Yokogawa EXA PH202 2-Wire pH/ORP Analyzer - Manual (Page 74)

IM 12B07D02-01E
Calibration 6-1
6. CALIBRATION
The EXA PH202 can be calibrated in three distinct ways.
6-1. Automatic calibration
This method uses internally programmed buffer tables, (from Service Codes 24, 25 and 26), to calculate
the buffer value at the actual temperature during the calibration. In addition, the stability of the reading is
automatically calculated, and when the reading has stabilized fully automatic adjustments of slope and
asymmetry are made. This eliminates the question of how long the operator should allow prior to adjust-
ment. A menu driven prompt system conducts the operator through the simple, foolproof routine.
Default settings for the buffer solutions are the standard NIST (formerly NBS) recognised solutions “4”,
“7” and “9”. These are known as primary buffers. They have a much better buffer capacity than the
“commercial” or adjusted buffers. Yokogawa strongly recommends the use of these buffers to provide
the best pH calibration.
6-2. Manual calibration
In this method, the operator decides on the actual value to enter. Manual calibration is most often used
for single-point adjustment of the asymmetry potential, by comparison method.
Manual calibration can also be used to perform a full 2-point calibration with solutions other than the
NIST buffers that are listed in the calibration tables. In this case, the solutions are applied sequentially as
in the AUT CAL method, but the user determines the adjustment of reading and stability.
NOTE:
During manual calibration the temperature coefficient is still active. This means that the readings
are referred to 25 °C. This makes grab sample calibration easy and accurate. However, if the
manual calibration technique is used for buffer calibration, the temperature coefficient must be set
to zero in maintenance mode in the “TEMP” routine (see chapter 5).
6-3. Sample calibration
The operator activates the “SAMPLE” calibration routine, at the same time as taking a representative
process sample. After determining the pH of this sample by independent methods, (in the lab for exam-
ple) the reading can be adjusted. While the sample is being analyzed, EXA holds the sample data in
memory, while continuing to control and read pH normally.
6-4. Data entry
In special circumstances, users can directly enter the calibration data in the service code menu (see
chapter 5). This is appropriate where the manufacturer provides calibration data for each probe (as with
the Pfaudler sensors) or where electrodes are laboratory calibrated for subsequent installation on the
plant.
Service Code 23 allows the values of ITP, asymmetry potential (or zero point) and slope to be entered.
NOTE:
· Be sure to use fresh, pure standard solution to ensure that the calibration is accurate.
· If the temperature of the standard solution changes then its pH will also change.
Ensure that the temperature of the standard solution is stable at calibration time.
· In using a pH sensor which isn't incorporated with a temperature element for automatic temperature
compensation, the temperature of measured solution should be set on the PH202 transmitter.
· When an antimony sensor is applied, measured temperature value also should be set.