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 small 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 880nm, it is on the order of 0.007 FNU = 7 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 7027 is less than 5 mFNU.
- 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 and contamination of the windows.
Fig. 22: 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 (for example as a result of window contamination). Thus the zero point set is uncertain and can yield negative values in the event of even slight overcompensation.
Fig. 23: Scattered light reading as a function of the formazine addition (FNU) for various inherent brightening effect corrections
SIGRIST turbidimeters indicate the inherent brightening effect unfalsified, something the user can check at any time with properly filtered water.