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MODEL DO-03/04 SECTION 10.0
TROUBLESHOOTING
10.4.6 Process readings are erratic.
A. Readings are often erratic when a new sensor or a rebuilt sensor is first placed in service. The current usual-
ly stabilizes after a few hours.
B. Is the sample flow within the recommended range? High sample flow may cause erratic readings. Refer to the
sensor instruction manual for recommended flow rates.
C. Gas bubbles impinging on the membrane may cause erratic readings. Orienting the sensor at an angle away
from vertical may reduce the noise.
D. The holes between the membrane and electrolyte reservoir might be plugged. Refer to Section 10.4.2.
E. Verify that wiring is correct. Pay particular attention to shield and ground connections.
F. Is the membrane in good condition and is the sensor filled with electrolyte solution? Replace the fill solution
and electrolyte. Refer to Section 9.2 for details.
10.4.7 Readings drift.
A. Is the sample temperature changing? Membrane permeability is a function of temperature. The time constant
for response to a temperature change is about 5 minutes. Therefore, the reading may drift for a while after a
sudden temperature change.
B. Is the membrane clean? For the sensor to work properly oxygen must diffuse freely through the membrane. A
coating on the membrane will interfere with the passage of oxygen, resulting in slow response.
C. Is the sensor in direct sunlight? If the sensor is in direct sunlight during air calibration, readings will drift as the
sensor warms up. Because the temperature reading lags the true temperature of the membrane, calibrating
the sensor in direct sunlight may introduce an error.
D. Is the sample flow within the recommended range? Gradual loss of sample flow will cause downward drift.
E. Is the sensor new or has it been recently serviced? New or rebuilt sensors may require several hours to sta-
bilize.
10.4.8 Sensor does not respond to changes in oxygen level.
A. If readings are being compared with a portable laboratory instrument, verify that the laboratory instrument is
working.
B. Is the membrane clean? Clean the membrane and replace it if necessary. Also, increase the frequency and
duration of the air blast. 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.
C. Replace the sensor.
10.4.9 Oxygen readings are too low.
A. 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 mini-
mum can lead to low results.
Example: the true residual (zero) current for a 499ADO sensor is 0.05 μA, and the sensitivity based on cali-
bration in water-saturated air is 2.35 μA/ppm. Assume the measured current is 2.00 μA. The true concentra-
tion is (2.00 - 0.05)/2.35 or 0.83 ppm. If the sensor was zeroed prematurely when the current was 0.2 μA, the
measured concentration will be (2.00 - 0.2)/2.35 or 0.77 ppm. The error is 7.2%. Suppose the measured cur-
rent is 5.00 μA. 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.
B. Sensor response depends on flow. If the flow is too low, readings will be low and flow sensitive. Move the sen-
sor to an area in the aeration basin where the flow or agitation is greater.