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Calibrating the controller diaLog DACb
Selecting the calibration process
72
Select the calibration process prior to initial calibration. This selec‐
tion is saved until you select a new process.
2-point calibration: This is the recommended calibration
n
process because it evaluates the sensor characteristic data:
asymmetric potential, slope and response speed. 2 buffer solu‐
tions are needed for 2-point calibration, e.g. pH 7 and pH 4 if
subsequent measurement is to be performed in an acidic
medium or pH 7 and pH 10, if subsequent measurement is to
be performed in an alkaline medium. The buffer gap should be
at least 2 pH units.
Samples (1-point) calibration: There are two options here.
n
Samples (1-point) calibration is only recommended with reser‐
vations. From time to time check the sensor with 2-point cali‐
bration.
–
The pH sensor remains in the sample medium and you
should calibrate a sample of the medium to be measured
against an external comparison measurement. Perform the
comparison measurement using an electrochemical
method. Deviations of up to ± 0.5 pH units can occur using
the phenol red method (photometer).
–
Calibration solely using a pH 7 buffer. This only calibrates
the zero point. The sensor is not checked for an acceptable
slope.
Data input: With this calibration method, using a comparison
n
measuring device, determine in advance the characteristic data
of the pH sensor (asymmetry and slope) at standard tempera‐
ture and enter this data into the controller. The comparative
calibration should not have been done more than a week
before, as the pH sensor's characteristic data changes if it is
stored for longer.
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