34
SECTION 6.0
CALIBRATION
6.1 INTRODUCTION
The calibrate menu allows the user to calibrate the turbidity sensor, to enter the equation the analyzer will use to
convert turbidity to a TSS reading, and to calibrate the outputs.
The turbidity sensor can be calibrated in one of three ways:
1.
against a user-prepared solution
2.
against a standard solution obtained from a commercial source
3.
against the results of a turbidity measurement made with a referee instrument.
6.2 CALIBRATING AGAINST A USER-PREPARED STANDARD
6.2.1 Definitions
This section describes how to calibrate the turbidity sensor against a
user-prepared standard. The calibration requires two steps. First,
immerse the sensor in filtered water having very low turbidity and meas-
ure 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. Figure 6-1 illustrates how the calibration works.
Before beginning the calibration, the analyzer does a dark current meas-
urement. 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.
6.2.2 Procedure
1. Obtain a quantity of filtered deionized water. Filtered deionized water can be prepared by using pressure or
vacuum to force deionized water through a 0.2 µm membrane filter. Collect the filtrate in a clean glass con-
tainer that has been rinsed at least three times with the filtrate. The turbidity of the water should be less than
0.5 NTU. Store the water in a clean container and keep it tightly capped when not in use. A freshly opened bot-
tle of commercially available distilled or deionized water is usually suitable for calibration.
2. Prepare a 20.0 NTU standard by diluting 4000 NTU of formazin suspension (PN 905-761854) with the filtered
water obtained in step 1.
NOTE
The standard does not actually have a turbidity of 20.0 NTU. Its turbidity is 20.0 NTU
greater than the turbidity of the filtered water from which it was prepared.
continued on following page
FIGURE 6-1. Calibration against
a user-prepared standard.
MODEL CLARITY II
SECTION 6.0
CALIBRATION
