Yokogawa EXAxt SC450 4-Wire Analyzer for Conductivity/Resistivity - Manual (Page 45)

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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 <10
µ
S/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