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20 Calibrating turbidity sensors
20.1 Important information
20.2 General information
Turbidity sensors are calibrated by means of two-point calibration in distilled wa-
ter and formazine reference solution. The sensor is calibrated ex works, mean-
ing that no calibration is required before initial startup. If the measured values
are seen to drift during operation, the sensor should be calibrated.
Preparatory cleaning
Rinse off the sensor with clear water. Organic deposits left on the sensor optics,
such as a biofilm or silt, may cause measurement errors. These deposits should
be removed carefully with warm soapy water and a soft sponge. Never use abra-
sive agents (e.g. scouring sponge)!
Calcium deposits can be removed by submersing the sensors in a diluted hydro-
chloric acid solution (concentration max. 5 %) for several minutes.
20.2.1 Calibration methods for turbidity sensors
Two-point calibration
With two-point calibration, the zero point and slope of the sensor are calibrated.
Distilled water is used as the test material for determining the zero point. For-
mazine is used as the test material for determining the second reference point.
The zero point and calculated percent deviation from the ideal slope are saved
as reference values.
WARNING!
During the calibration, the relays and analog output signals assume the config-
ured states! The response of the output signals is set for each output in its "Re-
sponse at calibration" configuration point.
⇨ Chapter 10.5 "Analog inputs", page 139
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