pH ARC SensorsModbus RTU Programmer’s Manual (EPHUM011)
624300/01 page 40/ 67
What happenst o the pH ARC Sensor’scalibration function upon product calibration (CP6)?
A product calibrationadds an offset to the linear calibration function defined by the standard calibration
at CP1 and CP2.
Productcalibration of AR Cp H sensors
-200
-150
-100
-50
0
50
100
150
200
5 6 7 8 9 10
pH
Electrical potential [mV]
StandardCalibration functio n
ProductCalibration fu nction
CP1
CP2
CP6
Figure 2.7.3.2.1:Effect of the product calibration CP6 on an existing standard calibration function
defined by CP1and CP2.
The operator startswith a Standard Calibration with calibration points CP1 and CP2:
CP1: pHv alueof calibration standard: 6 electrical potential:64.2 mV
CP2: pHv alueof calibration standard: 9.21 electrical potential:-125.2 mV
The sensori nternallycalculates the calibration function, using the calibration points CP1 and
CP2. The resulting calibrationf unction, compensatedto the standardtemperature 25°C, is
shownas a straight line. The calibration function is described by two parameters: the offset at pH
7 and the slope.
Some weekslater, the operatorbelieves that the StandardCalibration function is not correct
anymore. Ast heprocess is running and he is not able to perform a standard calibration under
defined conditionsin the lab, he decides to perform a product calibration CP6, in other words
adjusting the standardcalibration function to the process conditions:
CP6: pHv alueof product: 7.2 electrial potential:5 mV
The sensori nternallyadds an offset to the calibration curve. The slope remains unchanged.
Another specialfeature of this calibration point is to switch off and back on again a product calibration.
Thesef unctionsare called “restorestandard calibration”and “restoreproduct calibration”.
Note:
The sensor’sinternal criteria for a successful productcalibration are:
the sensori scurrently in an environment corresponding to the pH ARC Sensors measurement
range.
the manually assignedpH value does not deviate more than 2 pH units from the value
measuredpri orthe product calibration