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IM 12B06J03-02EN-P
6. GENERAL CALIBRATION & MAINTENANCE PROCEDURE
6.1 Calibration for pH measurement
To calibrate the FU20 pH/ORP sensor, two buffer solutions with known pH values are
required. It is recommended that one buffer solution has a value near to pH 7.00.
Depending on the process value to be measured, the second buffer solution should be
either acidic (below pH 7.00) or alkaline (above pH 7.00). Normally the IEC buffers (pH
4.01, 6.87 and 9.18) are used.
The following is a very general 2-point calibration procedure:
1. Clean the sensor using a 5% solution of HCl;
2. Rinse sensor thoroughly with demineralized water;
3. Immerse the sensor in the first buffer (pH 6.87 is recommended) and
execute calibration as described in the Instruction Manual of the analyzer.
4. Rinse sensor thoroughly with demineralized water;
5. Immerse the sensor in the second buffer (pH 4.01 or 9.18 is recommended)
and execute calibration as described in the Instruction Manual of the analyzer.
6. Rinse sensor thoroughly with demineralized water.
During calibration, the temperature compensation should be active. The EXA/FLXA
analyzer automatically compensates for the sensitivity change of the pH sensor at different
temperatures.
After calibration, replace or re-install the sensor into the process.
6.2 Calibration for ORP and rH measurement
For calibration of ORP and rH, the procedure for MANUAL CALIBRATION can be used as
described in the Instruction Manual of the analyzer.
The rH value is a function of the reference system and the pH value of the buffer solution.
The FU20 sensor has a reference system of saturated Silver/Silver Chloride (Ag/AgCl).
The commonly used standards for ORP and rH calibration are made from Chinhydron
(Quinhydrone) powder dissolved in pH buffer solutions (1 g / 200 ml). In Table 3 the
measurement values are given as function of the used pH buffer solution with Chinhydron
powder. The accuracy of the standards is approximately ± 10 mV.
Table 9: ORP, pH compensated ORP and rH as function of pH buffer solution with
Chinhydron powder.
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