Chlorine Analyzers For Residual Chlorine Monitoring – HaloSense

Free Chlorine and Total Chlorine analyzers and controllers measure the free residual chlorine (OCl and HOCl), and the total residual chlorine (free chlorine + combined chloramines). The Pi residual chlorine analyzers are amperometric membraned sensors that bring a number of advantages over traditional colorimetric analyzers, in that they are cheaper to own with no reagants and very little maintenance (once per year).

They are extremely stable and work almost anywhere you have a need to measure residual chlorine from drinking water, to pools and spas, produce washing, cooling tower disinfection and disinfection in paper and pulp industries.

Residual Chlorine Monitor 3D View

The following are available in the HaloSense range;
  • Online free chlorine – 0.005-2ppm, 0.05-5ppm, 0.05-10ppm, 0.05-20ppm, 0.5-200ppm
  • Online total chlorine – 0.005-0.5ppm, 0.005-2ppm, 0.05-5ppm, 0.05-10ppm, 0.05-20ppm
  • Online residual chlorine in seawater analyzers (free or total bromine) – 0.005-2ppm, 0.05-5ppm, 0.05-10ppm, 0.05-20ppm
  • Online zero chlorine (designed to measure the absence of free chlorine) – 0.005-2ppm for applications such as post activated carbon and pre-RO monitoring.

If you have any questions or would like to find out more about any aspect of residual chlorine monitoring get in touch with one of our technical experts.


After chlorine has been added and disinfected the process water, any surplus chlorine that remains is referred to as ‘Residual Chlorine’. 

When added to pure water, chlorine is present as hypochlorite (OCl) and hypochlorous acid (HOCl). Together these are referred to as ‘free chlorine’.

‘Combined’ chlorine collectively refers to three chloramine compounds: monochloramine, dichloramine and nitrogen trichloride. Monochloramine is formed from the reaction of hypochlorite and ammonia:

NH3 + OCl → NH2Cl + OH

If the conditions are acidic, further reactions occur that form dichloramine and nitrogen trichloride:

2NH2Cl + H+ → NHCl2 + NH4+
3NHCl2 + H+ → 2NCl3 + NH4+

The familiar ‘chlorine smell’ is often due to these chloramines rather than ‘actual’ chlorine.

Free chlorine and combined chlorine in water can be collectively referred to as ‘total chlorine’. It can be thought of as ‘free chlorine + combined chlorine = total chlorine’.

Our free and total chlorine sensors can be ordered in 0.005-2ppm, 0.05-5ppm and 0.05-20ppm ranges. The total chlorine sensor can also be provided with a range of 0.005-0.5ppm, and our free chlorine sensor is also available in a 0.5-200ppm version.

The simple answer is ‘yes’, but when chlorinating seawater a displacement reaction occurs due to the presence of bromine compounds in seawater. Using chlorine to disinfect seawater is common, but we would recommend talking to a member of our team or reading this Technical Note so that the correct instrumentation can be supplied.

Our chlorine sensors have a very low rate of drift, so the frequency of calibration depends upon the needs of your application. Different customers are known to calibrate weekly, monthly or even biannually.

Changing the electrolyte annually is recommended for our Total and Free Chlorine sensors, and every 3-6 months for our Zero Chlorine sensor.

Our sensors are resistant to changes in pH and will only vary by a very small amount. This is acceptable to most users.

There are known interferences with chlorine dioxide and ozone. A member of our team will be happy to discuss this with you; click here for more information.

Both the membrane and the electrolyte can be stored for two years in a cool, dry place.

All three sensors are constructed of stainless steel, PVC-U, silicon and hydrophilic membrane. The Total and Zero sensors also contain PEEK.

0 – 40°C for the Zero sensor, and 0 – 45°C for the Free and Total sensors.

A zero adjustment is not necessary because the sensor always operates at a positive voltage. Because of this any drift on the zero is negligible compared to the operating voltage.

Our sensors have an internal thermistor that allows for automatic temperature compensation, so there is no need to worry about temperature variation.

Many of our customers use handheld meters to determine the chlorine concentration for calibration, generally using colorimetric DPD analysis. These are available from a wide variety of suppliers.

Three things; timing, location and execution. Take your sample when the concentration is varying as little as possible, take it from the same location as the sensor and follow the instructions for your handheld device carefully.

The analyzer monitors the stability of the probe signal during the calibration process. If this varies by more than 10% during the calibration countdown, then the analyzer will not accept the result. This is to prevent the calibration routine from introducing inaccuracies into the sensor reading.

Focus On

Focus Ons are a series of short articles distributed by email providing technical information regarding instrumentation, process measurement in potable, waste, process and pool waters. If you would like to join the mailing list, please contact us.


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