YOKOGAWA FC400G, User Manual, Page: 67 / 91

< 7. Calibration >
7-6
IM 12F5A1-01E
7.3 Span Calibration
NOTE
Before performing span calibration, confirm that the electrode does not remain exposed to the air as
after zero calibration, and the electrode rotates normally in the measuring tank.
When both the zero point and span are calibrated, calibrate first the zero point and then the span.
There are two span calibration methods. One is a sampling method in which the reading of the
FC400G is adjusted to the value obtained by manual analysis. The other is a standard solution
method in which the reading is adjusted to the concentration value of calibration standard solution
(free chlorine concentration of approx. 80% of the output range).
Use of the sampling method is suitable to calibrate the FC400G more correctly, for example, when
always measuring near-zero low concentrations of free chlorine. In general, there is no practical
problem with span calibration using the sampling method.
The sampling method takes less effort than the standard solution method. Note that the electrode
sensitivity is required to be initialized at start-up or when performing calibration immediately after
the indicator electrode is replaced. The electrode sensitivity can be initialized by setting the service
code 08 to “1” for “initialize electrode sensitivity.” However, calibration after initializing the electrode
sensitivity is performed unconditionally. Before performing span calibration, be sure to confirm that
the electrode mechanism block has been placed back into the measuring tank and is ready for
measurement.
Note: To initialize the electrode sensitivity, press the
YES
key when “CHANGE” is displayed in the message display when entering
the <CAL> mode. (When “NO.CHGN” is displayed, press the
NO
key to display “CHANGE” and then press the
YES
key.)
7.3.1 Span Calibration Using Sampling Method
Span calibration using the sampling method can be performed as follows.
Note: Before performing span calibration, confirm that the electrode does not remain exposed to the air as after zero calibration, and
the electrode rotates normally in the measuring tank.
Start span calibration at the point when zero calibration using the method in
Sec.
7.2.1 or 7.2.2 is
finished and “SPAN” is displayed in the message display.
Note: When calibration described in Sec. 7.2.1 is performed, the FC400G is in the MEAS mode (or in the <HOLD> mode) and “SPAN”
is not displayed. Refer to (b) in Sec. 5.4.1 and display “SPAN” in the <CAL> mode.
(1) Press the
YES
key to enter the span calibration mode.
Free chlorine concentration is displayed in the data display.
(2) Collect sample water for performing manual analysis.
Confirm that the free chlorine concentration of the sample water remains stable before span
calibration is finished. Collect sample water for performing manual analysis by opening the drain
cock on the measuring tank.
Collect sample water while observing the liquid surface in the measuring tank to make sure the
tip of the counter electrode (silver chloride electrode) is not exposed to the air.
Note: If the counter electrode is exposed to the air, the reading will drop to zero, and when the counter electrode is re-immersed into the
sample water, it will take some time (usually several minutes) before the reading stabilizes. If it is accidentally exposed to the air,
confirm that the reading has returned to the original value and is stable and perform the operation in item (4).
(3) Measure the free chlorine concentration of the sample water using manual analysis (e.g., an
amperometric titration method).
The free chlorine concentration of the sample water changes over time. Perform manual
analysis immediately after collecting sample water.
(4) Enter the obtained value in the data display and perform span calibration.
Confirm that the obtained value has been entered correctly and press the
ENT
key. This causes