Turbidity (Turbidity) is a measure of how clear or cloudy a liquid (usually water) is due to the presence of suspended particles that scatter and absorb light.
Turbidity describes the optical property of water where tiny particles such as:
remain suspended in water and reduce its transparency.
Instead of measuring particles directly, turbidity measures how much they interfere with light passing through water.
When a light beam passes through water:
More scattering = higher turbidity.
Turbidity is measured using a turbidimeter or nephelometer, which:
Turbidity is widely used in:
High turbidity can:
| Parameter | Specification |
|---|---|
| Definition | Measure of cloudiness in water caused by suspended particles that scatter light |
| Symbol | NTU (reported unit) |
| Primary Units | NTU (Nephelometric Turbidity Units), FNU |
| Measurement Principle | Light scattering (nephelometry) at 90° angle |
| Instrument Type | Turbidimeter / Nephelometer |
| Light Source | Infrared LED (ISO standard) or tungsten lamp (older systems) |
| Detector Position | Typically 90° to incident light beam |
| Measurement Range | 0 – 4000 NTU (instrument-dependent; low-range meters: 0–1000 NTU) |
| Low Detection Limit | ~0.01 NTU (high-precision lab instruments) |
| Accuracy | ±2% of reading or ±0.01 NTU (whichever is greater) |
| Resolution | 0.01 NTU (low range), 0.1–1 NTU (high range) |
| Response Time | 2–10 seconds typical |
| Calibration Standard | Formazin primary standard |
| Calibration Range | 0, 20, 100, 800 NTU typical multi-point calibration |
| Temperature Range | 0–50°C (instrument dependent) |
| Temperature Compensation | Usually not required (optical measurement) |
| Sample Requirements | Free from air bubbles, homogeneous suspension |
| Interference Factors | Color, air bubbles, particle size distribution, fouling |
| Drinking Water Standard | <1 NTU (ideal), max ~5 NTU (regulatory limit in many standards) |
| Wastewater Range | 50 – 4000+ NTU |
| Natural Surface Water | 1 – 100 NTU (varies widely) |
| Key Applications | Drinking water quality, wastewater treatment, environmental monitoring, process water control |
| Parameter | Technical Details |
|---|---|
| Measured Property | Optical scattering caused by suspended particles in liquid |
| Measurement Principle | Nephelometry (light scattering at 90° angle) |
| Units | NTU (Nephelometric Turbidity Units), FNU (ISO equivalent) |
| Standard Method | ISO 7027 / EPA 180.1 |
| Light Source | Infrared LED (ISO 7027) or tungsten filament lamp (EPA method) |
| Detector Geometry | 90° scattered light detection (primary), sometimes multiple angles |
| Measurement Range | 0 – 4000 NTU (instrument dependent) |
| Low Detection Limit | ~0.01 NTU (high-end laboratory instruments) |
| Accuracy | ±2% of reading or ±0.01 NTU (whichever is greater) |
| Resolution | 0.01 NTU (low range), 0.1–1 NTU (high range) |
| Response Time | 2–10 seconds typical |
| Calibration Standard | Formazin primary standard or polymer bead standards |
| Calibration Points | 0, 20, 100, 800 NTU (typical multi-point calibration) |
| Sample Conditions | Homogeneous, bubble-free, stable suspension required |
| Temperature Range | 0–50°C (sensor dependent) |
| Temperature Compensation | Not generally required |
| Interferences | Air bubbles, color (in some systems), particle size, fouling of optics |
| Drift Characteristics | Low, but affected by optical window contamination |
| Drinking Water Limit | <1 NTU ideal, ≤5 NTU regulatory limit (varies by region) |
| Surface Water Range | ~1–100 NTU (high variability) |
| Wastewater Range | 50–4000+ NTU |
| Sensor Maintenance | Periodic cleaning of optical windows required |
| Primary Applications | Drinking water treatment, wastewater monitoring, environmental water quality, industrial process control |
