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Turbidity Testing

Turbidity measurement

The term “turbidity” is used to describe the cloudy or milky appearance of liquid or solid media such as water (drinking, mineral, bathing or waste water), beverages (beer, wine or soft drinks) or window glass (translucent glass).

In physical terms, turbidity is due to particles of varying sizes scattering or absorbing light, giving the medium in question a cloudy appearance. This turbidity is caused by suspended particles such as sludge, limestone, yeast or microorganisms. 

It is still common practice to test water samples using a “sight tube” made of plastic; the tube is lowered into the water until a mark can no longer be seen. The turbidity is then calculated on the basis of immersion depth. Today, the phenomenon of turbidity is measured using optoelectronic meters. An artificial light source emits a known intensity of light through a sample.

The suspended particles scatter or absorb the light. The scattered light is then recorded on a photodetector. Nowadays, the scattered light is generally measured at an angle of 90°. This measurement principle is known as nephelometry. A nephelometer is therefore a turbidity meter that measures scattered light at an angle of 90°. To obtain defined, reproducible results, turbidity meters are calibrated and adjusted using formazine solutions (reference standard).

These meters display their results in FNUs (Formazine Nephelometric Units). The result measured by a meter operating on the transmitted light principle is shown in FAUs (Formazine Attenuation Units). There are two standards for turbidity measurement that are widely accepted at an international level.   EN ISO 7027, “Water quality, determination of turbidity” outlines all the possible methods for turbidity measurement.  All optoelectronic methods require an infrared light source. This also permits testing of coloured samples. In its method 180.1, “Determination of turbidity by nephelometry”, the EPA in the US describes solely the nephelometric (scatter light) method using a so-called white light source(tungsten halogen lamp). The results measured by different units using the two aforementioned methods cannot be compared.

Measuring the turbidity of swimming pool water would be a suitable and traceable way of making sure you comply with the PWTAG recommendations of 0.5NTU (Nephlometric Turbidity Units). The Lovibond® TB 210IR Turbidity meter offers a convenient, handheld and easy way of measuring turbidity to ensure you comply to these recommendations and offer a safe swimming environment.

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