microsiemens
Electrical conductanceexact by definition
The microsiemens is one millionth of a siemens. It is the working unit of water quality instrumentation, where conductivity is reported as microsiemens per centimetre: ultrapure water sits near 0.055, distilled water a few, tap water in the hundreds, and boiler or cooling tower water in the thousands. The factor of 1e-6 is exact, being an SI prefix.
Watch out: Microsiemens per centimetre is a conductivity, a per-length property of the liquid, while the bare microsiemens is a conductance of a particular cell. A meter reading depends on its cell constant, and conductivity readings are also temperature compensated, usually to 25 degrees Celsius, so two numbers only compare if both the cell constant convention and the reference temperature match.
| 1 μS | 0.000001 S |
About the microsiemens
Water conductivity is the workhorse measurement of water treatment because it is cheap, continuous and closely tracks the total dissolved ionic load. Pure water conducts almost nothing: at 25 degrees Celsius, self-ionisation alone gives about 0.055 microsiemens per centimetre, the theoretical floor, which is the same sample described from the other side as 18.2 megaohm centimetres. Every ion you add raises it, so conductivity is used as a stand-in for total dissolved solids, for boiler cycles of concentration, and for the trigger on a cooling tower blowdown valve.
Two practical caveats travel with every reading. First, conductivity is temperature sensitive at roughly 2 percent per degree Celsius, so meters compensate to 25 degrees Celsius and a reading without that compensation is not comparable to a spec. Second, the conversion from microsiemens per centimetre to milligrams per litre of dissolved solids is an empirical factor, commonly between 0.5 and 0.7 depending on which salts dominate, not a physical constant. Treat any TDS number derived from a conductivity probe as an estimate with a named assumption behind it.