Definition: Specific conductance is a measure of the ability of water to conduct an electrical current. It is dependent on the amount of dissolved solids (such as salt) in the water.
Ideal range: Pure water, such as distilled or RO water, will have a very low specific conductance, and seawater will have a high specific conductance. Rainwater often dissolves airborne gasses and airborne dust while it is in the air, and thus often has a higher specific conductance than distilled water. Specific conductance is an important water-quality measurement because it gives a good idea of the amount of dissolved material in the water.
Method of measurement: To measure conductivity we use a machine called a conductivity meter. The actual amount of electricity that a given water solution will conduct changes with how far apart the electrodes are and what temperature the water is. This quantity is expressed in units called mhos (the unit of resistivity is the ohm; mho is ohm spelled backwards). The meter has a probe with two electrodes, usually 1 centimeter apart. Most of the modern ones sense the temperature as well and electronically correct for its effects. Since the meter gives a reading which is corrected for temperature and electrode separating distance, the number is called "specific conductance," expressed in mhos per centimeter at 25° C. The SI unit of conductivity is the siemen (S) named after the French physicist and equivalent to the mho. Thus 1 microsiemen per meter (mS/m) is equivalent to 100 mmho/cm. Very often, a meter will read out in mS/cm or mS/cm (or just mS or mS which are assumed to be per centimeter).
Method of control: Conductance can be adjusted by adding or removing dissolved solids (such as salt) in the water.
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