Problem See Section
Readings are too low. Readings are too low.Section 11.5.3
Current output is too low. Section 11.5.8
Alarm relays do not operate when setpoint is exceeded
or do not release when reading is below setpoint.
Section 11.5.9
11.5.1 Zero current is too high.
1. Is the sensor properly wired to the transmitter? See Section 4.4.
2. Is the zero solution chlorine free? Take a sample of the solution and test it for total
chlorine. The concentration should be less than about 0.05 ppm. Avoid using tap
water for zeroing the sensor. Even though the tap water contains no iodine, chlorine
oxidants present in the tap water may produce a sensor current as high as 100 nA.
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. Is the sensor fill solution fresh? An old, discolored fill solution may produce a high
zero current.
5. Is the membrane damaged? Inspect the membrane and replace it if necessary.
11.5.2 Zero current is unstable.
1. is the sensor properly wired to the transmitter? See Figure 4-4 and Figure 4-5. Verify
that all wiring connections are tight.
2. Readings are often erratic when a new sensor is first placed in service. Readings
usually stabilize after about an hour.
3. Is the space between the membrane and cathode mesh filled with electrolyte
solution, and is the flow path between the electrolyte reservoir and 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.
If shaking does not work, try clearing the holes around the cathode stem. Hold the
sensor with the membrane end pointing up. Unscrew the membrane retainer and
remove the membrane assembly. Use the end of a straightened paper clip to clear
the holes at the base of the cathode stem.
4. Verify that the sensor is filled with electrolyte solution. Refer to .
11.5.3 Sensitivity is low or readings are low.
1. Does the reagent carboy contain reagent? Is the reagent uptake tubing below the
level of the reagent? Has potassium iodide been added to the actic acid (vinegar)
reagent?
Troubleshooting
Instruction Manual 91
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