DOSApedia

Membrane-covered amperometric sensors

Bound chlorine

When most people hear the word "disinfection", they automatically think of chlorine. There are, however, several different kinds of chlorine, such as free chlorine and bound chlorine.
Bound chlorine is the reason for the pungent smell and unpleasant taste in the air – but has very little to do with disinfection or protection of people. Membrane-covered sensors are also capable of detecting this kind of chlorine.

As in many other fields of life, it is of great advantage to be able to measure all the parameters of a process online and to a high standard of accuracy. Too little disinfectant can endanger health or, in an industrial environment, production itself. If too much disinfectant is present, this may jeopardise human health but the most common result is a financial loss through excessive deployment of the substance. Dependable systems involving membrane-covered amperometric sensors are recognised by the German authorities for use in public swimming baths and are in use worldwide.

Effectiveness

Free chlorine also exists in a range of forms, and sometimes simultaneously in several forms. The composition will depend on the pH of the water in which it is found. The membranes and electrolytes enable measurement to take place in various pHs and, in many cases, even where the pH value fluctuates. In conjunction with a controller, such as the DCW 400ip, membrane-covered amperometric sensors are also capable of providing details on the effectiveness of free chlorine under the conditions pertaining.

Many disinfectants, many variations

The chlorine subgroups have been mentioned above, but this offered only a brief introduction to the possible variety of membrane-covered amperometric sensors available. Free chlorine may be bound to iso-cyanuric acid, which will interfere with measuring systems without a membrane. Similarly, some processes cannot work effectively in the presence of disinfectants. In some installations, it is essential to protect the sensitive filter membranes. But chlorine is not the be-all and end-all in the world of disinfectants

It is, indeed, possible to measure these chemicals:

- Chlorine

- Bromine

- Chlorine dioxide

- Ozone

- Chlorite

- Hydrogen peroxide

- Peracetic acid

Use without a controller

Membrane-covered amperometric sensors are equipped with the technology for automatically detecting the current temperature of the fluid, and the sensors process that information internally. As a result, these sensors are in a position to work anywhere in the world without a special-purpose controller even in conjunction with solar energy systems.

The following signals can be transmitted to the periphery:

- 0 to -2000 mV

- 4 to 20 mA

- ModBus RTU

Simplified maintenance

If used properly for the intended purpose, membrane-covered amperometric sensors have a virtually unlimited service life. The electrolyte and membrane cap will, however, require regular replacement. 

Two-electrode system

A 6 mm diameter nozzle in the flow cell at a distance of approximately 15 mm will provide a constant jet of disinfectant fluid (shown in the illustration by a blue arrow) perpendicular to the membrane surface (1).

Primarily, the membrane will only allow molecules of the disinfectant to pass through it. These molecules travel through the liquid electrolyte (2) to the working electrode (3). The working electrode (3) is made of a precious metal (gold or platinum) to prevent it corroding and is subject to a very low, precisely controlled potential difference. As a result, the disinfectant releases electrons towards the surface of the working electrode. The electrons flow to the reference electrode (4) where they generate a very low electric operating current. This operating current is measured and combined with the temperature to produce an electric signal (6). This signal may be a standard voltage signal from 0 to -2000 mV, a standard current signal from 4 to 20 mA or a standard bus signal for a ModBus RTU system. 

The reference electrode (4) is coated with a very fine layer of silver chloride. In conjunction with each other, the coating and the electrolyte generate an extremely accurate zero-point voltage. It is only this exact zero point that enables the precise potential difference to be set for the surface of the electrode.

Besides the signal connection (6) a supply voltage (7) is also required, depending on the type of output signal.

Three-electrode system

A 6 mm diameter nozzle in the flow cell at a distance of approximately 15 mm will provide a constant jet of disinfectant fluid (shown in the illustration by a blue arrow) perpendicular to the membrane surface (1).

