg/L is roughly 0.5 to 0.7 times the conductivity in μS/cm, and the factor depends on which salts are present, so a meter's TDS display is an estimate and not a measurement.
| 1 μS/cm | 1 μS/cm |
Older instruments and literature call this the micromho per centimetre. The mho is the ohm spelled backwards and was the pre-SI name for the siemens, so 1 μmho/cm and 1 μS/cm are exactly the same thing with no conversion required.
About the microsiemens per centimeter
Conductivity is the fastest useful measurement in water treatment because it responds to the total ionic content of the water without identifying anything. Pure water conducts almost nothing, at 0.055 μS/cm, which corresponds to a resistivity of 18.2 megohm-centimetres, the number semiconductor and pharmaceutical plants quote for their polished water. Every dissolved salt raises it. That makes conductivity a proxy for total dissolved solids, a leak detector for heat exchangers, and above all the control variable for cooling tower blowdown.
The cooling tower case is the clearest. Water leaves the tower as pure vapour and leaves the dissolved solids behind, so the concentration of the circulating water climbs until blowdown is opened. Cycles of concentration is defined as the ratio of circulating conductivity to makeup conductivity: makeup at 400 μS/cm running at 1600 μS/cm is at four cycles. A conductivity controller opening a bleed valve on a setpoint is a direct cycles controller, which is why almost every tower has one.
Two cautions matter in the field. Conductivity rises about 2 percent for every Celsius degree, so a sample measured hot reads high, and meters compensate to a 25 °C reference to make readings comparable. Confirm your meter is compensating and to what reference before comparing to a lab result. And the TDS conversion is an estimate: multiplying μS/cm by 0.5 to 0.7 gives mg/L only approximately, because sodium chloride, calcium bicarbonate and sulphate all carry current differently per unit mass. If TDS matters, evaporate a sample and weigh it, or run an ion balance. If a trend matters, conductivity alone is fine.