ELMETRON CX-401, User Manual

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User‘s manual for CX-401 multifunction meter v6.0x
22. CALIBRATION OF THE OXYGEN SENSOR
The meter cooperates with a galvanic membrane sensor of our production
with measurement accuracy equal ±1%, provided that the measurement is
performed at the same temperature as calibration. The bigger the difference
in the measurement and calibration temperature, the lower the accuracy. It is
<
3% when the temperature difference is
±
5
0
C and 5% for the difference
±10
0
C. The calibration is performed to eliminate the measurement error
arising from the individual characteristic of the sensor and should be
repeated always before measurement with new sensor, after replacing the
membrane or in case of high measurement accuracy requirements.
Characteristic feature of sensors is a „signal drift“, which means that the
longer interval between calibration and measurement, the lower
measurement accuracy. Calibration is also recommended if temperature of
the tested solution differs significantly from temperature in which the probe
was calibrated, because then an additional error arises. In such case it is
recommended to prepare calibration solutions having approximate
temperature to the predicted temperature of the tested solutions.
If it is impossible to calibrate the device, the membrane of the sensor should
be replaced according to the manufacturer’s instruction. This situation usually
takes place if the membrane is strongly polluted or ruptured (sometimes
almost invisibly). After replacing the membrane the sensor should be
conditioned in water for 24 hours. Applied oxygen sensors require 1- or 2-
point calibration in standard solutions. The meter has two values of the
calibration points recorded: P1=0% and P2=100% (or 20.9% Air). During
calibration in 0% oxygen concentration solution (e.g. saturated sodium
sulphite) its flow has to be provided by making round moves with the sensor.
Calibration of the 100% point is made in the air, after accurate washing the
remains of 0% solution from the sensor with distilled water. It is
assumed that O
2
content in the air corresponds to 100% of saturation, what
enables simplified calibration to be carried out.
1 point calibration is made only for 100% of oxygen concentration after a few
minutes of soaking the sensor in distilled water.
In case of measurement of oxygen content in air let the fresh air in into the
room. Usually, 1-point calibration in air is sufficient, as the zero point of the
meter is at its resolution level.
Entering the calibration mode under the chosen sensor number erases
the characteristic stored in the memory under this number