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
What is Turbidity meter?
A liquid looks cloudy (turbid) when it contains fine particles that scatter light. A turbidity meter measures this scattering. Instead of judging clarity by eye against reference suspensions, the instrument gives a number, usually in nephelometric turbidity units (NTU) or formazin nephelometric units (FNU).
In pharma QC, turbidity meters are used for the clarity and degree of opalescence test of solutions of APIs and excipients, for reconstituted injections and biologics, and for water. They make the result objective and allow trends to be followed.
The instrument is calibrated with formazin, a polymer suspension made from hydrazine sulphate and hexamethylenetetramine that gives a reproducible cloudiness. Ready-made stabilised formazin or polymer-bead standards and sealed secondary (gel or glass) standards are used for routine checks.
Used for
- Clarity and degree of opalescence of solutions of APIs and excipients
- Opalescence of reconstituted powders for injection and of protein solutions
- Turbidity of purified water and raw water
- In-process clarity checks after filtration
- Stability testing to detect haze or precipitation
How it works
- 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.
Advantages
- Objective, numerical result instead of a visual comparison
- Very sensitive to low levels of haze
- Fast and simple, needing only a few millilitres of sample
- Allows trending of results over time and between batches
- Ratio designs reduce the effect of sample colour
Limitations
- Does not tell you the size, number or identity of the particles
- Scratches, fingerprints and condensation on the cuvette scatter light and give false high readings
- Air bubbles look like particles to the instrument
- Coloured samples absorb light and can give low readings on simple instruments
- Results from instruments with different light sources or optics may not be identical
1 calibration procedure
Keep every Turbidity meter in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.