As water quality monitoring becomes increasingly critical, a wide range of sensors and signal conditioning circuits have been developed to meet the growing demand. Water quality assessments typically involve measuring parameters such as bacterial count, pH, chemical composition, turbidity, and **conductivity**. All aqueous solutions exhibit some level of electrical conductivity, which can be significantly increased by adding electrolytes like salts, acids, or bases to pure water. This article focuses on the measurement of **conductivity**, an essential parameter in various environmental and industrial applications.
Pure water has very low conductivity because it lacks significant amounts of electrolytes. When a voltage is applied, it conducts only a small current, resulting in a very low conductivity value. In contrast, when a solution contains a high concentration of electrolytes, it conducts more current, leading to higher conductivity. Conductivity is inversely related to resistance, and while we often refer to resistance in practical measurements, the two are mathematically reciprocal.
The resistivity (Ï) of a material or liquid is defined as the electrical resistance between opposite faces of a cube-shaped sample with 1 cm sides. The formula for calculating resistance in other shapes is:
$$ R = \frac{\rho L}{A} $$
Where:
- $ L $ is the distance between the electrodes.
- $ A $ is the area of the electrode surface.
Resistivity is measured in ohm-centimeters (Ω·cm). If a 1 Ω resistance is observed across a 1 cm³ cube, its resistivity is 1 Ω·cm. Conductance, the inverse of resistance, is measured in Siemens (S), while conductivity, the inverse of resistivity, is expressed in S/cm, mS/cm, or μS/cm.
In many cases, the unit is simplified to S, mS, or μS for convenience. Conductivity is usually denoted as Y, and accurate measurement requires proper calibration and temperature compensation, as conductivity has a strong temperature dependence—up to 4% per degree Celsius.
To measure conductivity, a **conductivity cell** is used, which consists of two or four electrodes immersed in the solution. These cells are connected to a circuit that applies an alternating voltage and measures the resulting current. Temperature compensation is essential, and additional sensors are often included to adjust readings to a standard reference temperature, such as 25°C.
Conductivity cells come in different configurations, such as **bipolar** and **quadrupole** designs. Bipolar sensors are suitable for low-conductivity samples like pure water, while quadrupole sensors are better suited for high-conductivity solutions like seawater. The electrode constant (K), defined as the ratio of electrode spacing to area ($ K = L/A $), determines the sensitivity and accuracy of the measurement.
Modern systems use advanced signal conditioning and microcontroller-based processing to ensure high precision. For example, the EVAL-CN0359-EB1Z board provides a fully integrated solution for conductivity and RTD (Resistance Temperature Detector) measurements. It features programmable excitation voltages, automatic gain control, and precise sampling techniques to minimize errors.
The system also includes a user-friendly interface with an LCD display and encoder knob for easy navigation and configuration. Communication with external devices is possible via RS-485, making it ideal for remote monitoring and data logging.
Powering the system is straightforward, with all necessary voltages generated from a single 4–7 V supply. The design uses efficient regulators and layout techniques to reduce noise and improve stability.
Overall, this comprehensive approach ensures reliable and accurate conductivity measurements, even in challenging environments. Whether for laboratory analysis, industrial process control, or environmental monitoring, the technology offers a robust and flexible solution for modern water quality assessment.
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