Yokogawa EXAxt PH450 4-Wire Analyzer for pH and ORP - Manual (Page 49)

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IM 12B07C05-01E
6
CALIBRATION
When the right buffer tables are entered,
automatic calibration is the easiest and most
reliable calibration method to use.
The calibration is performed in several steps,
each clearly indicated by the user interface.
Each measurement point needs to be stable
before proceeding. The parameters for this
stability check are set in Commissioning >>
Calibration settings >> Limits and timing
We advice to leave the sensors for 3~5 min-
utes in the buffer solution before proceeding,
even when the measurement is stable. This will
give reliable and accurate calibration results.
6-4. Sample calibration mode
This mode is used first to record an instantane-
ous value for a grab sample. The sample value
is held in memory, and normal measurement
and control can continue, while the sample is
analyzed. Following the analysis re-enter the
“Sample” calibration mode. The original value
(from memory) is displayed. The recorded
reading is simply adjusted to agree with the
analyzed value. The sample mode eliminates
the calculation usually needed for this kind
of calibration. A sample calibration is a single
(zero) point calibration.
6-5. Temperature calibration
In order to make the most accurate meas-
urements, it is important to have a precise
temperature measurement. Measure the
temperature with a high precision thermometer.
Adjust the sensor reading accordingly. For best
accuracy this should be done as near to the
normal operating temperature as possible.
ORP & rH calibration modes
6-6. ORP & rH calibration
The calibration modes for ORP or rH are the
“Manual” and the “Sample” modes.
“Manual” calibration can be used for either
single or two point calibrations.
“Sample” calibration is only a single point as it
is with in pH measurement.
Note! The non-availability of well defined
buffer solutions for ORP and rH
eliminates the automatic calibration
option.
Refer to the user manual of the ORP electrode
for the proper calibration method.
6-7. Operation of hold function during cali-
bration
EXAxt PH450G 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 main-
screen, to remove the HOLD. The route for
HOLD setup is Commissioning >> Output
setup>> Configure Hold
6-8. Contact output setup
Wash
Wash functionality is more than activating the
cleaning system. Wash can be seen as an inter-
ruption of the normal measuring mode to clean
the electrode system. The wash cycle first cleans
the sensor system (either chemical or mechani-
cal) during “wash time” (T
W
). Next the sensor
system is left to recover during “wash recovery
time” (T
R
). After the sensor system is recovered,
the wash cycle has ended and converter returns
to the normal measuring mode. The input contact
is always enabled when an output contact is
configured as wash. The input contact can be
used to enable a wash when a high impedance
error occurs on the reference electrode. Then one
wash cycle is started.
Hold during wash
When enabled, the mA-outputs will be frozen
to a pre-defined “last” or “fixed” value. All
contacts are de-energized except the one(s)
configured as “wash” contact. Disabled, mA-
outputs and contacts will not be affected by
wash cycles.