Turbidity measurement is highly sensitive and can fluctuate over time, often resulting in discrepancies between field and process instruments. Here's how to minimize this phenomenon and enhance result reliability.
The primary causes of variance between field and process measurements include: the use of non-representative samples, failure to adhere to correct analysis procedures in both field and process settings, inadequate calibration frequency, reliance on instruments or standards with low reliability, and improper installation in unsuitable locations. Therefore, it's essential to ensure all requirements are met before conducting an effective comparison between field and process measurements. To achieve this, understanding the proper methods for conducting measurements is imperative.
How to measure turbidity with:
Field and bench meters
To ensure measurements with metrological reliability, it's essential to follow good laboratory practices:
- Analysis vials must be clean and free of scratches. They should always be cleaned with soft paper before measurements and inserted correctly according to the manufacturer's instructions.
- For measurements and calibration, matched vials should always be used to reduce errors caused by differences in thickness or shape.
- Regular checks with reliable standards and calibration are crucial to ensure the meter's accuracy. The ideal frequency of checks should be determined using a control chart for each application.
- Calibration should be performed at a point close to the analysis range.
- Samples must be homogeneous and free of air bubbles. Vibrating areas should be avoided.
- A standardized procedure for homogenization during sample and standard readings should be established. Shaking should be gentle enough to prevent microbubble formation while ensuring homogeneity.
Check out the available models at: https://www.digimed.ind.br/en/products/turbidity/benchtop_meters_or_portables
Process analyzers
For process analyzers, proper installation and sampling points are paramount. Most process turbidity analyzers are extractive, requiring a sample aliquot to be taken to the instrument. Here are the instructions for obtaining accurate measurements:
- Evaluate the most representative sampling point. In a pipe, for instance, the sample should be taken at a point without turbulence and in the center of the pipe, avoiding ends that may contain air bubbles (if sampled from the top) or settled material (if sampled from the bottom), both of which can result in higher turbidity readings than the actual value.
- Extractive analyzers should ideally have a degassing chamber to remove air bubbles that could be misinterpreted as turbidity. Sample flow should be low to avoid disturbances.
- Install the instrument in a vibration-free, well-aligned location without tilting.
- Conduct checks with reliable standards at a frequency sufficient to ensure the analyzer operates without deviations. This frequency can be established using a control chart and should vary depending on the application.
- For immersion, insertion, or submersion analyzers, adhere closely to the correct installation method and manufacturer guidelines.
Check out the available models at: https://www.digimed.ind.br/en/products/turbidity/analyzers
NOTE: Performance comparison of instruments should rely on reliable standards, not on other instruments.
Finally, after confirming correct analysis procedures, ensure that both field and process analyses are conducted at the same sampling point with the shortest possible time interval. Prolonged field collection or inadequate homogenization can lead to errors due to settled particulate material, resulting in misinterpretation of measurement results. Whenever feasible, use the same batch of standards for instrument calibration to eliminate errors from standard preparation.
By following these recommendations, the comparison results between field and process measurements will be more reliable, enabling more accurate assessment of process conditions and implementation of actions for improvement and system maintenance.
See also:
Why we should not control the dose of coagulants through Turbidity?
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