Turbidity Analyzer

Overview

A turbidity analyzer measures suspended solids concentration in water by detecting light scattered by particles. It uses an infrared IR LED Light Source because visible light is blocked by colored water (tannins, iron) but near-infrared penetrates better and does not trigger human eye sensitivity. The LED beam passes through a Flow Cell Assembly containing sample water; particles scatter photons in all directions. A 90-Degree Photodiode mounted at 90 degrees to the incident beam collects scattered light. In clean water (few particles), little light scatters sideways and the detector receives low signal. In turbid water (many particles), much light scatters and signal rises proportionally. The Signal Processor Electronics converts photocurrent to nephelometric turbidity units (NTU), a logarithmic scale that matches particle-counting methods. A motorized Wiper Cleaner Assembly periodically wipes the optical windows to prevent biofouling and mineral buildup.

How it works

Suspended particles, clay, bacteria, algae, floc, scatter light according to Mie scattering theory. For particles much larger than the light wavelength (>1 µm), scattering intensity is proportional to particle volume and count. The Transimpedance Amplifier converts the small photodiode current (nanoamps in clean water, microamps in turbid water) to a voltage via a 10¹⁰ Ω feedback resistor, providing massive gain with minimal noise. The Gain Select digitally switches this feedback resistor to adapt to different turbidity ranges: high gain for clear water, low gain for murky water.

The Microcontroller acquires ADC samples at 1 kHz or higher, averaging 1000 readings per second to suppress optical noise. It linearizes the raw photocurrent to NTU using a two-point factory calibration (zero: pure water at 0 NTU, calibration: 4.0 NTU standard suspension). Between calibrations, response is stable because LED intensity and photodiode responsivity drift together, and the logarithmic nephelometric scale naturally compresses errors.

The Wiper Cleaner Assembly rotates a soft nylon brush against the sample-facing sapphire windows every 10–30 minutes (user-adjustable) to prevent biofilm and mineral scale from accumulating on optics. Fouling reduces signal and causes errors; the wiper adds perhaps 1% maintenance cost but extends useful life between recalibration from weeks to months. The Inlet Tube draws sample through a Intake Strainer preventing sand and large particles from blocking the cell; the Pressure Relief Valve protects against backpressure spikes.

Turbidity is widely used to detect treatment plant filter breakthrough (sudden particle breakthrough when sand beds clog), to monitor drinking-water quality (clear water = <0.5 NTU per most standards), and to control flocculation dosing in water treatment. Infrared turbidity is unaffected by color, making it superior to visible-light turbidimeters for colored or tannin-stained water. Real-time feedback enables operators to adjust coagulant and clarifier settings and detect inlet contamination immediately.