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pH ARC Sensors Modbus RTU Programmer’s Manual (EPHUM011)
624300/01 page 37 / 67
Perform the following steps to do a standard calibration at CP1 and CP2:
Step1: Select the desired set of calibration standards (see chapter 2.7.11)
Step 2: Immerse the sensor into one of the calibration standards available in the selected set
Attention:
It is important that the pH ARC Sensor is immersed in a defined calibration standard at least 3
minutes BEFORE the calibration is started.
Step 3: Choose one of the calibration points CP1 or CP2.
Attention:
The assignment of both CP1 and CP2 to the same pH value is rejected.
Try to avoid the following situations:
For example, the sensor was calibrated earlier at pH 4 (CP1) and at pH 7 (CP2):
You want to perform a new calibration at CP1: it is possible to assign CP1 to pH 4 or to
pH 10, but not to pH 7 (already used for CP2).
You want to perform a new calibration at CP2: it is possible to assign CP2 to pH 7 or
pH 10, but not to pH 4 (already used for CP1).
If you calibrate now CP1 at pH 10, pH 4 will later be free for selection at CP2. In this
case you can calibrate CP2 at pH 4 or pH 7, but not at pH 10 (CP1).
Step 4: Start the calibration (automatic recognition or manual selection)
The calibration is initiated at CP1 by writing to register 5162 or at CP2 by writing to register 5194.
Start
register
Number of
registers
Reg1 / Reg2 Modbus
function code
Read
access
Write
access
5162 2 pH value at CP1 (unit is always pH) 16 none A/S
5194 2 pH value at CP2 (unit is always pH) 16 none A/S
Figure 2.7.3.1.3: Definition of register 5162 and 5194. You have two options to enter the pH value:
Automatic recognition: pH value=0: the sensor tries to assign the measured electrical potential to
one of the calibration standards available for automatic calibration.
Manual selection: enter the actual pH value at 25°C (the value must be within the tolerance
range of the nominal value of one of the calibration standards available for manual selection)
Step 5: Read the calibration status (see chapter 2.7.4)
Step 6: Check the pH ARC Sensor’s quality indicator