Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 77)

<5. CALIBRATION OF pH/ORP>
5-3
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
pH
3cal
= ITP -
(273.15 + t
2
)(ITP - pH
2
) + (1 - ) x (273.15 + t
1
)(ITP - pH
1
)
(273.15 + t
3
)
t
3
- t
1
t
2
- t
1
( ) t
3
- t
1
t
2
- t
1
pH
n
: pH value of n-th buffer solution
T
n
: Temperature of n-th buffer solution (ºC)
ITP: ITP value displayed in calibration settings (see section 4.2.4)
Assign 7.00 as the ITP value when a specific value is not available or for the first
calibration of a sensor.
Do not use the 3rd solution whose pH value is within pH
3cal
± 1.
l zero/slope1,2(3point)
This calibration type is the line-segment type three-point calibration.
If the relation between electromotive force and pH is not in proportion for a wide range, divide the
relevant range into two sections and obtain the zero (asymmetry) and slope (sensitivity) in each
section to perform calibration.
Limitations
• Three different buffer solutions whose difference in pH value between buffer solutions is 1
pH or more should be used.
(1st buffer < 2nd buffer < 3rd buffer or 1st buffer > 2nd buffer > 3rd buffer)
• The temperature difference between the 1st and 2nd buffer solutions should be 20ºC or
less.
• The temperature difference between the 2nd and 3rd buffer solutions should be 20ºC or
less.
5.1.2 Automatic calibration
Calibration can easily be performed by following the calibration menus.
Pre-select the buffer solution to be used from among NIST/DIN 19266, DIN 19267, US, and User
defined buffer in Commissioning → Measurement setup → Calibration settings → pH settings →
Buffers (select set). See also Appendix table 1.
If you select User defined buffer, calibration is performed based on the conditions registered
in buffer tables 1 to 3. Use of the proper buffer table allows the system to perform reliable
calibration.
In the same way as manual pH calibration, select the calibration type from among [zero/slope],
[zero/slope/ITP(3point)], and [zero/slope1,2(3point)].
Calibration is performed stepwise; follow the prompts displayed on the screen.
A stability check is conducted at each measurement point. Proceed to the next step only after the
reading has stabilized.
NOTE
When a sensor or an electrode is exchanged or replaced, sensor wellness data should be reset.
When a sensor is replaced, the replacement can be recorded manually into a logbook. (Refer to
the figure 3.9.)
PH
5