5 main reasons why turbidity measurement is challenging, and how these issues are no longer
Turbidity has long been one of the key indicators in determining water quality. Turbidity can be a complex measurement, especially at the very low levels required in UK drinking water production. Measurement can be affected by: 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; resulting 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. You don’t have to be able to see the bubbles in order for them to influence turbidity measurement sub 1NTU levels. Because turbidity measurement is carried out by measuring scattered light at 90o to the incident light, at low levels of turbidity, the detector is picking up a very small signal. Any bubbles present, especially micro bubbles, lead to a large amount of noise and the operator’s accusation of “variable results”.
No Longer
To eliminate bubbles from entering the measurement chamber, the new instrument design incorporates an integrated patented bubble trap mechanism, using both horizontal and vertical flow to trap and remove micro bubbles from the instrument prior to measurement.
2. Avoiding the issues of glass measurement cells
Any surface 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, these 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. Added 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.
No Longer
By submerging the detector into the measurement chamber, the use of glass cells has been avoided. The new instrument also features heated optics to further prevent condensation. This design also eliminates the need to use desiccant, something that is often required to minimize condensation in systems with glass measuring cells.
3. Minimising stray light
Stray light is another problem which can be reduced by a well thought-out instrument design.
Essentially, it is the light that is detected by the instrument that isn’t associated with the scattering by the particles in the sample. Again, very low level turbidity measurement is prone to error due to even the tiniest amounts of light reflected in the sample chamber.
No Longer
The new instrument has been designed to remove stray light. The shape of the measurement chamber combined with advanced optical features captures stray light and prevents it from reflecting inside the measurement chamber. This eliminates false positive results.
4. Keeping it clean
It seems obvious but one of the challenges of turbidity measurement is ensuring the sample chamber is kept clean; any surface in contact with water is prone to picking up any dirt and microbiological fouling.
No Longer
Specially designed wetted parts are used and the measurement chamber design is completely smooth to minimise any places where sediment can build up. Additionally, the measurement chamber is designed to simplify access as making cleaning easy for operators is the key to avoiding contamination issues.
No nooks or crannies.
5. Calibration and the hazards of Formazin
Turbidity calibration is all based around the response of the instrument to formazin. Formazin is a polymer which has relatively consistent light scattering properties and is the ONLY primary calibration standard available. Every other standard for a turbidity instrument is a secondary standard which relates back to formazin. Unfortunately, formazin has two major drawbacks: correct preparation of standards is difficult and low level formazin standards are not very stable. Given that drinking water works are working below 1NTU, then the solutions you would use to calibrate would have to be made up fresh before use.
No Longer
By using an LED light source, the instrument offers long and stable performance over time.
In order to verify the optical system, a solid standard has been developed to enable operators to check their system, but the Lovibond® instrument goes one step further and enables easy, accurate and safe calibration on the primary calibration solution for turbidity instrumentation, a stabilized formazin.
Due to the difficulties in preparing Formazin standards and their instability at low concentrations, the experts at Lovibond have developed TCALplusTM standards. The packaging eliminates common interferences, guarantees the accuracy of the diluted formazin and avoids direct exposure to the operator to the polymer. Simply squeeze and manipulate the bag for approximately 1 minute and it is ready to use – no dilution required. Following calibration, the solution can be completely removed from the sample chamber – all contained for proper disposal.
And that’s not all
With over 100 years of skill, knowledge and comprehension of the users’ challenges , the Lovibond® team of experts has not only overcome many of the technical issues associated with turbidity measurement, the team has also designed it with the water treatment works in mind: ensuring only small volumes of sample are required to reduce water consumption; easy servicing; and introducing an optional App-based interface to facilitate servicing, enable verification and downtime tracking, allow meaningful reporting and, thereby, saving overall TOTEX costs.
Written by Tom Lendrem, Sales Engineer for PMA, the distributor of the Lovibond® PTV 1000 in the UK and Elizabeth Wilkinson, Marketing Manager for The Tintometer Ltd.
