What factors affect the results of residual chlorine testing?

In the case of continuous use, on-line residual chlorine analysis will inevitably encounter zero drift, system contamination, and interference factors caused by changes in sample flow rate, pH, temperature, etc., especially the residual chlorine concentration at the end of the pipe network is very low, and the residual chlorine content in the water is only around the method detection limit. Therefore, accurately monitoring residual chlorine and making the data credible is not only a test of instrument performance, but also a challenge for technicians.

1. Analyze the common interference of the online residual chlorine analyzer in principle

Understanding the common problems and interferences of in-line chlorine meters in principle is essential for accurate measurement of residual chlorine. In particular, trace chlorine detection at the end of the network is even more important. The summary is as follows:

Environmental conditions: Measurement disturbances due to pH, temperature, pressure, flow rate, etc.

System drift: electronic signal drift, sample contamination caused by system background drift;

Correct calibration: Since the standard sample configuration of residual chlorine is not easy, it is difficult to verify the degree of linearity of the calibration curve and the bias induced.

They are analyzed below:

1.1 System drift

Zero drift is also called zero drift, and the zero drift phenomenon refers to the phenomenon that when the input signal of the amplifier circuit is zero (i.e., there is no AC input), due to the influence of temperature changes, power supply voltage instability and other factors, the static working point changes, and is amplified and transmitted step by step, resulting in the voltage at the output end of the circuit deviating from the original fixed value and drifting up and down.

Drift of the output voltage

When the drift phenomenon is severe, the effective signal in the circuit is often “drowned”, making the amplification circuit not work properly. So, can such a bad phenomenon be eliminated to ensure that the circuit works well?

If you want to solve the problem once and for all, you need to start at the root of the problem. According to the definition of zero drift, we can know that the phenomenon of zero drift is generated in the amplification circuit, and most of the operational amplifiers in the amplification circuit are directly coupled. The Q points of the various stages of the direct coupled amplifier circuit are mutually influential, and due to the amplification of the various stages, the slight change of the first stage will cause a great change in the output stage. Therefore, when the input is short-circuited (due to a slight change in the Q point of the input stage for some reason, such as temperature), the output will change slowly over time, resulting in zero drift.

There are many reasons for zero drift, and any change in component parameters can cause output voltage drift, including but not limited to temperature changes, supply voltage instability, etc. A large number of experiments and practices show that the change of temperature is the main reason for the zero point drift, and because the conductivity of semiconductor components is very sensitive to temperature, and the temperature is difficult to maintain constant, the temperature change is a major problem that is very difficult to overcome in the current technology when solving the zero drift problem. However, while it is not possible to completely overcome all the factors that cause zero drift, there are some special techniques and measures that can be used to suppress this drift.

In practical applications, the technologies and measures to suppress zero drift are mainly as follows:

1. Choose high-quality silicon tubes

The reverse saturation current of the collector junction of the silicon tube is several orders of magnitude smaller than that of the germanium tube, so almost all of the current high-quality DC amplification circuits use silicon tubes.

In addition, the manufacturing process of transistors is also very important, even if the same type of transistor, if the process is not strict enough, the semiconductor surface is not clean, will increase the degree of drift. Therefore, qualified semiconductor devices must be strictly selected.

2. Introduce DC negative feedback into the circuit to stabilize the static working point

3. Temperature compensation

The method of temperature compensation is adopted, and the thermal element is used to compensate for the change of the amplifier tube. Compensation means that the drift of another component is used to offset the drift of the amplifier circuit, and if the parameters are properly matched, the drift can be suppressed within a lower limit. In circuits composed of discrete components, diode compensation is often used to stabilize the static operating point. This method is simple and practical, but the effect is not ideal, and it is suitable for circuits that do not require high temperature drift.

4. Modulation

Modulation refers to the conversion of DC changes into other forms of change (such as changes in the amplitude of sine waves), which are amplified by a resistor-capacitance coupling circuit with little drift, and then try to restore the amplified signal to the change of DC components. This method has complex circuit structure, high cost and poor frequency characteristics. As a result, the cost of implementing this approach is high.

5. Differential amplification circuit

Inspired by the temperature compensation method, people use two transistors with the same model and characteristics to compensate, and obtain a better effect of suppressing zero drift, which is the differential amplification circuit. The most widely used unit circuit in integrated circuits is the differential amplifier circuit based on the principle of parameter compensation. In a direct-coupled amplifier circuit, the most effective way to suppress zero drift is to use a differential amplifier circuit.

1.2 Temperature

The temperature of the water sample can have an impact on the results of the chlorine measurement. In general, for every 1°C increase in temperature, the measured value increases by about 5%. In order to get more accurate results, the temperature of the water sample should be between 15~20°C.

1.3 pH issues

The pH of the sample can have a big impact on the test results of residual chlorine, especially when the pH is less than 5 or greater than 10. In order to avoid affecting the accuracy of the test results, samples within this range should be avoided as much as possible in practical applications.

