Turbidity meter
Measures how cloudy a liquid is by shining light through it and measuring the light scattered by tiny particles. Results are given in NTU or FNU and are used for clarity and opalescence tests.
- Pharmaceutical
- Biotech
- Water treatment
- Food & beverage
- Brewing
- Environmental testing
How Turbidity meter works
Light from a lamp or LED passes through the sample in a glass cuvette. Particles scatter some of the light in all directions. A detector placed at 90° to the beam measures the scattered light (nephelometry). Ratio turbidimeters also measure transmitted and forward-scattered light and use the ratio to correct for colour and lamp changes. The signal is converted into NTU using a calibration with formazin standards.
The path of a sample
- 01
Light source
A tungsten lamp (white light) or an LED, often at 860 nm for ISO 7027 instruments, produces a focused beam.
- 02
Sample cuvette
The beam passes through a clean, matched glass cuvette filled with the sample.
- 03
Scattering
Particles in the sample scatter part of the light sideways.
- 04
90° detector
A detector at right angles to the beam measures the scattered light; other detectors may measure transmitted light.
- 05
Ratio and calibration
The instrument calculates the signal (or ratio) and converts it to NTU using the formazin calibration.
- 06
Result
The turbidity is displayed and recorded, and compared with the specification or with reference suspension values.
Main parts and what they do
Light source
Tungsten lamp or infrared/visible LED that illuminates the sample.
Optics and lenses
Focus the beam through the cuvette and onto the detectors.
Sample cuvette
Optical-quality glass vial with an orientation mark. Cuvettes are matched and must be free from scratches.
90° (nephelometric) detector
Measures light scattered at right angles to the beam.
Transmitted and forward-scatter detectors
Used in ratio designs to correct for colour and lamp ageing.
Cuvette holder with light shield
Holds the cuvette in a fixed orientation and keeps out room light.
Calibration and verification standards
Formazin (or stabilised formazin) standards for calibration, and sealed secondary standards for daily checks.
Display and data interface
Shows NTU and sends results to a printer or LIMS.
Terms every analyst should know
Turbidity
The cloudiness of a liquid caused by suspended particles that scatter light.
NTU / FNU
Nephelometric turbidity unit (white light, 90° detection) and formazin nephelometric unit (860 nm LED, ISO 7027). Both are based on formazin.
Formazin
The primary turbidity standard: a suspension formed by reacting hydrazine sulphate with hexamethylenetetramine. A freshly made stock is defined as 4000 NTU.
Nephelometry
Measurement of light scattered at 90°. It is very sensitive at low turbidity.
Opalescence
The milky appearance of a solution. Pharmacopoeias compare it with formazin-based reference suspensions or measure it instrumentally.
Stray light
Light reaching the detector that is not scattered by the sample, for example from scratches. It sets the lowest turbidity the meter can measure.
Keep every Turbidity meter in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.