Emerson Rosemount FCL, Справочное руководство

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9.5.7 Chlorine readings spike following sudden changes in pH
(automatic pH correction).
Changes in pH alter the relative amounts of hypochlorous acid (HOCl) and hypochlorite
ion (OCl¯) in the sample. Because the sensor responds only to HOCl, an increase in pH
causes the sensor current (and the apparent chlorine level) to drop even though the actual
free chlorine concentration remains constant. To correct for the pH effect, the transmitter
automatically applies a correction. Generally, the pH sensor responds faster than the
chlorine sensor. After a sudden pH change, the transmitter will temporarily over-
compensate and gradually return to the correct value. The time constant for return to
normal is about 5 minutes.
9.5.8 Chlorine readings are too low.
Recommended actions
1. Test the sample immediately after collecting it. Avoid exposing the sample to
sunlight.
Chlorine solutions are unstable.
Zeroing the sensor before the residual current has reached a stable minimum value
can cause low readings. Residual current is the current the sensor generates even
when no chlorine is in the sample. Because the residual current is subtracted from
the measured currents, zeroing before the current is a minimum can lead to low
results.
Example: The true residual current for a free chlorine sensor is 4 nA, and the
sensitivity is 350 nA/ppm. Assume the measured current is 200 nA. The true
concentration is (200-4)/350 or 0.56 ppm. If the sensor was zeroed prematurely
when the current was 10 nA, the measured concentration will be (200-10)/350 or
0.54 ppm. The error is 3.6%. Now, suppose the measured current is 400 nA. The
true concentration is 1.13 ppm, and the measured concentration is 1.11 ppm. The
error is 1.8%. However, the absolute difference between the readings remains the
same, 0.02 ppm.
2. Verify that the chlorine sensor is installed in the correct flow cell, that the liquid
level in the constant head sampler is level with the central overflow tube, and that
excess sample is flowing down the tube. If necessary, disassemble and clean the
overflow sampler.
Sensor response depends on flow. If the flow is too low, readings will be low and
flow sensitive. See
Figure 2-1 and Figure 2-2 . See Constant head flow controller .
9.6 Troubleshooting when no error message is
showing - pH
Problem See Section
Calibration Error warning during two-point calibration. Calibration error during two-point calibration.
Offset Error warning during standardization. Calibration error during standardization.
Reference Manual Troubleshoot
00809-0100-3412 May 2019
Rosemount FCL 1056 97