Emerson Rosemount 56, Instruction Manual

Page: 92 / 136

10.6.4 pH-Independent Free Chlorine Measurement
A free chlorine sensor generates a current directly proportional to the concentration of free
chlorine in the sample. Calibrating the sensor requires exposing it to a solution containing no
chlorine (zero standard) and to a solution containing a known amount of chlorine (full-scale
standard). The zero calibration is necessary because chlorine sensors, even when no chlorine
is in the sample, generate a small current called the residual current. The analyzer
compensates for the residual current by subtracting it from the measured current before
converting the result to a chlorine value. New sensors require zeroing before being placed in
service, and sensors should be zeroed whenever the electrolyte solution is replaced.
To calibrate the chlorine sensor, access the Calibration screen by pressing ENTER/MENU from the
main screen, select S1 or S2 Measurement and press ENTER/MENU. Press INFO at any time to
learn more about this procedure. A yellow screen will appear with detailed instructions and
information.
The following calibration routines are
covered:
1. Zero Cal Zeroing the sensor in solu-
tion with zero total chlorine
2. Grab Cal Standardizing to a sample
of known chlorine concentration
1. To Zero Calibrate the analyzer with
the sensor attached, follow the step-
by-step
procedures displayed on-screen.
2. To perform a Grab Calibration by
Standardizing the sensor, follow the
step-by-step procedures displayed
on-screen.
10.7 Calibration — Oxygen
Oxygen sensors generate a current directly proportional to the concentration of dissolved
oxygen in the sample. Calibrating the sensor requires exposing it to a solution containing no
oxygen (zero standard) and to a solution containing a known amount of oxygen (full-scale
standard). The Zero Calibration is necessary because oxygen sensors, even when no oxygen
is present in the sample, generate a small current called the residual current. The analyzer
compensates for the residual current by subtracting it from the measured current before
converting the result to a dissolved oxygen value. New sensors require zeroing before being
placed in service, and sensors should be zeroed whenever the electrolyte solution is replaced.
The recommended zero standard is 5% sodium sulfite in water, although oxygen-free nitrogen
can also be used. The 499A TrDO sensor, used for the determination of trace (ppb) oxygen
levels, has very low residual current and does not normally require zeroing. The residual
current in the 499A TrDO sensor is equivalent to less than 0.5 ppb oxygen. The purpose of the In
Process Calibration is to establish the slope of the calibration curve. Because the solubility of
82 Calibration
Section 10: Calibration Instruction Manual
April 2017 LIQ-MAN-56