A conductivity cell measures the electrical conductivity of a substance. Two and four electrode cells are common, with the cell constant (K) determining the type. Calibration is necessary to account for changes in the cell constant and temperature. Two-electrode cells are supported by most conductivity meters, while four-electrode cells simplify measurement. A complete measuring system includes a transmitter, controller, and patch cord.
A conductivity cell is a device made up 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. The size of the electrodes, the distance between each one and the pattern of the electric field present determine this cell constant. It is highest for cells with small electrodes that are spaced apart, and lowest for those that have larger electrodes that are spaced apart.
To obtain a conductivity reading, it is necessary to multiply the cell constant and the conductance of the material. The effect of the fringe field must also be taken into account in the equation, which is simplified by also carrying out a measurement of a solution whose 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 an actual value based on the ambient temperature at the time of measurement.
A two-electrode conductivity cell has electrodes made of platinum, gold-plated nickel, titanium, or graphite. Available in immersion or flow-through configurations, the cells can be made of glass or epoxy. Some are unaffected by fringe fields, especially if the measuring field remains within the electrode body. Most types of conductivity meters are supported by the two-electrode configuration.
Another popular type of conductivity cell is the four-electrode version. This design simplifies measurement because there are fewer errors related to polarization or fouling of the electrodes. Current does not flow within the measuring circuit, so voltage is measured accurately across the inner loops of the device. The alternating current flows through an outer set of rings on the cell. A full range of conductivities can be measured with this conductivity flow cell design.
The conductivity cell is only one component in a measuring system. To measure conductivity, the user must also have a transmitter and controller for signal conditioning, as well as a patch cord. An entire system can include everything, complete with a microprocessor to help automate the conductivity measurement process. In addition to temperature compensation, the cell should also be used with an automated range selection method and with conversion factors representing solutions other than plain or salt water.
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