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pH ARC Sensors Modbus RTU Programmer’s Manual (EPHUM011)
624300/01 page 35 / 67
2.7.3 Calibration Procedure
2.7.3.1 Calibration at CP1 and CP2 (Standard Calibration)
The ARC Sensor family has a unique calibration routine. When initiating the calibration, the data set of
the sensor is automatically traced back within the last 3 minutes and a decision is made immediately if
the calibration is successful or not. The operator therefore gets an immediate result. The criteria for a
successful calibration are:
the stability of pH value and temperature over the last 3 minutes (see register 5128)
the currently measured pH value fits to one of the calibration standards defined in the selected
set of calibration standards
the limits of slope and offset at pH 7 have to be met
Standard Calibration at 2 calibration points
-200
-150
-100
-50
0
50
100
150
200
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
pH value
electrical potential glass vs. ref [mV]
CP2
CP1
Figure 2.7.3.1.1: Standard Calibration using CP1 and CP2.
CP1 and CP2 define a linear relationship between the electrical potential and the pH value. This
linear calibration function is defined by an offset at pH 7 [mV] and a slope [mV/pH]. These two
values are stored in register 5448.
Note: the pH value of CP1 can be lower or higher than the pH value of CP2. However, the
difference in pH between CP1 and CP2 must always be greater than 1 pH unit.
Notes:
In order to perform a standard calibration at CP1 and CP2, it is necessary to use commercially
available calibration standards. The operator is restricted to use those standards that are
defined in the six sets of calibration standards (see chapter 2.7.11).
There are two ways of performing a standard calibration:
- standard calibration with automatic recognition of the calibration standard:
in this case, the sensor decides on itself in what calibration standard it is immersed.
The criteria to decide on is the electrical potential measured. The sensor checks the list
of calibration standards that are available for automatic recognition (see chapter
2.7.11)
- standard calibration with manual selection of the calibration standard:
the operator is selecting the calibration standard in which the sensor is immersed.
Factory calibration is pH 4 at CP1 and pH 7 at CP 2.