Conductivity is an important measurement of water quality used widely throughout industrial process control. Applications for electrical conductivity measurement include boiler water treatment, cooling tower water treatment, and reverse osmosis monitoring.
There are several different kinds of sensors for measuring conductivity. Choosing a sensor that fits your application will improve accuracy and ensure longevity of your equipment. One important factor to understand before choosing a conductivity sensor is how to select a conductivity cell constant.
Conductivity sensor working principle
Conductivity can be measured using either a contacting conductivity sensor or an electrodeless, toroidal conductivity sensor. The conductivity sensor working principle depends on the sensor type. Understanding conductivity cell constants is important when selecting a contacting conductivity sensor due to its working principle.
A contacting conductivity cell has two or more surfaces of known area spaced a known distance apart. Electrodes in conductivity cells are constructed of a conductive material, such as graphite, stainless steel, or platinum. An AC voltage waveform is applied between the cells, and the resulting current is measured. Conductive ions, such as salts and metals, produce a path for current to flow. Therefore, high conductivity indicates high ionic concentration.
Specific conductivity and cell constants
Measured conductivity is typically measured in milliSiemens (mS) or microSiemens (µS). When using a contacting conductivity sensor, conductivity cell geometry affects the conductivity reading. In order to ensure standardization of electrical conductivity measurements, units of specific conductivity are used. Specific conductivity is expressed as milliSiemens per centimeter (mS/cm) or microSiemens per centimeter (µS/cm).
What is a conductivity cell constant?
Specific conductivity compensates for variations in conductivity cell geometry by multiplying measured conductivity by a factor called the cell constant. Cell constant (k) is directly proportional to the distance separating the two conductive plates and inversely proportional to their surface area.
K = L/a, where a(area) = A x B.
Specific conductivity = Measured conductivity (G) * Cell Constant (k)
Conductivity cell constant determination
A cell constant of 1.0 will produce a measured conductivity (G) approximately equal to the specific conductivity of a solution. However, a cell constant of 1.0 is not always an appropriate choice. For example, in solutions with very low conductivity, measuring surfaces must be placed closer together in order to produce a good signal to the conductivity meter. When the path length between conducting plates is reduced, the cell constant is also reduced to 0.1 or even 0.01. Conversely, when measuring high conductivity solutions, a longer path length (higher cell constant) of 10 or 100 typically produces a more accurate reading.
Select a conductivity sensor with a cell constant that is appropriate for the conductivity range of the solution you will be measuring. Conductivity ranges of typical solutions and the optimum cell constant for each are given in the table below.
|Solution||Conductivity Range||Optimum Cell Constant|
|Ultra pure water||0.05 μS/cm||0.01|
|Power plant or boiler water||0.05-1 μS/cm||0.01 or 0.1|
|Drinking water||150-800 μS/cm||1.0|
|Cooling tower water||0-5mS/cm||1.0|
|Wastewater effluent||0.9-9 mS/cm||1.0|
|Ocean water||53 mS/cm||10 (consider toroidal measurement)|
|29% Nitric Acid||865 mS/cm||100 (consider toroidal measurement)|