Ph Sensor Calibration (Cal) With An External Sample (1-Point) - ProMinent DULCOMETER Assembly And Operating Instructions Manual

Calibration
9.1.3

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 sig‐
nificantly during the process, then the pH value may change by a variable amount in
the period between sampling, 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 installa‐
tions 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.
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