Calibrating Against A Commercial Standard - Emerson Clarity II Instruction Manual

MODEL CLARITY II

6.3 CALIBRATING AGAINST A COMMERCIAL STANDARD

6.3.1 Purpose
The turbidity sensor can also be calibrated against a commercial standard. Stable 20.0 NTU standards are avail-
able 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 gen-
erated by the detector even when no light is falling on it. The analyzer subtracts the dark current from the raw scat-
tered 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.
6.3.2 Procedure
1. Rinse the calibration cup several times with clean water, then several times with 20.0 NTU standard. Fill the
calibration cup to the level of the groove cut inside the cup.
Turbidity standards other 20.0 NTU can be used to calibrate the sensor. However, for greatest
accuracy, particularly when measuring water having low turbidity, 20.0 NTU standard is recom-
mended.
2. Remove the sensor from the flow chamber. If the sensor is dirty, clean it with a soft, damp cloth. Rinse the sen-
sor with several portions of 20.0 NTU standard and place it in the calibration cup. Swirl the sensor to remove
air bubbles. It is not necessary to completely screw the gray coupling nut onto the calibration cup.
BEFORE REMOVING THE SENSOR, be absolutely
certain the process pressure is reduced to 0 psig
and the process temperature is at a safe level.
C C a a l l i i b b r r a a t t e e
Program Display
Calibrate?
S S e e n n 1 1
Sen2
Calibrate?
S S e e n n s s o o r r
C C a a l l i i b b r r a a t t e e
Enter TSS Data
Slope
S S t t a a n n d d a a r r d d
Sensor in Std?
Press ENTER
Dark cal in
progress...
3. Press MENU. The main menu screen appears. Choose Calibrate.
Hold
4. If the sensor has dual input, choose Sen1 (sensor 1) or Sen2 (sensor 2).
Output
If the analyzer has single input, choose Sensor.
Output
5. Choose Calibrate.
>>
6. Choose Standard. If Lamp appears in this screen, refer to Section 6.5.
Grab
7. The analyzer prompts the user to put the sensor in the standard solution.
Press ENTER.
8. The screen at left appears showing that the sensor dark current is being
measured. This step takes 30 seconds. If the reading is not stable after
about 60 seconds, press ENTER. The analyzer will use the current reading.
procedure continued on following page
NOTE
CAUTION
SECTION 6.0
CALIBRATION
37