Residual chlorine is composed of three parts: dissolved chlorine, hypochlorous acid and hypochlorite. The content of three of these components varies according to the pH value of the water.

According to the measuring principle, the measurement of the residual chlorine concentration is the measurement of the concentration of hypochlorous acid, which is then converted into the residual chlorine concentration. As you can see in Figure 1, an important condition for the detection of hypochlorous acid in water is that the sample pH is between 5-7. Because the concentration of hypochlorous acid in the water is relatively high in this pH range, which can be greater than 80%, the instrument detection signal can be maximized, and the interference can be relatively minimized. This is because sample pre-acidification is the most important condition for the accuracy of residual chlorine detection.

Typically, the pH range of drinking water is between 7.3 and 8.5, and in this pH range, hypochlorite concentrations are very high, even much higher than those of hypochlorous acid. Therefore, the pH value of the sample is kept stable, and the sample water is properly pre-acidified, and the pH is kept between 5-7, which is an important guarantee for the accurate monitoring of the residual chlorine meter. Otherwise, the accuracy of the instrument’s measurements will be in question, even if the analytical system has automatic pH compensation.

1.4 Interference of flow rate and pressure

In the process of measuring residual chlorine, the sample flows through the surface of the probe continuously, and the change of flow rate and pressure of the sample will bring deviations to the measured value of residual chlorine.

The change of sample pressure/flow rate mainly interferes with the residual chlorine measurement by the electrode method. The reason for this is that, for example, the electrode surface is covered with a membrane, and changes in sample pressure can change the thickness of the electrolyte between the electrode surface and the membrane, as well as small changes in the tension of the covering membrane and the porosity of the membrane, which are enough to cause an incorrect response from the residual chlorine monitoring probe.

When the pressure is too high and the flow rate is too fast, the sensor will not be able to react in time, which will reduce the accuracy of the measurement results. When the pressure is too low and the flow rate is too slow, it will cause the hypochlorous acid and other substances in the sample water to be replenished in time, which will affect the sensitivity of the detection. In general, the flow rate of the sample water should be kept between 0.5 and 1 litre per minute, and the flow rate should be between 5 and 25 cm per second, which is more appropriate.

1.5 The content of ammonia and chloride in water

In practical application, it is sometimes found that when the amount of chlorine added increases, the residual chlorine content in the water sample decreases, and only after the chlorine gas is added will the residual chlorine content increase again. This is what we often call the phenomenon of breaking point chlorination. Usually, this is due to the high concentration of ammonia nitrogen in the water, which causes chlorine to react with ammonia nitrogen in the water and form chloramines.

1.6 The sampling point and sampling pipeline

sampling point should be selected at 10~20 times the diameter of the pipeline after the dosing point to ensure that the chlorine gas is fully mixed after dosing. At the same time, it should be ensured that there are no air bubbles in the pipeline that affect the measurement. Because the metal pipeline will rust when used for a long time in the case of high residual chlorine, the rust in the pipeline will reduce the residual chlorine concentration, and at the same time, the rust adsorption on the surface of the sensor will also affect the measurement accuracy, therefore, the material of the sampling pipeline should be non-metallic pipeline as much as possible, such as ABS pipeline, UPVC pipeline, etc. The length of the sampling line should not be too long, especially if the residual chlorine meter is involved in automatic dosing. When the sampling line has to be very long, a bypass line should be installed as close to the instrument as possible, and the bypass valve should be opened as wide as possible. At the same time, measures are taken accordingly in the software settings of the meter and the automatic chlorinator. The author has encountered such a situation twice, the filtered water residual chlorine meter and the factory water residual chlorine meter are relatively stable and can track the operation of the chlorinator, the test data and instrument display of the two meter water outlets are also basically – caused, but the factory water residual chlorine is higher than the filtered water residual chlorine. The result of the consultation is the negligence of the above-mentioned installation and use.

1.7 Total organic carbon (TOC) concentration

TOC is the total amount of organic matter in water. High TOC concentrations may react with residual chlorine to form chlorine organics, thereby reducing the effective concentration of residual chlorine. This factor needs to be taken into account when performing residual chlorine measurements in water samples containing high TOC.

1.8 Initial chlorine concentration

The initial chlorine concentration is the concentration of chlorine added to the water sample. If the initial chlorine concentration is low, then after a period of time, the concentration of residual chlorine may decrease. When performing residual chlorine measurements, the initial chlorine concentration should be recorded and taken into account when calculating the residual chlorine concentration.

1.9 Biofilm

Biofilm is a layer of bacteria and microorganisms that form inside pipes, vessels, or other water treatment equipment. Microorganisms in the biofilm can react with residual chlorine, reducing its concentration. Therefore, attention should be paid to the cleaning and maintenance of pipes and equipment when performing residual chlorine measurements.

1.10 Pipeline corrosion and corrosion products

Pipeline corrosion and corrosion products may react with residual chlorine, affecting its concentration. Regularly inspect the pipes to remove corrosion products to ensure the accuracy of residual chlorine measurements.

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