IM 12A06S01-00EN-P
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5 CALIBRATION OF pH/ORP
For accurate pH measurement it is important to calibrate a pH sensor regularly. The sensor
is calibrated before shipment, and the calibration data is stored in the ID-chip of the sensor
as factory defaults. It is recommended to re-calibrate the sensor before first use.
5.1 Temperature calibration
For the accuracy of calibration of pH/ORP, it is important to have a precise temperature
measurement. Measure the temperature with a high-precision thermometer and adjust the
sensor reading (temp. offset) accordingly. For best accuracy, this should be done as near to
the normal operating temperature as possible.
5.2 pH calibration
There are four methods of calibration: Manual, Automatic, Sample and Fully Automatic
calibration.
Calibration is performed stepwise, and a stability check is conducted at each measurement
point. For setting of this check see section 4.1.9. At calibration, we advise leaving the
sensors for three to five minutes in the buffer solution before proceeding to the next step.
This will give reliable and accurate calibration results.
The SA11 has automatic buffer recognition for easy use and to prevent wrong sequence of
actions. This is active only in case of fully automatic calibration.
For buffer selection see section 4.1.10.
Note:
When a sensor is replaced or exchanged, a reset of the wellness data is not
necessary. If a user wants to reset this data anyhow, this is possible by the Yokogawa
HOST system. For details see the belonging User’s Manual.
5.2.1 Manual calibration
The calculated output data for pH is adjusted to match the value of the buffer standards or a
process solution with a known pH value. Setting is done by HOST for pH value, temperature
compensation and stability (see sections 4.1.3, 4.1.4 and 4.1.5).
Calibration type can be set for:
•
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.
•
ZERO/SLOPE/ITP
This calibration type is ITP-type three-point calibration.
If ITP does not have default pH 7, three-point calibration is performed to obtain the ZERO
(asymmetry), SLOPE (sensitivity), and ITP (isothermal point).
Limitations:
o 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)
o The 2nd buffer solution should be pH 7 ± 2.
o The temperature difference between the 2nd and 3rd buffer solutions should be 20ºC
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.