Turbidity is a measurement of suspended particles in water that scatter or absorb light. For example, the more suspended solids there are, the murkier the water will appear and the higher the turbidity is. Turbidity will also depend on the size of the particles, as this will affect the scattering or absorption of light.
Why measure turbidity?
Turbidity is a useful measurement to evaluate water quality as it can indicate the presence of pathogenic microorganisms and other sources of turbidity such as clay, silt, organic and inorganic matter, algae, plankton, and other microscopic organisms. A major cause of these particles in water is construction sites, but most human activity will increase the turbidity in water. For water to be safe for drinking, WHO has published guidelines recommending turbidity values <1 NTU for safe drinking, so measuring turbidity is important in drinking water treatment plants and disinfection sites.
Turbidity is measured using optoelectronic meters. An artificial light source emits a known light intensity through a sample; the particles in this sample will then absorb or scatter the light. The intensity of the scattered or absorbed light is detected by a photometer, which correlates to turbidity. Several factors change the intensity of scattered light:
- Type of particle (absorbance)
- Concentration (number of particles)
- Size and shape of particles (absorbance – reflection)
- Wavelength of light
- The angle between a light source and the detector
- Geometry/dimensions of the test tube and optical pathway
Lovibond BLAC multipath 90o technology
Lovibond has developed a new method to test turbidity called BLAC technology – Backscattered Light Absorbing Cavity- used in their new TB 350 meters. The arrangement of the two detectors allows samples with high and low turbidity to be measured accurately over the complete measuring range up to 4000 NTU. The detection angle will stay at 90o over the entire measurement range – the method remains nephelometric. This gives consistent results at any time, regardless of the size and shape of the turbidity particles. The BLAC (light-absorbing trap) eliminates stray light to provide extremely accurate results down to 0.01 NTU.
Nephelometric turbidity measurement
A nephelometer (turbidimeter) consists of a light source and a detector positioned at right angles to the incident light beam. The incident beam will pass through the samples and scatter in all directions. In nephelometers, only the light scattered at 90o is measured for turbidity. This angle is recommended for low turbidity, <400NTU, according to ISO 7027 and US EPA regulatory standards.
Turbidity measurements are usually expressed as NTU (nephelometric turbidity units) or FNU (nephelometric formazin units) and refer to the method as well as the standards used for the meter.
Choosing a light source
The choice of a light source will depend on what compliance you are adhering to. ISO 7027 requires an infrared light source, like the TB 350 IR, while US EPA requires a tungsten or white light source, like the TB 350 WL. US EPA also allows other light sources, such as red LEDs or lasers.
TB 350 IR and WL
- Full-colour touchscreen user interface that provides animated and guided procedures with straightforward data management
- Multi-lingual graphical interface
- Delivers superior accuracy at low and high turbidities
- A flexible state-of-the-art optical system
- 90o nephelometric detection over complete measuring range of 0.01 to 4000 NTU
- Visual alert if reading is out of tolerance
- High-performance optics for accuracy
- Supplied ready-to-use with sample cells, silicone oil and T-CAL calibration standards
- 3 different modes – single, signal averaging, fast-settling
More information on turbidity and determining the right Lovibond meter for your application can be found here
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