Emerson ROSEMOUNT 5081-A Instruction Manual page 110

MODEL 5081-A
15.4.3 Sensor current during air calibration is substantially different from the value in Section 9.3.
1. Is the sensor properly wired to the transmitter? See Section 3.0. Verify that all connections are tight.
2. Is the membrane dry? the membrane must be dry during air calibration. A droplet of water on the membrane during
air calibration will lower the sensor current and cause an inaccurate calibration.
3. If the sensor current in air is very low and the sensor is new, either the electrolyte flow has stopped or the membrane
is torn or loose. For instructions on how to restart electrolyte flow see Section 15.4.2 or refer to the sensor instruction
manual. to replace a torn membrane, refer to the sensor instruction manual.
4. Is the temperature low? Sensor current is a strong function of temperature. the sensor current decreases about 3%
for every °C drop in temperature.
5. Is the membrane fouled or coated? A dirty membrane inhibits diffusion of oxygen through the membrane, reducing the
sensor current. Clean the membrane by rinsing it with a stream of water from a wash bottle or by gently wiping the
membrane with a soft tissue. 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 more information.
15.4.4 Process and standard instrument readings during in-process calibration are substantially different.
this error warning appears if the current process reading and the reading it is being changed to, ie, the reading from the
standard instrument, are appreciably different.
1. Is the standard instrument properly zeroed and calibrated?
2. Are the standard and process sensor measuring the same sample? Place the sensors as close together as possible.
3. Is the process sensor working properly? Check the response of the process sensor in air and in sodium sulfite solu-
tion.
15.4.5 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 stabilizes after
a few hours.
2. Is the sample flow within the recommended range? High sample flow may cause erratic readings. Refer to the sensor instruc-
tion manual for recommended flow rates.
3. Gas bubbles impinging on the membrane may cause erratic readings. orienting the sensor at an angle away from vertical
may reduce the noise.
4. the holes between the membrane and electrolyte reservoir might be plugged (applies to Models 499A do and 499A trdo
sensors only). Refer to Section 15.4.2.
5. Verify that wiring is correct. Pay particular attention to shield and ground connections.
6. Is the membrane in good condition and is the sensor filled with electrolyte solution? Replace the fill solution and electrolyte.
Refer to the sensor instruction manual for details.
15.4.6 Readings drift.
1. 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 five (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.
2. 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.
3. 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.
4. Is the sample flow within the recommended range? Gradual loss of sample flow will cause downward drift.
5. Is the sensor new or has it been recently serviced? New or rebuilt sensors may require several hours to stabilize.
104
SECTION 15.0
TROUBLESHOOTING