Turbidity

The turbidity (including turbidity, from Latin turbidus, cloudy ) of a liquid is a subjective visual impression. Turbidity is caused by small particles in transparent solids in a liquid or a gas, which have a different refractive index from the carrier or cause absorption.

Historical

Formerly, the turbidity of a liquid in a transparent container was measured on the back table was a view (similar to the vision test at the eye doctor ) that has been considered by the liquid. The number that was barely readable, corresponded to the measured turbidity. However, this method was very inaccurate and also the subjective perception of the subject -dependent.

Measurement in liquids

In order to measure turbidity comparable to the turbidity standard liquid formazine was created. All turbidity units refer to dilutions of the liquid formazine. The most commonly used turbidity units are defined as follows:

  • FAU Formazine Attenuation Units - transmitted light measurement ( angle 0 °) in accordance with the requirements of ISO 7027
  • FNU Formazine Nephelometric Units - measuring scattered light ( angle 90 °) in accordance with the requirements of ISO 7027
  • Formazine FTU Turbidity Unit - unit used in water treatment
  • NTU Nephelometric Turbidity Unit - measurement at 90 ° in accordance with the provisions of the United States, the same as FTU
  • TE / F turbidity unit / formazine - German unit that is used in water treatment
  • EBC turbidity unit - European / international unit that is used in brewing for beers

FAU = FNU = FTU = NTU = TE / F of this applies only to formazine. All other liquids provide different values ​​for the different measurement angles. For example, delivers milk, the fat drops of a similar particle size as formazine have ( approximately 1 micron ), at 0 ° transmitted light measurement and 90 ° scattered light measurement is a difference of up to 50 %.

Measurement methods

Nowadays, the turbidity of a liquid is indeed still determined optically, however, measured by electronic evaluation. The wavelength of the measuring radiation is usually in the infrared region at 860 nm ( according to ISO 7027 ).

In general, a distinction is made between two methods of measurement:

  • The attenuation of a light radiation ( light)
  • The side scatter of light radiation (scattered light )

In general, leave small opacities in the scattered light is best detect and larger opacities in transmitted light.

Typical turbidity values

Turbidity values ​​below 0.016 are not optically measurable, as the pure water molecules already have an influence on the light radiation.

Turbidity in gases

See, among others emissions test. The opacity of the exhaust is often specified in HSU ( Hartridge Smoke Units, without a unit or as a percentage) or as turbidity value K in m-1.

Measured the transmittance T of light of a certain wavelength over a gauge length l based on the extinction E of Light

With T - transmittance (ratio of light passing to irradiated light )

The absorbance A for a given wavelength is related as follows to the smoke density, and the gauge length

With l - measuring length in m and K - absorption coefficient, turbidity value or smoke density in m -1.

The smoke opacity in Hartridge Smoke Units is the opacity O as a percentage, based on a measurement wavelength of 430 nm at atmospheric pressure and 100 ° C

Is measured with green light (520 nm).

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