Working Principles of Online Colorimetric Sensors for Water Quality Monitoring

Online colorimetric sensors are essential tools for real-time water quality assessment, specifically designed to measure the color or hue of water, which often indicates the presence of dissolved organic matter, minerals, or industrial contaminants. Unlike subjective visual comparison, these sensors provide objective, continuous data. Their operation is primarily based on optical principles, with two main methodologies being prevalent.

The most common approach is Spectrophotometric Measurement. This method involves passing a specific wavelength of light, typically in the visible range (often around 455 nm for yellow hue or 590 nm for red/platinum-cobalt standard), through a water sample in a flow cell. A photodetector on the other side measures the intensity of the transmitted light. The sensor calculates the absorbance (or attenuation) of light, which is directly proportional to the concentration of color-causing substances according to the Beer-Lambert law. The result is usually expressed in standard units like Hazen (Pt-Co) or Gardner scales.

Another widespread technique is the Scattered Light Method (often using a dual-light source). Here, the sensor emits light into the water and measures the intensity of light scattered at a specific angle (e.g., 90° or 135°). The presence of colored substances affects the scattering characteristics. This method is particularly effective for monitoring slight color variations and can be less susceptible to interference from suspended particles when combined with specific optical designs.

Modern online sensors integrate advanced features such as automatic calibration cycles, cleaning mechanisms to prevent fouling, and temperature compensation. They convert the raw optical signal into a digital color value, transmitting it directly to monitoring systems. 

This enables immediate detection of pollution events, such as sewage leaks or industrial discharge, ensuring prompt response and continuous compliance with water quality standards.