One detector is located at a 90° beam scatter to measure turbidity, the other detector is located 140° from the beam as a back scatter reference.
Turbidity measurement is done by a factory calibration using the 90° angle.
Suspended solids measurement uses both detectors 90° and 140°, which are used to calculate the solids content based on calibration curves.
Measuring suspended solids requires calibration to the actual sample to optimize the compensation for the particle size and shape typical at the measuring site.
If the solids readings is incorrect by large factors which calibration cannot correct, then clean the optical device with 10% HCl.
Make sure that grabbing and analyzing samples by laboratory methods is done properly.
If the issue is still ongoing, the second detector is not operating correctly and the optics must be serviced by Hach service personnel in either the field or on the service bench.