Disinfection: FCLi Free Chlorine Measuring System

Source: Emerson


Measuring free chlorine for on-line continuous applications can be accomplished in fresh water applications using the free chlorine system FCL without the use of chemical reagents or pre-treatment. The complete system includes a sensor, cable, analyzer, and flow controller. Continuous pH correction eliminates expensive and messy reagents and troublesome sample conditioning systems in samples having pH as high as 9.5. The full feature-packed analyzer includes dual outputs, three programmable alarm relays and a two-line display with a choice of six languages.

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Datasheet: FCLi Free Chlorine Measuring System

The FCLi free chlorine system is intended for the determination of free chlorine (hypochlorous acid plus hypochlorite ion) in fresh water. Unlike free chlorine analyzers from other manufacturers, the Model FCLi does not use expensive sample conditioning systems or messy reagents to control pH. Nor, does it require an auxiliary pH sensor for pH correction. Instead, the pH adjustment takes place inside the sensor, producing a signal that changes less than 4% per unit change in pH between pH 6.5 and 10. Below pH 6.5 the change is less than 1%. The linear range of the sensor is 0 to 20 ppm (mg/L).

The FCLi is not intended for the determination of total or combined chlorine (like monochloramine). Nor, can the FCLi be used for the determination of chlorine in seawater.

The FCLi uses a three electrode, membrane-covered amperometric sensor. The sensor consists of a hydrophilic membrane stretched over a gold mesh cathode. A silver/silver chloride reference electrode and an external copper auxiliary electrode complete the circuit. The fill solution is saturated succinic acid slurry. During operation, an electrochemical reaction, driven by the polarizing voltage, consumes free chlorine at the cathode surface. The auxiliary electrode provides the electrons for the cathode reaction, and a current proportional to the reaction rate flows between the electrodes. Because the concentration of chlorine at the cathode is zero, free chlorine in the sample continuously diffuses through the membrane and is destroyed at the cathode. Thus, the cathode current is proportional to the diffusion rate, which is proportional to the concentration of free chlorine in the sample.

Unlike other free chlorine sensors, the FCLi sensor requires neither sample pretreatment nor pH correction. All amperometric free chlorine sensors generate a raw current that depends primarily on the concentration of hypochlorous acid. Because the fraction of free chlorine present as hypochlorous acid is a function of pH, readings will be in error if the sample pH changes from the value it had during calibration. To correct for pH changes, some manufacturers treat the sample with acid to convert hypochlorite to hypochlorous acid. Others continuously measure the pH and use the pH value to correct the chlorine sensor reading. The Model FCLi is different. The sensor uses a highly buffered acidic fill solution for internal pH adjustment. The fill solution converts all the free chlorine entering the sensor as well as much of the free chlorine at the outside surface of the membrane into hypochlorous acid. Thus, the sensor response is practically independent of pH.

For customers who wish to measure pH, an option that includes a pH sensor and flow cell is available.

Maintenance is fast and easy. Replacing a membrane requires no special tools or fixtures. A screw cap holds the pre-tensioned membrane in place. Replacing the membrane and fill slurry takes only a few minutes.

The Model FCLi includes the easy-to-use Model 1055 analyzer. The analyzer features two fully programmable 4-20 mA analog outputs and three fully programmable alarm relays. Programming and calibration is simple and intuitive. The backlit, two-line display allows the user to read chlorine (andpH) at a single glance.

Valves, rotameters, and pressure regulators to control sample flow are things of the past with the Model FCLi. A constant head overflow sampler ensures the correct flow to the sensor no matter how much the sample flow or pressure changes. To eliminate wiring hassles, quick disconnect Variopol cable is standard.

Stable free chlorine standards do not exist. The chlorine sensor must be calibrated using the results of a laboratory test on a grab sample.

  • COMPLETE SYSTEM INCLUDES sensor, connecting cable, analyzer, and flow controller.
  • VARIOPOL QUICK-DISCONNECT FITTINGS makes sensor replacement easy.

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Datasheet: FCLi Free Chlorine Measuring System

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