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Water monitoring To further minimise the impact of air bubbles,


the sensor should never be positioned vertically. Having the sensor mounted vertically severely limits its performance capabilities due to increased risk of air bubbles and other particulates coming into contact with the sensor face. This can cause damage that can ultimately impact data quality through incorrect measurement readings. As well as having the correct sensor orientation,


it is important to place the sensor downstream. Having the sensor installed facing away from the directional flow reduces turbulence and stops other particles from crashing into the sensitive face of the sensor. Much like a vertical installation, readings can become impaired, resulting in inferior data.


3. When is it appropriate to use the hot tap retractable assembly method?


When space is at a premium and operators cannot achieve the required angle via the dip mount method or chain mount immersion, another method is available. The hot tap retractable method places a sensor directly into an existing pipeline without shutting it off, eliminating the time, cost and disruption out of installing a sensor. To create a secure connection the retractable


sensor insertion assembly is bolted to the pipe flange. This allows the sensor to be removed or inserted at pressures up to 10 bar, without the need to reduce or stop the line pressure. Inserting and removing the sensor is a


straightforward task. When inserting the sensor, the capstan handle needs to be turned until it reaches the first stop position. Once in place, the ball valve can be opened, allowing the sensor to reach its final position. The sensor is removed by turning the capstan in the opposite direction and closing the ball valve.


4. a clean sensor is an accurate sensor


It is no surprise that the nature of measuring turbidity and suspended solids can be quite murky.


At low TSS levels plenty of sunlight reaches the aquatic plantlife, mean- ing plenty of dissolved oxygen for fish and other waterborne animals


While in long term operation, the sensor can get extremely dirty due to the accumulation of bio- film, manganese and iron oxide. This problem can be overcome to some


extent by correct installation of the sensor, positioning it so that potential contaminants dissipate as much as possible. In addition, it is also ideal to have some form of cleaning mechanism in place to automatically remove contaminants before they can accumulate. The automatic cleaning method is a vital


component which eliminates the issue of optical contamination and prolongs the sensor lifespan without the need for any manual cleaning. By reducing fouling and particulate contact with the sensor face, both potential measurement errors and unwanted downtime can be kept to a minimum.


5. maintain calibration


To safeguard against inaccurate data, maintaining the turbidity sensor to a high working order is critical. With the complex nature of replicating turbidity and suspended solids characteristics, two main calibration methods have become commonplace. The first method of calibration involves


formazine, which is a synthetic insoluble solution created by combining hydrazine sulfate and hexamethylenetetramine. By creating a wide range of particles shapes and sizes, formazine is the closest method to real samples as it accurately replicates real-life characteristics when light is scattered through the solution. However, formazine can be dangerous


to operators as it is a known carcinogen. Robust health and safety measures are needed if this method of calibration is to be considered. Additional care is also needed due to formazine’s instability at low concentrations which negatively impacts its shelf life. Formazine solutions that are recorded above


400 NTU are considered stable for over a year but its longevity dramatically reduces as the NTU drops. Tests have shown that between 20 and 400 NTU, formazine is only stable for around one


As TSS levels rise the amount of sun- light getting to the plants is reduced. Dissolved oxygen levels fall as plants begin to die away. Fewer fish can survive


month, with stability plummeting further to less than 24 hours when standards drop to between two and 20 NTU. If standards fall below two NTU, the formazine solution is only stable for one hour or sometimes less. Monitoring the shelf life of formazine is important, but costs quickly rise when replacements are needed. If unstable formazine is used in any calibration method, data is dramatically compromised, rendering it unusable. The second method uses a dry standard,


which simplifies calibration whilst improving operator safety by removing the need to use carcinogenic formazine. However, each dry standard calibration is tested against a primary formazine standard to boost accuracy before a hockey puck-like disc is used. Once verified against the result, clear labelling is necessary to ensure operators can accurately check the calibration of the sensor. This method also has the added benefit of reducing operator error. The discs that are used are sealed with a polymer gel that accurately maintains its NTU to ensure precise calibration, helping to avoid data corruption.


summary


The important role played by continuous water quality analysers in safeguarding the quality of both potable and treated waste water makes it critical to ensure they are properly installed and maintained in top working order. Following the steps outlined in this article will provide a useful starting point to help ensure that turbidity and total suspended solids levels are measured as accurately as possible. As a manufacturer and supplier of continuous


water analysers for a wide range of measurement parameters, ABB Measurement and Analytics can help you to achieve optimum performance from your turbidity measurement equipment throughout its operational life.


ABB www.abb.com


At high TSS levels the water is so cloudy no plants or fish can survive. The aquatic eco-system has died


36


November 2018 Instrumentation Monthly


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