MODEL SOLU COMP II
SECTION 8.0
TROUBLESHOOTING
8.3.6 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 (applies to Models 499A DO and
499A TrDO sensors only). Refer to Section 8.3.3.
E. Verify that wiring is correct. Pay particular attention to shield and ground connections.
8.3.7 Readings drift.
A. Is the sample temperature changing? Membrane permeability is a function of temperature. For the 499ADO
and 499ATrDO sensors, 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. The time constant for the Gx438 and
Hx448 sensors is much shorter; these sensors respond fairly rapidly to temperature changes.
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.
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