Chlorine measurement and control instruments

Chlorine Instruments

Chlorine is an excellent oxidising agent which is commonly used in a number of different processes. Often found in the pulp & paper sector as a bleaching agent, as well as an oxidising biocide in water treatment applications. As an excellent sanitising agent, we find chlorine used to treat swimming pool water, to treat drinking water and used commonly used in cooling towers to prevent legionella and other pathogens.

Chlorine is a gas at room temperature (it becomes a liquid at ~ -33°C) and can be tricky to administer into a system in it's gaseous form. As such a number of other solutions exist to ensure that chlorine can introduced into a process or system, this is most commonly in the form of Sodium Hypochlorite.

As chlorine sanitises a system or a process it is consumed during the process. The rate of chlorine consumption is referred to as the chlorine demand

Free Chlorine

When Sodium Hypochlorite or chlorine gas is introduced into water it dissolves to form the following two compounds:- 

  • Hypochlorous acid (HCLO)
  • Hypochlorite (CLO-)

These compounds together are referred to as Free Chlorine.

Free Chlorine may also react with naturally present our artificially introduced nitrogen compounds to form Monochloramine plus dichloramine, which is referred to as Total Chlorine

The ratio of Hypochlorous Acid to Hypochlorite that is produced when chlorine or Sodium Hypochlorite is introduced into water is heavily pH dependant as per the below chart.

 At a pH of approximately 7.5 pH the ratio of Hydrochlorous Acid (HOCL) and Hypochlorite (OCL-) is approximately 1:1. 

Considering the logarithmic nature of pH - when the pH value of the solution is just 6.5 pH the ratio changes to 9 : 1 in favour of Hydrochlorous Acid (HOCL).

The ratio inverts if we move into the alkaline spectrum, with a pH of just 8.5 the ratio changes to 9 : 1 in favour of Hypochlorite (OCL-). 

Hydrochlorous Acid works as a very effective sanitising agent, which is up to 10 times more powerful the Hypochlorite.

Residual Chlorine

When used as a disinfectant, Free Chlorine is consumed after it has sanitised a processes. As it's almost impossible to dose the precise amount of chlorine or Sodium Hypochlorite required for a disinfection process, it's preferable to dose sodium hypochlorite so that a residual remains, which we refer to as Residual Chlorine

The residual chlorine is the result of the free chlorine less the consumed chlorine.

The residual chlorine or unconsumed free chlorine is the primary measurement for most chlorine instrumentation. By monitoring the residual chlorine we can ensure that we have enough present in a system to act effectively as a disinfectant and that we're not dosing too much chlorine into a system, which is both wasteful and costly.

Chlorine Measurements

There are two very different methodologies which are used when measuring free or residual chlorine. These two measurement types are:-

  • DPD Colorimetric Detection for offline or sample measurements. 
  • Amperometric Sensors for online or continuous measurements.

A DPD Colorimetric Chlorine Meter normally involves taking a sample of the solution to be measured and adding a small amount of DPD reagent (N,N-Diethyl-p-Phenylenediamine sometimes referred to as Würster Solution).

The DPD reacts with the Free Chlorine to change the colour of sample to a pinkish colour. The depth of the pink colour is proportional to the amount of free chlorine within the sample. By using light absorption from a photometer within the Colorimetric Meter, we're able to accurately calculate the amount of Free Chlorine contained with the sample.

An Amperometric Chlorine Sensor is designed to give continuous or online Chlorine measurements. These are installed where continual residual chlorine measurements are paramount, such as in drinking water and water treatment applications. The Amperometric system measures a change in current directly proportional to the chlorine present in the sample.

Our SZ283 is a special Chlorine sensor that follows the amperometric principle. It is referred to as a Potentiostatic Chlorine Sensor, as it uses a triple electrode sensor arrangement with an electronically controlled potential. This provides a linear relationship between current and concentration and also provides exceptional accuracy at very low concentration levels of chlorine. 

Chlorine Instrument & Chlorine Sensor Range


Portable Chlorine Meter

Portable Chlorine Meter

Our Chlorine Meter range includes a range of portable DPD Colorimetric chlorine meters.

By taking a sample of the solution to be measured and treating it with a reagent, the portable chlorine meter uses the colorimetric prinicple to accurately measure the concentration of free chlorine or total chlorine within the sample.

Our Chlorine meters are designed for taking accurate clorine measurements as either independent stand alone measurements or for use calibrating in-situ equipment. For swimming pool and water treatment professions, a portable DPD colorimetric chlorine meter is a real "must have".

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Chlorine Controller

Chlorine Controller

Our range of Chlorine Controllers offer accurate, dependable and repeatable chlorine measurements as well as a range of control and alarm options.

The chlorine controller range includes instruments suitable as swimming pool chlorine controllers or as water treatment chlorine controllers.

Designed as either stand alone devices or for integration into larger systems our chlorine controller range are available in both surface mounting and panel mounting configurations.

The chlorine concentration is visible in PPM or Mg/L and suitable for use with both amperometric and potentiostatic types of Chlorine sensor.

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Chlorine Sensor

Chlorine Sensor

Our Chlorine Sensor range is designed to connect to our range of Chlorine Controllers to allow continuous online chlorine measurements.

Within our range of Chlorine sensors we are able to offer open amperometric Chlorine sensors as well as our special closed amperometric potentiostatic chlorine sensor.

Our chlorine sensor range are able to measure total chlorine, free chlorine, residual chlorine and we can supply sensors to measure chlorite, chlorine dioxide, and other fluorides and oxidisers such as bromine and hydrogen peroxide.

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