Yokogawa EXA PH202 2-Wire pH/ORP Analyzer - Manual (Page 90)

IM 12B07D02-01E
10-4 Appendix
10-3. Setup for sensor compatibility
10-3-1. General
The inputs of the EXA transmitter are freely programmable for ease of installation. Standard glass pH
electrodes, Ag/AgCl reference electrodes and Pt100 and Pt1000 temperature sensors need no special
programming. The EXA indicates a fault with a signal in the display field if there is a mismatch of sen-
sors in the connection.
10-3-2. Selection of measurement and reference electrode
The EXA PH202 is preprogrammed to accept industry standard glass electrodes and reference elec-
trodes. The unit initiates checks for asymmetry and slope during calibration. The on-line impedance
checking function has been upgraded in this most recent EXA release.
The EXA is universally compatible with all types of electrodes, such as enamel and antimony. In such
systems, however, the specific isothermal point of intersection (ITP), slope (pH/mV) and asymmetry
potential can be set for the type of electrode.
10-3-3. Selecting a temperature sensor
The EXA PH202 reaches its highest accuracy when used with the Pt1000 temperature sensor. This ele-
ment offers a 10-fold increase in resistance dependence over the Pt100 sensor. Choice of temperature
sensor is made in the Service Codes found in Chapter 5 of this manual.
• ITP
Most Yokogawa sensor systems use an Iso-thermal point (ITP) of pH7 and a zero point at pH7.
This is the default condition for which the transmitter is set. It is only necessary to consider this
adjustment when installing a system with a different ITP. Antimony systems and Pfaudler probes are
good examples of systems with different ITP values. Service code 23 is used. This also permits the
setting of calibration data for precalibrated sensors.
• Temperature sensor
The Pt 1000Ω RTD sensor is now becoming the most commonly used for temperature compensation.
The transmitter accepts inputs from several different temperature sensors to suit most sensor
systems. Service code 10-19 are used to set the temperature parameters and the process
temperature coefficient.
• Temperature calibration
For best accuracy, the temperature sensor should be calibrated to compensate for connection cable
errors. See Service code 12.
• pH Calibration
Traditionally, users select buffer solutions to suit the chosen output range. This is merely a
continuation of the days of analog instruments that used indicators driven by the mA output. With
digital technology, it is better to choose good buffer solutions and make an effective calibration than
to use commercial (adjusted) buffers which may have round number values, but are less effective
buffers with lower buffer capacity. It is for this reason that Yokogawa recommends that the NIST 4,
7 and 9 standard buffers be used to calibrate solutions. The temperature responses of these are
pre-programmed into Service codes 24, 25, and 26 in the EXA PH202. Where other buffers are used
with the semi-automatic calibration function, their temperature response should be programmed into
the relevant code.