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Calibrate
July 2020
•
Standardize Calibration: Standardizing the sensor to a known turbidity.
•
Grab Calibration: Standardizing the sensor to a known turbidity based on a reference
turbidmeter.
7.2.1
Calibrate slope
Conduct a two-point calibration of the turbidity sensor against a user-prepared
nephelometric turbidity unit (NTU) standard.
Procedure
1. Immerse the sensor in filtered water having very low turbidity and measure the
2. Increase the turbidity of the filtered water by a known amount, typically 20 NTU,
Sensitivity is the sensor output (in mV) divided by turbidity. A typical new sensor has a
turbidity of about 10 mV/NTU. As the sensor ages, the sensitivity decreases. Dark current
is the signal generated by the detector when no light is falling on it. The transmitter
substracts the dark current from the raw scattered light signal and converts the result to
turbidity. In highly filtered samples, which scatter little light, the dark current can be a
substantial amount of the signal generated by the detector.
7.2.2
Calibrate against a standard
You can calibrate the turbidity sensor against a commercial standard.
Stable 20.0 nephelometric turbidity unit (NTU) standards are available from a number of
sources. Calibration using a commercial standard is simple. Filtered deionized water is not
required.
Prerequisites
Before beginning the calibration, the transmitter does a dark current measurement. Dark
current is the signal generated by the detector even when no light is falling on it.
Procedure
1. Select Standard Calibration.
2. Press ENTER/MENU.
46
sensor output.
and measure the sensor output again.
The transmitter takes the two measurements, applies a linear correction (if
necessary), and calculates the sensitivity.
The transmitter subtracts the dark current from the raw scattered light signal and
converts the result to turbidity. In highly filtered samples, which scatter little light,
the dark current can be a substantial amount of the signal generated by the sensor.
Reference Manual
00809-0100-3565
Emerson.com/Rosemount
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