7.1.7 Oxygen sensor check in the absence of oxygen
To guarantee maximum accuracy in results, it is a good practice to test the response of the sensor once
a month in the absence of oxygen. Nitrogen is recommended for this check; should not Nitrogen be
available, an aqueous solution, chemically reduced, can be alternatively used. To carry out this test, it
is important that the membrane cap should have been replaced for at least 15 minutes, thus allowing a
complete sensor polarization.
Procedure
1) Connect a cuvette (body of a syringe) to a Nitrogen supply using a flexible tube.
2) Purge the line and adjust the gas flow rate at 200 ml/min. approx.
3) Calibrate the oxygen channel by exposing the sensor to room air.
4) Insert the sensor completely into the cuvette. The reading should rapidly decrease and within a few
seconds to one minute, it should be less than 0.2 ppm. If the reading is more than 0.2 ppm, re-expose
the sensor to room air and repeat the operation.
Should the inconvenience persist, replace the membrane cap and/or the electrolyte.
Due to the high-quality construction of this oxygen sensor, which reduces to insignificant the
background current, no electronic zero calibration is necessary and possible
7.2 REFERENCE SENSOR
❖ During all periods of inactivity, the reference sensor must be always hydrated with the IDRONAUT
REFERENCE SENSOR STORAGE SOLUTION or, if not available, even with KCl saturated solution,
using the plastic hydrating cap. Fill the cap to about one third with the solution. The cap should be
squeezed to allow an easy insertion.
❖ Before starting the measurements, the plastic hydrating cap must be removed.
❖ If the sensor has been exposed to air without its protective cap or if the solution in the cap is not
present, the solid electrolyte of the sensor may have contracted forming internal air bubbles, in
particular by the junction hole. In this case, it is necessary to fill the cavity with the IDRONAUT
REFERENCE SENSOR STORAGE SOLUTION. Take a small syringe, i.e. 1.5 ml and aspirate a small
amount of the IDRONAUT REFERENCE SENSOR STORAGE SOLUTION (supplied with the probe).
Place the probe on a table in a horizontal position taking care that it does not rotate. Carefully insert
the needle of the syringe in the junction hole of the reference sensor without bending the needle to
avoid breaking the glass sensor. Gently press the syringe plunger to inject some drops of solution
inside to eliminate the air bubbles. If the cavity is too big and the electrolyte added easily leaks, or
should the cavity appear contaminated by foreign material, then the replacement of the reference
sensor may become necessary.
❖ If the sensor is left immersed in the measuring environment for an indefinite period, there will be a
slow progressive loss of the KCl from the solid electrolyte. In such working conditions, the reference
sensor needs to be replaced within 6 months 1 year .
7.3 pH SENSOR
The glass membrane of the pH sensor must be always kept hydrated. If the sensor is stored dry for an
extended period (more than half a day), the sensor’s performance may deteriorate. The electrode
sensitivity diminishes, the response times increase, and signals tend to drift during measurements and
calibrations.
Before using the sensor after long storage periods, it is advisable to check the electrode performance
using pH7 and pH4 buffers. Following calibration of the sensor with pH7 buffer (see the relevant
section), wash the sensor and calibration cup with distilled water.
Then perform the following as described:
1) Dry the electrode with a soft tissue, making sure not to rub on the pH sensor tip.
2) Fill the calibration cup with pH4 buffer and dip the pH and reference electrodes in the cup.
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