Ph Sensor Calibration (Cal) With An External Sample - ProMinent DULCOMETER Assembly And Operating Instructions Manual

9.1.3

pH sensor calibration (CAL) with an external sample

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 few disadvantages in compar‐
ison with the buffer solution calibration method. If the pH value fluctuates consider‐
ably during the process, then in the period between sampling, sample measurement
and entry of the pH value into the controller, the pH value may change by a variable
amount. In this case, the result may be that the pH value entered in the controller
does not correspond to the actual pH value. Consequently the result is a linear dis‐
placement 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, then this cannot be detected by calibration using an
external sample. In the calibration method with two buffers (e.g. pH 7 and pH 4) this
becomes apparent if the pH sensor does not detect any change in the pH value.
The calibration method with an external sample should only be used with installa‐
tions with difficult to access pH sensors and process pH values that are always the
same or very uniform. In addition the pH sensor should be regularly serviced or
replaced.
Calibrating the controller
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