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7.2.1 Slope Calibration – Turbidity
This section describes how to conduct a 2-point calibration of the turbidity sensor against a
user-prepared 20NTU standard. The calibration requires two steps. First, immerse the sensor in
f
iltered water having very low turbidity and measure the sensor output. Next, increase the
turbidity of the filtered water by a known amount, typically 20 NTU, and measure the sensor
output again. The analyzer takes the two measurements, applies a linearization correction (if
necessary), and calculates the sensitivity. Sensitivity is the sensor output (in mV) divided by
turbidity. A typical new sensor has a sensitivity of about 10 mV/NTU. As the sensor ages, the
sensitivity decreases. The figure below illustrates how turbidity calibration works. Before
beginning the calibration, the analyzer does a dark current measurement. Dark current is the
signal generated by the detector when no light is falling on it. The analyzer 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 detector.
7.2.2 Standardize Calibration – Turbidity
The turbidity sensor can also be calibrated against a commercial standard. Stable 20.0 NTU
standards are available from a number of sources. Calibration using a commercial standard is
simple. Filtered deionized water is not required. Before beginning the calibration, the analyzer
does a dark current measurement. Dark current is the signal generated by the detector even
when no light is falling on it. The analyzer 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.
7.2.3 Grab Calibration – Turbidity
If desired, the turbidity sensor can be calibrated against the turbidity reading from another
instrument. The analyzer treats the value entered by the user as though it were the true
turbidity of the sample. Therefore, grab sample calibration changes the sensitivity, it does not
apply an offset to the reading.
26 Calibration
Instruction Manual Clarity II T56
PN-51-T56 May 2012