Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 76)

<5. CALIBRATION OF pH/ORP>
5-2
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
5.1 pH calibration
There are the Manual, Automatic, and Sample modes of pH calibration.
5.1.1 Manual calibration
The unit is adjusted to match the value of the buffer standards or a process solution with a known
pH value (buffer solution).
The user determines the pH value, temperature influence, and stability.
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.
At calibration, we advise leaving the sensors for three to five minutes in the buffer solution before
proceeding to the next step even when the reading has stabilized. This will give reliable and
accurate calibration results.
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.)
l zero/slope
This calibration type is one-point or two-point calibration.
One-point calibration performs the zero adjustment only. Two-point calibration performs the zero
and slope adjustments.
l zero/slope/ITP(3point)
This calibration type is ITP-type three-point calibration.
If ITP does not have pH 7, three-point calibration is performed to obtain the zero (asymmetry),
slope (sensitivity), and ITP (isothermal point) for 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 2nd buffer solution should be pH 7 ± 2.
• The temperature difference between the 2nd and 3rd buffer solutions should be 5ºC or less.
The temperatures of the 2nd and 3rd buffer solutions should be at least 20ºC higher or lower
than the temperature of the 1st buffer solution.
• To calculate the pH value of the 3rd buffer solution (pH
3cal
), insert the pH and temperature
values of the 1st and 2nd buffer solutions and the temperature value of the 3rd buffer
solution into the following equation.