What is Turbidity? Turbidity is the cloudiness or haziness of a fluid caused by suspended solids and dissolved colored materials. It is an optical measurement of water clarity, not a chemical one, and is a key indicator of water quality.
How is it Measured? The most common method is nephelometry, which uses a light source and a photo detector at a 90-degree angle to measure scattered light. This is most sensitive for lower turbidity levels.
Standards and Norms:
USEPA Method 180.1 (US): Uses a tungsten lamp and measures in Nephelometric Turbidity Units (NTU).
ISO 7027 (EU): Uses an infrared light source and measures in Formazine Nephelometric Units (FNU).
EU Pharmacopoeia: Provides reference suspensions for visual comparison of opalescence.
Instrumentation:
Ratio Optics: For higher turbidity levels (above 40 NTU), instruments with additional detectors at different angles are used to extend the measurement range.
Benchtop vs. Portable: Benchtop instruments are more sensitive and have a wider range than portable ones.
Newer Technology (TU5 Series): These instruments have a 360-degree mirror to detect scattered light, allowing for comparable results between lab and online process instruments.
Practical Considerations:
Stray Light: This is a major source of error and can be caused by scratched or dirty vials, fingerprints, or dust.
Calibration: Instruments are calibrated using formazine standards. It is important to use stable standards and to properly mix them before use.
Reading Modes: Different reading modes are available for different types of samples, such as those that settle quickly.
