de Glossar

Calibration (turbidity)

To facilitate practical interpretation of the turbidity value, turbidimeters are calibrated with a standard suspension. Hence the reading is shown not in the form of the measured light intensity, but as a concentration of the calibration suspension. When a given suspension is measured, therefore, the reading shown means that the liquid in question scatters the same amount of light as the standard suspension of the concentration indicated.

The internationally accepted turbidity standard is formazine. The most common units are:

  • ASBC - "American Society of Brewing Chemists".
    American unit used in breweries.
  • EBC - "European Brewery Convention".
    European unit used in breweries.
  • FAU - "Formazine Attenuation Units".
    Unit used in water treatment for transmitted light measurement (angle 0°) as prescribed by standard ISO 7027.
  • FNU - "Formazine Nephelometric Units".
    Unit used in water treatment for measurement at 90° as prescribed by standard ISO 7027.
  • FTU - "Formazine Turbidity Unit".
    Unit used in water treatment.
  • NTU - "Nephelometric Turbidity Unit".
    Unit used in water treatment for measurement at 90° as prescribed by the U.S. EPA (Environmental Protection Agency).
  • TE/F - "Trübungseinheit/Formazin".
    German designation, now replaced by the FNU unit.

Conversion table: turbidity units based on formazine:

1 EBC

1 FNU
1 NTU
1 FTU

1 ASBC

 

1

0.25

0.014

EBC

4

1

0.057

FNU
NTU
FTU

70

17.5

1

ASBC

An older calibration suspension that has now fallen into disuse is that of diatomaceous earth (unit "ppm SiO2"). The reproducibility of its light scattering properties does not meet the requirements expected of standard suspensions, because the particle size distribution is not standardized. On the other hand, diatomaceous earth has the advantage that its optical properties are very similar to those of the natural turbidity substances found in water.

The readings are not directly comparable with those of formazine. Taking the measurement angle into account, however, the conversion factors listed below constitute a close approximation.

Scatter angle

1 FTU

 

25°

2.25

ppm SiO2

90°

3.25

ppm SiO2

135°

7.5

ppm SiO2

Index

Contact

{{selectedCountry.Name}} {{country.Name}}
{{vendor.BusinessAreasString}} {{area.Name}}
{{vendor.BusinessAreasString}}

Main Office