Definition
An online residual chlorine meter is an analytical instrument used to continuously monitor the residual chlorine content in water bodies. Residual chlorine usually refers to the active chlorine components remaining in the water after chlorination and disinfection, including free residual chlorine and synthetic residual chlorine. This instrument can achieve real-time, automatic measurement and data feedback, and is one of the key equipment to ensure water quality safety and process control.
Principle
The measurement of an online residual chlorine meter is mainly based on electrochemical or optical principles. The working process of the electrochemical principle is as follows: the diffusion of residual chlorine molecules in the water sample through the selective permeation membrane, electrochemical reaction occurs in the electrolyte inside the sensor, and produces a microcurrent signal proportional to the residual chlorine concentration, which is converted into a concentration reading after circuit amplification and processing. Instruments with optical principles mostly use the colorimetric method, through the reagent to react with the residual chlorine in the water sample, and then use photoelectric components to measure the absorbance at a specific wavelength, and calculate the concentration according to Lambert-Beale's law. Its basic formula can be expressed as:
A = εbc
where A is the absorbance, ε is the molar absorbance coefficient, b is the length of the optical path, and c is the residual chlorine concentration.
Measurement method
According to different measurement principles, common online measurement methods mainly include constant voltage amperage method, colorimetric method, and some technologies using fluorescence method. The constant voltage amperage method measures the current generated by residual chlorine reduction by maintaining a constant voltage between the working electrode and the reference electrode. Colorimetric methods are performed by automatically adding specific reagents (such as DPD reagents) to make water samples color-developing and then photometry. These methods can achieve continuous sampling and automatic analysis, and the measurement cycle and cleaning frequency can be set through the instrument to adapt to different working conditions.
Factors that affect measurement
The measurement accuracy of an online residual chlorine meter is affected by a variety of factors. Water temperature and pH value affect the morphology and chemical activity of residual chlorine, and usually the instrument is equipped with a temperature compensation function. The flow rate and pressure of the water sample need to be stable to avoid fluctuations in sensor response. Interfering substances in water, such as oxidizing substances such as manganese and chromium, or certain organic matter, can cross-interfere with electrochemical sensors or chromogenic reactions. In addition, sensor membrane contamination, electrolyte consumption, or optical window fouling can also cause measurement drift, so regular maintenance and calibration are necessary to ensure data reliability.
Applications
Online residual chlorine meters are widely used in occasions where continuous monitoring of residual chlorine is required. In drinking water treatment and distribution systems, disinfectant residues used to monitor the end of the pipe network and secondary water supply ensure water supply safety. In industrial circulating cooling water treatment, it is used to control the dosage of biocides to balance microbial inhibition with corrosion control. The instrument also plays an important role in process monitoring and regulation in swimming pool water treatment, food and beverage processing water, and some light chemical process water.
Selection considerations
When selecting a model, it is necessary to comprehensively consider the measurement range, accuracy requirements, response time and adaptability to on-site working conditions. For occasions with low concentration and high precision requirements (such as drinking water), membrane electrochemical sensors with low detection limits or high-precision colorimetric instruments can be considered. For industrial wastewater or circulating water with complex water quality composition and interfering substances, attention should be paid to the anti-interference ability and automatic cleaning function of the instrument. The installation environment, such as the degree of protection, the type of output signal, the ease of maintenance, and compliance with relevant industry standards (e.g. those in the field of drinking water) are also important decision-making factors. It is recommended to test and compare according to the actual water sample characteristics to confirm the suitability of the instrument.