The Hach 2100N Turbidimeter is designed for measurement of turbidity from 0 to 4000 NTU (Nephelometric Turbidity Units) with automatic range selection. Samples with higher turbidity levels must be diluted.
The instrument permits turbidity measurements of less than 40 NTU to be performed with the 'ratio' control toggled off, using only the 90 degree detector. At higher turbidities, the ratio-control should be turned on, so as to use the complete set of detectors. With 'ratio' on, the instrument uses signals from the three detectors to calculate the turbidity. The benefits of using Ratio-on for measurements include long-term calibration stability, wide measurement range with excellent linearity, and the ability to measure turbidity in the presence of color.
As shown below, The optical system includes a tungsten-filament light source, lenses and apertures to focus the light, and three separate detectors: 1. a 90 degree detector, 2. a forward-scatter light detector, and 3. a transmitted-light detector. The 90-degree scatter detector receives light scattered by particles. The 'transmitted-light and forward-scatter detectors receive light that passes through the sample.