7.3.1.1.3 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 deacti‐
vated. 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 suc‐
cessfully, 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 sampling, sample measurement
and entry of the pH value into the controller. This
could mean that the pH value entered into the con‐
troller 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 calibra‐
tion method with an external sample. With the cali‐
bration 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 serv‐
iced or replaced.
Correct sensor operation
–
Correct measuring, control and metering is
only possible if the sensor is working perfectly
–
Refer to the sensor's operating instructions
Tab. 4: Valid calibration values
Evaluation
Zero point
Slope
Very good
-30 mV … +30 mV
56 mV/pH … 60 mV/pH
Good
-45 mV … +45 mV
56 mV/pH … 61 mV/pH
Acceptable
-60 mV … +60 mV
55 mV/pH … 62 mV/pH
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