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 Zero reading unstable 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% with 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.
Process and standard readings different
This error warning appears if the current process reading and the reading it is being
changed to, i.e., 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 solution.
Process readings 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 instruction manual for 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 499ADO and 499ATrDO sensors only). Refer to Zero reading unstable.
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.
Diagnostics and troubleshooting
Instruction Manual 101
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