User guide

37
IM 12D08N05-01E
6
CALIBRATION
6-7. Temperature calibration
In order to make the most accurate measure-
ments, it is important to have a precise temper-
ature measurement. This affects the display of
temperature, and the output signal when used.
More important, however, is the temperature
compensation, and calibration accuracy.
The temperature of the sensor system should
be measured independently with a high preci-
sion thermometer. The display should then be
adjusted to agree with the reading (zero offset
calibrationonly).Forbestaccuracythisshould
be done as near to the normal operating tem-
perature as possible.
6-8. Operation of hold function during cali-
bration
EXAxtSC450GhasaHOLDfunctionthatwill
suspend the operation of the control/alarm
relays and mA-outputs.
During calibration, the user may choose to
enableHOLDsothattheoutputsignalsare
frozentoa“last”or“fixed”value.Someusers
will choose to leave the outputs “live” to record
the calibration event. This has implications
for pharmaceutical manufacture, for example,
where an independent record of calibrations is
mandatory.
PressHOLDbuttononmainscreen,toremove
theHOLD.
TherouteforHOLDsetupis
Commissioning >> Output setup>> Config-
ure Hold
6-9. General comments on SC calibration
a) SCsensorsexperiencenodriftexceptif
they are damaged or dirty
b) There are no good calibration solutions
(like pH buffer solutions)
c) SolutioncalibrationofSCdemands
laboratory technical skills
d) Solutionscanbeusedtogiveafair
calibration check at higher conductivity
e) SolutionscanNOTbeusedtocheck
calibration at low conductivity.
f) Lowconductivitysolutions<10µS/cm
absorbCO
2
from the air very fast
g) Lowconductivitymeasurementmustbe
made only with air excluded
h) Apparatus must be scrupulously clean to
avoid contamination
i) Sensorlinearityisneveraproblemfor
lower values
j) A dirty sensor is prone to polarization
k) Polarization shows as a low side error at
higher conductivity
l) A dirty sensor will often read perfectly at
low conductivity
m) Wet calibration tests are best done
towards the top of a sensor’s range
n) If the system responds correctly to the
highest trip point, all is well