What is turbidity and how do you measure it ?

The meaning of turbidity in a liquid

The turbidity of a liquid is the degree to which small particles in the liquid scatter the light that runs through it, making the liquid almost opaque. Turbidity is being caused by small unresolved floating particles that are generally invisible to the naked eye. Therefore they use turbidity sensors to measure the right concentration.

  • Turbidity is not a physical size.
  • Previously turbidity was determined by using a turbidity view panel.
  • Today it is referenced to the turbidity standard formazin.

In which process do I use a turbidity measurement?

Turbidity measurement is often used in the beverage, food and dairy industry as well as in the chemical and pharmaceutical industry, to monitor continuous process results or to indicate changes. The absorption measurement is probably the most widely used method of measuring turbidity. This is due to the large measurement range and the different types of immersion probes, probe tubes and rod probes. The most common applications with this technique are:

  • phase separation between different products;
  • measurement of lauter or the concentration of yeast in breweries;
  • detection of oil in water or the other way around;
  • centrifuge monitoring;
  • waste water measurements.

What happens when shining light through liquids?

When light shines through a liquid, a part of the light can get lost, it is absorbed. 100% transmission means that no light is lost, 0% transmission means that no light reaches the light detector. The transmission is in percentages (%T).

The law of Lambert-Beer describes the relation between the absorption and the transmission of light as follows: when a light beam with a particular intensity shines through an absorbing  material, the beam will have a lower intensity when leaving that material. A part of the light beam will be absorbed. The extinction depends on the absorbing particles in the material but also from the molar concentration from the absorbing particles and the thickness of the material.

An example of Beer–Lambert law, a green laser lighting in a solution of Rhodamine 6B, the beam becomes weaker as it progresses inward

Conclusion: increases in turbidity of the medium inevitably increases the absorption!

What is Formazine ?

For turbidity measurement, a formazin suspension is made by mixing solutions of hydrazine sulfate and hexamethylenetetramine with ultrapure water. The average particle size is 1,5 microns with a standard deviation of 0,6 microns. For the calibration of a turbidity sensor a formazin suspension of 1000EBC/TEF4000 is recommended. This relatively high concentration is characterized by improved storage capacity of up to 6 months.

How is turbidity measured

From the transmitting light a bundled beam of light is transmitted through the liquid. This beam is scattered on particles and recorded by the scattered light detectors. At the same time the transmitted light is measured. The quotient of scattered light and transmitted light is the turbidity value. The turbidity of a liquid can be measured with 4 different methods.

  • The transmitted light method: measures the weakening of the transmitted light in an angle of 180°. It is especially suitable for high turbidity and samples with large particles. The measurement results can be expressed in FAU (Formazin attenuation units).

The scattered light method (nephelometric): Measures the intensity of the scattered light in an angle of 90°. It is suitable especially for low turbidity. Measurement results can be expressed in FNU (Formazin nephelometric units) or NTU (Nephelometric Turbidity Units)

The reflection measurement: measures the intensity of the scattered light directly reflected from the medium. Here the transmitter and the receiver are in a line. The light must therefore be reflected by 180°. This method can only be applied with high turbidity from about 2000 NTU and in a restricted range of measurement. The measurement results can be expressed in NTU.

  • The ratio method: Shows the relation between the transmitted light measurement and the stray light measurement. It is not compliant with international standards. Measuring instruments based on the ratio-based measuring principle can minimize stray light and colour interferences.

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