Conductivity is the measurement of the waters ability to conduct electric current. The basic unit of Conductance(G) is Siemens(S), formally called the mho.

The principle by which instruments measure conductivity is simple - two conductive plates are placed in the water, a sinewave voltage is applied across the plates and the current is measured.

The measuring probes cell geometry affects the conductivity readings. The AAS Conductivity System uses a cell with a geometry of 1cm^-1 and therefore the readings values are displayed as uS/cm or mS/cm.

For example, the conductivity of Ultra Pure water is 0.055uS/cm and that of good city water is 50uS/cm.

The AAS Conductivity cell used can withstand pressures to 150psi. The reading range is from 0 to 100mS/cm and they have a durable CPVC Body with Graphite Electrodes.

Conductivity FAQ Page
the relationship between Conductivity and Water Hardness
and even more Conductivity Technical Information