The
SI unit of conductivity is
S/m and, unless otherwise qualified, it refers to 25 °C. Often encountered in industry is the traditional unit of μS/cm. 106 μS/cm = 103 mS/cm = 1 S/cm. The numbers in μS/cm are higher than those in μS/m by a factor of 100 (i.e., 1 μS/cm = 100 μS/m). Occasionally a unit of "EC" (electrical conductivity) is found on scales of instruments: 1 EC = 1 mS/cm. Sometimes encountered is a so-called
mho (reciprocal of ohm): 1 mho/m = 1 S/m. Historically, mhos antedate Siemens by many decades; good vacuum-tube testers, for instance, gave transconductance readings in micromhos.
The commonly used standard cell has a width of 1 cm, and thus for very pure water in equilibrium with air would have a resistance of about 106 ohm, known as a
megohm, occasionally spelled as "megaohm".
[3] Ultra-pure water could achieve 18 megohms or more. Thus in the past megohm-cm was used, sometimes abbreviated to "megohm". Sometimes, a conductivity is given just in "microSiemens" (omitting the distance term in the unit). While this is an error, it can often be assumed to be equal to the traditional μS/cm.
The conversion of conductivity to the total dissolved solids depends on the chemical composition of the sample and can vary between 0.54 and 0.96.
Typically, the conversion is done assuming that the solid is sodium chloride, i.e., 1 μS/cm is then equivalent to about 0.64 mg of NaCl per kg of water.
Molar conductivity has the SI unit S m2 mol−1. Older publications use the unit Ω−1 cm2 mol−1.