Maintenance; General Cleaning; General Sludge And Loosely Adhering Matter; Heavy, Non-Greasy Deposits - ABB AP300 User Manual

5 Maintenance

5.1 General Cleaning

Warning. Before removing a sensor from a flow line,
ensure that all isolating valves have been closed.
To ensure accurate monitoring, keep the sensor free of
contaminants by periodic cleaning, the frequency of which
depends on the particular application.
Methods of removing various types of deposit are detailed
below. Replace the sensor if its performance does not improve
after cleaning.

5.1.1 General Sludge and Loosely Adhering Matter

Rinse off the excess matter and wipe the sensor with a soft
cloth or tissue before calibrating.

5.1.2 Heavy, Non-Greasy Deposits

For example: lime, salts, etc. Immerse the sensor in 1 to 2 M
hydrochloric acid until the deposit has dissolved. Rinse the
sensor with water and allow to settle in buffer solution before
calibrating.

5.1.3 Greasy or Organic Deposits

Wipe the glass membrane with a detergent or acetone-based
solvent. Rinse with water before calibrating.

5.2 Fault Finding

Listed below are some common symptoms of sensor
malfunction together with possible cures.
Short scaling (Low Slope) or sluggish response
1. Glass sensor membrane dirty or coated – refer to Section
5.1 for cleaning.
2. Poor insulation on cable connectors, possibly due to
moisture – dry connectors with warm air.
Replace the sensor if no improvement is seen. It may also
be necessary to replace the extension cable if used.
No response to pH buffer or sample
1. Check the sensor has been correctly wired to the
transmitter as detailed in Section 3.1, page 12 and the
relevant transmitter instruction manual.
2. Check the glass sensor membrane is not broken or
cracked.
Unstable readings or drift
1. Check the sensor has been correctly wired to the
transmitter as detailed in Section 3.1, page 12 and the
relevant transmitter instruction manual.
2. Dry or dirty reference junction – clean the junction as
detailed in Section 5.1.
Replace the sensor if no improvement.
Stable but incorrect readings
1. Recalibrate using fresh buffer solutions.
2. Check temperature compensation settings are correct –
manual temperature is correct, or automatic temperature
compensation is reading correctly.
3. If the sensor responds correctly to pH changes, but there
is an offset of <1.0 pH to >0.2 pH, perform a one-point
process calibration (see Section 4.1, page 14).
Note. All the above symptoms could be caused by a
faulty extension cable. Check and replace it, if necessary.

5.3 Storage of the Electrode

Caution. Failure to ensure that the glass membrane and
reference junction do not dry out may irreversibly affect the
response of the electrode.
If it is necessary to remove the electrode from the sample line,
fill the retained protective cap with buffer solution and cotton
wool, or equivalent, and fit it to the sensor.
AP300 | Combination pH/Redox (ORP) sensors | IM/AP300 Rev. J 15