de Glossar

Inherent brightening effect

Quantity of scattered light occurring in a medium entirely free of turbidity.

Certain applications of turbidity measurement, such as the monitoring of iron oxide content in power plant water circuits, the monitoring of drinking water filtration, and the measurement of dust particles in exhaust gases, require the detection of extremely low solids concentrations.

Because the intensity of the light scattered by the few particles present is very low, the limitations of the scattered light principle are shown up by these applications. For the reasons stated in the following, it is not possible to reach the zero value, i.e. the point at which all light is absent. For one thing a p amount of light will always occur in the medium measured as a result of molecular scatter and the presence of residual particles, and for another the flow cell will always be susceptible to so-called instrument stray light.

  • Molecular scattered light occurs because of scatter on the water molecules, or strictly speaking the density fluctuations created by molecular motion. At 550nm, it is on the order of 0.016 FNU = 16 mFNU.
  • The residual particle scattered light is created by particles remaining in the liquid because they cannot be filtered out. The empirical value of water which is filtered according to the ISO standard is less than 5 mFNU. Together with the molecular scattered light, this means that such water exhibits total scattered light of 20 mFNU ±10% at 550 nm and 9 mFNU ± 10% at 860 nm.
  • The instrument stray light is the quantity of light produced in the flow cell even in the case of a theoretically pure medium, mainly because of reflection at the windows.

Creation of stray light (S) at flow cell windows.

The amount of instrument stray light is an indication of the optical quality of a turbidimeter. In the Sigrist process turbidimeter for pure water, the instrument stray light is virtually eliminated by using a suitably designed flow cell.

Certain turbidimeters permit the electronic suppression of the inherent brightening effect by means of zero-point correction with "ultrapure water". The trouble with this system is that residual particle concentration and the instrument stray light can vary. Thus the zero point set is uncertain and can yield negative values in the event of even slight overcompensation.


Scattered light reading (S) as a function of the formazine addition (FNU)
for various inherent brightening effect corrections.

(a) water turbidity not corrected (Sigrist)

(b) water turbidity corrected exactly: 0.019 FNU (theoretical)

(c) water turbidity over-corrected: 0.03 FNU (zero-point setting)

(d) water turbidity over-corrected: 0.05 FNU (zero-point setting)

Sigrist turbidimeters indicate the inherent brightening effect unfalsified, something the user can check at any time with properly filtered water.



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