8.4.5 Free chlorine measurement and calibration problems
Problem See Section
Zero current is substantially outside the range -10 to 10 nA. Zero current outside range
Zero reading is unstable. Chlorine zero reading unstable
Sensor current during calibration is substantially less than
about 350 nA/ppm at 25 °C (77 °F) and pH 7.
Sensor current too low
Process readings are erratic. Process readings erratic
Readings drift.
Sensor does not respond to changes in chlorine level. Sensor not responding to chlorine
changes
Chlorine reading spikes following rapid change in pH (auto-
matic pH correction only).
Chlorine readings spike
Readings are too low. Chlorine readings too low
Zero current outside range
The zero current is substantially outside the range -10 to 10 nA.
1.
Is the sensor properly wired to the transmitter?
2. Is the zero solution chlorine-free? Take a sample of the solution and test it for free
chlorine level. The concentration should be less than 0.02 ppm.
3. Has adequate time been allowed for the sensor to reach a minimum stable residual
current? It may take several hours, sometimes as long as overnight, for a new sensor
to stabilize.
4. Check the membrane for damage and replace if necessary.
Chlorine zero reading unstable
1. Is the sensor properly wired to the transmitter? Verify that all wiring connections are
tight.
2. Readings are often erratic when a new or rebuilt sensor is first placed in service.
Readings usually stabilize after about an hour.
3. Is the conductivity of the zero solution greater than 50 µS/cm? Do not use deionized
or distilled water to zero the sensor. The sensor should contain at least 0.5 grams of
sodium chloride per liter.
4. Is the space between the membrane and cathode filled with electrolyte solution,
and is the flow path between the electrolyte reservoir and the membrane clear?
Often the flow of electrolyte can be started by simply holding the sensor with the
membrane end pointing down and sharply shaking the sensor a few times as though
shaking down a clinical thermometer.
Diagnostics and troubleshooting
Instruction Manual 103
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