Readings drift.
1. 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 a slow response.
2. 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 behind the true temperature of the membrane, calibrating the sensor
in direct sunlight may introduce an error.
3. Is the sample flow within the recommended range? Gradual loss of flow will cause
downward drift.
4. Is the sensor new or has it been recently serviced? New or rebuilt sensors may
require several hours to stabilize.
Sensor not responding to oxygen changes
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 as 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.
Oxygen readings 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.
Eaxample: The true residual (zero) current for a 499ADO sensor is 0.05 µA, and the
sensitivity based on calibration in water-saturated air is 2.35 µA/ppm. Assume the
measured current is 2.00 µA. 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 µA, the
measured concentration will be (2.00 - 0.02)/2.35 or 0.77 ppm. The error is 7.2%.
Suppose the measured current 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.
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.
Diagnostics and troubleshooting
102 Rosemount 5081
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