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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 Buffer calibration
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 manual
calibration procedure with buffer solution:
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.
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.
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 advice
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.
17
IM 12B06J03-05E-E
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