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MODEL 54eA SECTION 14.0
TROUBLESHOOTING
15.5.6 Sensor does not respond to changes in chlorine level.
A. Is the grab sample test accurate? Is the grab sample representative of the sample flowing to the sensor?
B. Is the pH compensation correct? If the controller is using manual pH correction, verify that the pH value in the
controller equals the actual pH to within ±0.1 pH. If the controller is using automatic pH correction, check the
calibration of the pH sensor.
C. Is the membrane clean? Clean the membrane and replace it if necessary. Check that the holes at the base of
the cathode stem are open. Use a straightened paper clip to clear blockages. Replace the electrolyte solution.
D. Replace the sensor.
15.5.7 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 sam-
ple. Because the sensor responds only to HOCl, an increase in pH causes the sensor current (and the appar-
ent chlorine level) to drop even though the actual free chlorine concentration remained constant. To correct for
the pH effect, the controller automatically applies a correction. Generally, the pH sensor responds faster than
the chlorine sensor. After a sudden pH change, the controller will temporarily over-compensate and gradually
return to the correct value. The time constant for return to normal is about 5 minutes.
15.5.8 Chlorine readings are too low.
A. Was the sample tested as soon as it was taken? Chlorine solutions are unstable. Test the sample immediate-
ly after collecting it. Avoid exposing the sample to sunlight.
B. Low readings can be caused by zeroing the sensor before the residual current has reached a stable minimum
value. Residual current is the current the sensor generates even when no chlorine is in the sample. Because
the residual current is subtracted from subsequent measured currents, zeroing before the current is a mini-
mum 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%. 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 now 1.8%. The absolute difference between the reading
remains the same, 0.02 ppm.
C. Sensor response depends on flow. If the flow is too low, readings will be low and flow sensitive. Verify that the
flow past the sensor equals or exceeds the minimum value. See the sensor instruction manual for recom-
mended flows.
15.6 TROUBLESHOOTING WHEN NO FAULT MESSAGE IS SHOWING - TOTAL CHLORINE
Refer to the instruction manual for the SCS921 for a complete troubleshooting guide.
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