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21. Appendices: pH and Conductivity Sensor Theory
21.1 Care, Cleaning and Maintenance of Conductivity Sensors
21.1.1 Care of Conductivity sensors
The conductivity section of the sensor supplied with your
WP-81
consists
of two platinum wires that are plated with a layer of “platinum-black”. This
is quite a soft layer and is required for stable, accurate measurements. In
time, the platinum-black layer may wear off in some applications, at which
time the sensor will require replatinising (see section 21.2). You can help
to maintain the platinum-black layer by following these simple rules:
1.
NEVER
touch or rub the sensor wires with your fingers, cloth etc.
2. Avoid using the sensor in solutions that contain a high concentration of
suspended solids, such as sand or soil, which can abrade the sensor
wires. Filter these types of solutions first, if possible.
3. Avoid concentrated acids. If you must measure acids, remove the
sensor immediately after taking the measurement and rinse well with
distilled water.
Conductivity sensors can be stored dry. Ensure that the sensor is stored
in a covered container, to avoid dust and dirt build-up.
21.1.2 Cleaning of Conductivity of Sensors.
Platinised platinum Conductivity sensors can only be cleaned by rinsing in
a suitable solvent.
DO NOT wipe the sensor wires
, as this will remove
the platinum-black layer.
1. Rinsing in distilled water will remove most build-ups of material on the
sensor wires.
2. Films of oils or fats on the sensor wires can usually be removed by
rinsing the sensor in methylated spirits.
3. Stubborn contamination can be removed by soaking the sensor in a
solution of 1 part Concentrated HCl and 10 parts distilled water. The
sensor should not be soaked for more than approximately 5 minutes,
otherwise the platinum-black layer may start to dissolve.
4. If all of these methods fail, then the last resort is to physically scrub the
sensor wires, which will remove the contaminant and the layer of
platinum-black. Use only a cloth or nylon scouring pad. DO NOT USE
STEEL WOOL. The sensor will then need to be cleaned in HCl, as per
step 3 and replatinised, as per section 21.2.