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9
Calibration
9.1
General information
The actual electrical characteristics of analysis sensors always deviate somewhat from the nominal
specifications. The reasons for this include:
•
Like every measuring instrument, analysis sensors always have a certain uncertainty of measure-
ment that results from manufacturing tolerances.
•
During use, analysis sensors are exposed to chemical processes. Deposits and wear phenomena
caused by these processes result in changes of the electrical characteristics of sensors.
To optimize the accuracy of measurements, analysis sensors must be calibrated. Calibrations are re-
quired:
•
during installation or when changing a sensor
•
regularly at time intervals that must be specified by the user
•
if implausible measured values are displayed
•
if process conditions change (e. g. as the result of equipment modification)
Each successfully completed calibration of the relative cell constant and TC calibration is recorded in the
calibration logbook. The calibration logbook can be viewed on the PC using the JUMO DSM software.
9.2
Calibration methods for CR conductivity sensors (conductive)
Rel. cell constant
The deviation from the nominal cell constant of a Ci sensor is described by the relative cell constant. The
relative cell constant is determined by making a measurement in a test solution with a defined conduc-
tivity.
Depending on the mode set for the relative cell constant in the configuration of the conductivity input,
either a common relative cell constant is used for all 4 measuring ranges or a relative cell constant is
determined separately for each measuring range. If "One cell constant per measuring range" has been
set, a separate calibration of the cell constant must be performed for each measuring range.
chapter 12.3.1 "CR input (resistivity conductivity)", page 59
Temperature coefficient
The temperature coefficient is a measure of the temperature dependence of the electrolytic conductivity
of a liquid. It is used to compensate for the effect of temperature when measuring the electrolytic con-
ductivity. When performing a temperature-compensated conductivity measurement, the conductivity val-
ue measured is always indicated with reference to the fixed reference temperature. With the aid of the
temperature coefficient, the value of the electrolytic conductivity displayed at the reference temperature
is calculated from the current measured values of conductivity and temperature of the liquid.The refer-
ence temperature is set in the configuration.
chapter 12.3.1 "CR input (resistivity conductivity)", page 59
The temperature coefficient is determined from 2 measurements in a sample of the process medium
from your system at different temperatures (reference temp. and operation temp.). The reference tem-
perature is obtained from the configuration. The operation temp. (usual temperature of the process in
your system) is entered by the user during the calibration or acquired automatically. The two tempera-
tures must differ from each other by more than 6 °C.
TC curve (for nonlinear temperature coefficients)
If the conductivity of a liquid whose temperature coefficient changes with temperature has to be mea-
sured, this method can determine 6 temperature coefficients for 6 temperature intervals. In this way, it is
possible to determine a good approximation of the temperature coefficient curve. While the operator
brings the sample solution to the temperature values requested by the device, the device determines the
9 Calibration
47
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