Ph Sensor Calibration (Cal) With An External Sample (1-Point) - ProMinent DULCOMETER diaLog DACb Assembly And Operating Instruction

10.1.3

pH sensor calibration (CAL) with an external sample (1-point)

Measuring and control behaviour of the controller during calibration
During calibration the actuating outputs are deactivated. Exception: a basic load or a manual
control variable has been set. This remains active. The measured value output
[standard signal output mA] is frozen, corresponding to its settings in the mA output menu.
When calibration/testing has been completed successfully, all of the error checks relating to
the measured value are restarted. The controller saves all the determined data for zero point
and slope upon a successful calibration.
NOTICE!
Poor sensor operation and fluctuating pH values during the process
The calibration method with an external sample has a number of disadvantages compared
with the buffer solution calibration method. If the pH value fluctuates significantly during the
process, then the pH value may change by a variable amount in the period between sam‐
pling, sample measurement and entry of the pH value into the controller. This could mean
that the pH value entered into the controller does not correspond to the actual pH value in
the process. Consequently the result is a linear displacement of the pH value across the
entire measuring range.
If the pH sensor no longer reacts to changes in the pH value and only gives out a constant
uniform mV signal, this cannot be detected using the calibration method with an external
sample. With the calibration method with two buffers (e.g. pH 7 and pH 4), this becomes
apparent if the pH sensor does not detect any changes in the pH value.
The calibration method with an external sample should only be used with installations where
there is poor access to the pH sensor and the identical or very uniform pH values are used
in the process. In addition the pH sensor should be regularly serviced or replaced.
Calibration
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