Hach-Lange 1720E Low Range Turbidimeter - Manual (Page 11)

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General Information
2.2 Theory of Operation
The 1720E Turbidimeter measures turbidity by directing a strong beam of collimated light
from the sensor head assembly down into the sample in the turbidimeter body. Light
scattered at 90° relative to the center line of incident light by suspended particles in the
sample is detected by the submerged photocell (
Figure 1 ).
The amount of light scattered is proportional to the turbidity of the sample. If the turbidity of
the sample is negligible, little light will be scattered and detected by the photocell and the
turbidity reading will be low. High turbidity, on the other hand, will cause a high level of
light scattering and result in a high reading.
Sample enters the turbidimeter body and flows through the baffle network of the bubble
trap. The flow allows bubbles to either cling to surfaces of the baffle system or rise to the
surface and vent to atmosphere. After traveling through the bubble trap, sample enters the
center column of the turbidimeter body, rises into the measuring chamber and spills over
the weir into the drain port. A reading is taken once per second.
Figure 1 90 Degree Detector
1. Lamp 3. Aperture 5. 90° Detector
2. Lens 4. Water Level