Primarily, the membrane will only allow molecules of the disinfectant to pass through it. These molecules travel through the liquid electrolyte (2) to the working electrode (3). The working electrode (3) is made of precious metal (gold or platinum) to prevent it corroding. The working electrode (3) is subject to a very low, precisely controlled potential difference. As a result, the disinfectant releases electrons towards the surface of the working electrode. In a three-electrode system, the electrons flow to the return electrode (5) where they generate a very low electric operating current. The return electrode (5) is a stainless steel ring. This configuration ensures the reference electrode (4) is kept current-free. The operating current is measured and combined with the temperature to produce an electric signal (6). This signal may be a standard voltage signal from 0 to -2000 mV, a standard current signal from 4 to 20 mA or a standard bus signal for a ModBus RTU system. 

The reference electrode (4) is coated with a very fine layer of silver chloride. In conjunction with each other, the coating and the electrolyte generate an extremely accurate zero-point voltage. It is only this exact zero point that enables the precise potential difference to be set for the surface of the working electrode.

Besides the signal connection (6) a supply voltage (7) is also required, depending on the type of output signal.

Membrane-free amperometric sensors

Primarily, the membrane will only allow molecules of the intended disinfectant to pass through it. This kind of selection is not possible without a membrane. In systems without a membrane, it is also not possible for the electrolyte to deliver some of its functionality, to buffer the pH or to distinguish between individual disinfectants. What is more, the all-important working electrode is subject to contamination from the soiling in the water, which makes it necessary to implement cleaning mechanisms or clean it using electric circuits.

Sensors without membranes are also less susceptible to changes in pressure. Sensors of the DOSASens AS family do not require any specially adapted electronic circuitry in the signal transmitter

.

Copper-platinum sensors

The benefits of the membrane and electrolyte are not provided by copper-platinum sensors, because they have neither. Measurement of a disinfectant using the amperometric principle requires a precise potential difference between the reference electrode and the working electrode. In membrane-covered sensors, this potential difference is provided by the electronic circuitry itself and is set to an exact value.

By contrast, the copper-platinum sensor uses the natural voltage difference between the two metals copper and platinum. However, this natural potential difference is also dependent on the size of the surfaces, the water flow rate and the degree of oxidation. In standard systems of this kind, the copper surface is kept free of oxide by means of glass beads.

All in all, this system generates a potential difference that cannot be allocated unambiguously to a specific disinfectant. The electric currents flowing between the copper and platinum are, as a result, very low and require sophisticated processing. A copper-platinum sensor cannot be used without a special-purpose transmitter.

DPD photometer measurement

This method makes use of manual measurement by means of a photometer or a visual colour comparison, where continuous measurements are also an option. However, this method does not distinguish between different disinfectants because the system only measures or compares the same colour each time. In addition, due to the fact that the method is a manual one, considerable individual errors can occur – the result depends to some extent on the person conducting the measurement. Despite that, the procedure is needed as a comparison for the purpose of calibrating the transmitter.

DPD measurement is a patented process that is based on measuring the effect of the disinfectant on a dye. The dye is called dipropyl-p-phenylenediamine. The DPD process is adjusted to the intended disinfectant by means of conversion factors and supplemental substances.

Legionella

Every pipe contains a biofilm colonised by legionella bacteria. Legionella are highly resistant bacteria, approx. 1 µm big, that multiply particularly effectively in warm water. They thrive in slightly larger amoebae, where they also reproduce. The bacteria are transmitted to humans by the inhalation of contaminated aerosols and are a serious risk to older people and those with weakened immune systems.

If contaminated water, in the form of water vapour, or aerosol, reaches the air, the legionella can find their way through the respiratory tract in the body and trigger legionnaires' disease. Legionnaires' disease has been known since 1976. At an American Legion convention in Philadelphia, 221 attendees caught the disease and 34 of them died. This is why it was given the name "legionnaires' disease". After that outbreak, intensive research was carried out and the cause was found to be the legionella bacterium.

In accordance with German standards (Drinking Water Act, Sec. 14(3)), the population of legionella in water supply systems must be monitored. If the limit value of 100 CFU per 100 ml is exceeded, the operators of the system are required to initiate technical countermeasures.