Photometric Optical Sensors

Photometric optical sensors are among the most commonly used measurement principles for online analysers. These methods are based on measuring the interaction of light with the sample and the measurement principle is usually based on Beer – Lambert Law which states about a linear relationship between absorbance and concentration of absorbing species. According to the law:

They are often employed for their simplicity and rapid response. The majority of commercially available photometric sensors fall into main three categories.

1. Colorimetric
2. UV Absorption and
3. UV Visible absorption

Optical sensors, such as non-destructive sensors arrays, have advantage over ISE (ion selective electrodes) for water quality monitoring as in these analysers gross fouling and potential problems or contamination can be detected by colour changes in the sample.

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ISEs are commonly employed as online sensors due to their speed and simplicity as well as low maintenance and purchase cost. With careful use, frequent calibration ideal measurement conditions being maintained, they can achieve higher accuracy and precision. Hence, in theory the configuration and embodiment of an ISE makes it an ideal sensing probe for on-site, real time
monitoring applications.

Data obtained from online ISE measurements, although continuous, is usually semi-quantitative in nature. This is because the analytical signal of the ISE is known to be affected by matrix interferences, drift, contamination (e.g. by organic molecules), non-linear responses at the lower end of the detection, temperature in the non ideal measurement conditions. These factors in turn cause measurements errors. To improve its reliability, especially in the field, an ISE must be regularly calibrated with standards and samples should be analysed according to prescribed measurement conditions. However, the economic costs associated with calibration especially for instruments that require ex-situ calibration with skilled personnel, have greatly limited its adoption.