Water & Wastewater Asia Mar/Apr 2018

WATER & WASTEWATER ASIA March / April 2018 42 / OPINION PTV 1000 series instrument Here are five main reasons why turbidity measurement is challenging, but are issues no longer… By Tom Lendrem, Sales Engineer for PMA, Distributor of Lovibond ® PTV 1000 in the UK; and Elizabeth Wilkinson, Marketing Manager of The Tintometer Ltd s one o f t he ke y indicators indetermining water quality, turbidity can be a comp l ex m e a s u r e m e n t , especially in low-levels required in drinking water production. F a c t o r s a f f e c t i n g t h e measurement include the size, shape and colour of the material scattering the light, the sample colour, and the particle size distribution of the material in the sample. These can result in different instruments giving different results on the same sample. The common causes of variation in readings between instrumentation are well known, but it is worth listing some of them to understand how the new Lovibond® PTV 1000 instrument design sought to overcome each one. 1. Avoiding bubbles Bubbles are the bane of turbidity measurement. They need not be visible for them to influence turbidity measurement sub 1NTU levels. Turbidity measurement is carried out by measuring scattered light at 90o to the incident light, and at low levels of turbidity, the detector picks up a very small signal. Hence, the presence of bubbles, especially micro bubbles, can lead to a large amount of noise and the operator’s accusation of “variable results”. To eliminate bubbles from entering the measurement chamber, Lovibond ® PTV 1000 is designed to incorporate an integrated patented bubble trapmechanism. It uses both horizontal and vertical flow to trap and remove micro bubbles from the instrument prior to measurement. 2. Avoiding issues of glass measurement cells Any surface contact that comes between the incident light and the detector is not ideal. Benchtop turbidity measurement is inherently prone to error due to defects on the cell caused by cleaning and use, which can cause false positive turbidity results. So why use them in online instrumentation if you don’t need to? Glass cells are prone to scratching and fouling and adding to this is the issue of condensation, a particular problem when using glass cells. Condensation can also affect online instruments when it builds up around the light source. By submerging the detector into the measurement chamber, the use of glass cells is avoided. In addition, the new Lovibond ® instrument also features heated optics to further prevent condensation. This design eliminates the need to use desiccant, something that is often required to minimise condensation in systems with glass measuring cells.