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pH ARC Sensors Modbus RTU Programmer’s Manual (EPHUM011)
624300/01 page 40 / 67
What happens to the pH ARC Sensor’s calibration function upon product calibration (CP6)?
A product calibration adds an offset to the linear calibration function defined by the standard calibration
at CP1 and CP2.
Product calibration of ARC pH sensors
-200
-150
-100
-50
0
50
100
150
200
5 6 7 8 9 10
pH
Electrical potential [mV]
Standard Calibration function
Product Calibration function
CP1
CP2
CP6
Figure 2.7.3.2.1: Effect of the product calibration CP6 on an existing standard calibration function
defined by CP1 and CP2.
The operator starts with a Standard Calibration with calibration points CP1 and CP2:
CP1: pH value of calibration standard: 6 electrical potential: 64.2 mV
CP2: pH value of calibration standard: 9.21 electrical potential: -125.2 mV
The sensor internally calculates the calibration function, using the calibration points CP1 and
CP2. The resulting calibration function, compensated to the standard temperature 25°C, is
shown as a straight line. The calibration function is described by two parameters: the offset at pH
7 and the slope.
Some weeks later, the operator believes that the Standard Calibration function is not correct
anymore. As the process is running and he is not able to perform a standard calibration under
defined conditions in the lab, he decides to perform a product calibration CP6, in other words
adjusting the standard calibration function to the process conditions:
CP6: pH value of product: 7.2 electrial potential: 5 mV
The sensor internally adds an offset to the calibration curve. The slope remains unchanged.
Another special feature of this calibration point is to switch off and back on again a product calibration.
These functions are called “restore standard calibration”and “restore product calibration”.
Note:
The sensor’s internal criteria for a successful product calibration are:
the sensor is currently in an environment corresponding to the pH ARC Sensors measurement
range.
the manually assigned pH value does not deviate more than 2 pH units from the value
measured prior the product calibration