A conductivity cell is a device that is comprised of electrodes that sense the electrical conductivity of a substance, such as water. Common configurations include two electrode cells and four electrode conductivity cells. The main trait that differentiates each type is the cell constant of the conductivity cell, identified as the letter K. Electrode size, the distance between each, and the pattern of the electrical field present determine this cell constant. It is higher for cells with small electrodes that are spaced far apart, and is lower for ones that have larger electrodes spaced closer together.
To get a conductivity reading, the cell constant and the conductance of the material have to be multiplied. The fringe field effect must also be factored into the equation, which is simplified by also performing a measurement of a solution in which the electrical conductivity is known. By calibrating a probe with a conductivity cell, it is possible to account for an unknown cell constant that changes as the electrode ages. The reading is also adjusted to a real value based on the ambient temperature when the measurement is taken.
A two electrode conductivity cell features electrodes that are made of platinum, gold-plated nickel, titanium, or graphite. Available in dip or flow-through configurations, the cells can be made of glass or epoxy. Some are not affected by fringe fields, especially if the measuring field remains inside the body of the electrode. Most kinds of conductivity meters are supported by the two electrode configuration.
Another popular type of conductivity cell is the four electrode version. This design makes measuring easier because there are fewer errors related to polarization or electrode fouling. Current does not flow within the measuring circuit, so voltage is accurately measured through the device’s inner rings. Alternating current flows through an outer set of rings on the cell. An entire range of conductivity can be measured with this design of conductivity flow cell.
The conductivity cell is just one component in a measuring system. To measure the conductivity, the user must also have a transmitter and controller for signal conditioning, as well as a connecting cable. A whole system can include everything, complete with a microprocessor to help automate the conductivity measuring process. In addition to compensating for temperature, the cell should also be used with an automated method of selecting a range and with conversion factors that represent solutions different from natural or salt water.
By Andrew Kirmayer