Free Chlorine Measurement And Calibration Problems - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

MODEL 5081-A
16.4.7 Sensor does not respond to changes in oxygen level.
1. If readings are being compared with a portable laboratory instrument, verify that the laboratory instrument is working.
2. 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.
3. Replace the sensor.
16.4.8 Oxygen readings are too low.
1. 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 oxygen is in the sample. Because the residual current is subtracted
from subsequent measured currents, zeroing before the current is a minimum can lead to low results.
Example: the true residual (zero) current for a 499ADO sensor is 0.05 mA, and the sensitivity based on calibration in water-
saturated air is 2.35 mA/ppm. Assume the measured current is 2.00 mA. The true concentration is (2.00 - 0.05)/2.35 or
0.83 ppm. If the sensor was zeroed prematurely when the current was 0.2 mA, the measured concentration will be
(2.00 - 0.2)/2.35 or 0.77 ppm. The error is 7.2%. Suppose the measured current is 5.00 mA. The true concentration is 2.11
ppm, and the measured concentration is 2.05 ppm. The error is now 3.3%. The absolute difference between the readings
remains the same, 0.06 ppm.
2. 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 recommended flows. If the sensor is in
an aeration basin, move the sensor to an area where the flow or agitation is greater.

16.5 FREE CHLORINE MEASUREMENT AND CALIBRATION PROBLEMS

Problem
Zero current is substantially outside the range -10 to 10 nA
Zero reading is unstable
Sensor current during calibration is substantially less than about 250 nA/ppm at 25°C and pH 7
Process readings are erratic
Readings drift
Sensor does not respond to changes in chlorine level
Chlorine reading spikes following rapid change in pH (automatic pH correction only)
Readings are too low
16.5.1 zero current is substantially outside the range -10 to 10 nA.
1. Is the sensor properly wired to the transmitter? See Section 3.0.
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 it if necessary.
84
SECTION 16.0
TROUBLESHOOTING
See Section
16.5.1
16.5.2
16.5.3
16.5.4
16.5.5
16.5.6
16.5.7
16.5.8