Emerson ROSEMOUNT 5081-A Instruction Manual page 112

MODEL 5081-A
15.5.2 zero reading is unstable.
1. Is the sensor properly wired to the transmitter? See Section 3.0. 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 mS/cm? do Not uSE dEIoNIZEd oR dIStILLEd WAtER to
ZERo tHE SENSoR. the zero solution 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 elec-
trolyte reservoir and membrane clear? often the flow of electrolyte and 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 ther-
mometer.
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. Be sure the wood ring remains
with the membrane assembly. use the end of a straightened paper clip to clear the holes at the base of the cathode
stem. Replace the membrane.
Verify that the sensor is filled with electrolyte solution. Refer to the sensor instruction manual for details.
15.5.3 Sensor current during calibration is substantially less than 250 nA/ppm at 25°C and pH 7.
1. Is the temperature low or is the pH high? Sensor current is a strong function of pH and temperature. the sensor cur-
rent decreases about 3% for every °C drop in temperature. Sensor current also decreases as pH increases. Above
pH 7, a 0.1 unit increase in pH lowers the current about 5%.
2. Sensor current depends on the rate of sample flow past the sensor tip. If the flow is too low, chlorine readings will be
low. Refer to the sensor instruction sheet for recommended sample flows.
3. Low current can be caused by lack of electrolyte flow to the cathode and membrane. See step 4 in Section 15.5.2.
4. Is the membrane fouled or coated? A dirty membrane inhibits diffusion of free chlorine through the membrane, reduc-
ing the sensor current and increasing the response time. Clean the membrane by rinsing it with a stream of water from
a wash bottle. do Not use a membrane or tissue to wipe the membrane.
5. If cleaning the membrane does not improve the sensor response, replace the membrane and electrolyte solution. If
necessary, polish the cathode. See the sensor instruction sheet for details.
15.5.4 Process readings are erratic.
1. Readings are often erratic when a new sensor or a rebuilt sensor is first placed in service. the current usually stabi-
lizes after a few hours.
2. Is the sample flow within the recommended range? High sample flow may cause erratic readings. Refer to the sensor
instruction sheet for recommended flow rates.
3. Are the holes between the membrane and the electrolyte reservoir open. Refer to Section 15.5.2.
4. Verify that wiring is correct. Pay particular attention to shield and ground connections.
5. If automatic pH correction is being used, check the pH reading. If the pH reading is noisy, the chlorine reading will also
be noisy. If the pH sensor is the cause of the noise, use manual pH correction until the problem with the pH sensor
can be corrected.
6. Is the membrane in good condition and is the sensor filled with electrolyte solution? Replace the fill solution and elec-
trolyte. Refer to the sensor instruction manual for details.
106
SECTION 15.0
TROUBLESHOOTING