YOKOGAWA FU20-FTS, Инструкция по эксплуатации, Страница: 27 / 32

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6. GENERAL CALIBRATION & MAINTENANCE PROCEDURE
Calibration of the FU20-FTS/MTS sensor has to be done with the pH analyzer connected.
Normally the pH standards that are preprogrammed in the pH analyzer can be used for
calibration with the pH analyzer set to
“AUTOCAL”
. The FU20-FTS/MTS however is a
differential pH sensor which needs pH buffers that have the same ionic strength because
the sodium reference will change as the ionic strength changes. These pH buffers are not
preprogrammed in the pH analyzer, which means the calibration has to be done with
specific buffer solutions (see Section 6.1) and the pH analyzer set to
“MANUAL”
calibration. Refer to the analyzer Instruction Manual for details. Calibration data of the
sensor is stored into the pH analyzer.
6.1 Calibration for pH measurement
To calibrate the FU20-FTS/MTS 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). Buffers which are available are:
pH 4.00, pH 7.00 and pH 9.00. See Section 8 for ordering information.
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 tap water (
DO NOT use demineralized water);
3. Immerse the sensor in the first buffer solution (pH 7.00 is recommended);
Set the pH analyzer to
“MANUAL”
calibration. Wait until the pH reading is stable.
Adjust the pH reading in the pH analyzer to the value indicated on the bottle (in this
case 7.00). Go to solution 2 to do a 2-point calibration;
4. Rinse sensor thoroughly with tap water;
5. Immerse the sensor in the second buffer (pH 4.00 is recommended); Wait until the pH
reading is stable. Adjust the pH reading in the pH analyzer to the value indicated on the
bottle (in this case 4.00). Quit the manual calibration routine by accepting the new
calculated calibration data of zero and slope.
6. After calibration, re-install the sensor into the process.
Note: It is important to understand it is possible the FU20-FTS/MTS differential sensor
does not show the correct pH value in process after buffer calibration. The reason is
that the ionic strength of the buffers is 1 mol NaCl, which can deviate from the ionic
strength of the process. For an accurate pH reading an extra 1 point process
calibration has to be done (see Section 6.2) to compensate for differences in ionic
strength.
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 Process calibration
The following is a specific 1-point manual calibration procedure for the process to
measure: Set the pH analyzer to
“MANUAL”
calibration. Wait until the pH reading is
stable. Adjust the pH reading in the pH analyzer to the value of the process sample that is
analyzed using a laboratory measurement. For the laboratory measurement, we advise to
use the Yokogawa PH72 personal pH meter. Quit the manual calibration routine by
accepting the new calculated calibration data of zero. Slope value will be unchanged.