Yokogawa EXA PH202 2-Wire pH/ORP Analyzer - Manual (Page 92)

IM 12B07D02-01E
10-6 Appendix
10-5. Set up for Pfaudler Type 18 sensor
The PH202 is intended to measure with all sorts of pH sensors, including the Pfaudler Type 18 sensor.
The Pfaudler design of dual membrane system uses two enamels of differing sensitivity. The first a pH
sensitive membrane, and the second one that responds to Na+ and K+ and acts as a reference.
The analyzer has dual high impedance inputs which measure perfectly even with very high impedance
sensors. However, the impedance measuring system (diagnostics) needs to be set up for best perform-
ance.
10-5-1. General set up
1. Set impedance measuring hardware. This is done by the use of links on the terminals adjacent to
the input terminals. For the Pfaudler system, this means that the terminals should have the links
disconnected in order to set for HIGH/HIGH impedance measuring.
2. Set the impedance check in software. Use codes 03 & 04 to enable the measurement and set for
high impedance and configure appropriate limits.
Code 03 set to 1.0.1 Iow limit 1 Megaohm
high limit 1 Gigaohm
Code 04 set to 1.0.1 Iow limit 1 Megaohm
high limit 1 Gigaohm
3. Set the temperature compensation sensor as 100 Ohm Platinum RTD with service code 10.
Code 10 set to 1 100 Ohms Pt.
The system will now respond properly to the Pfaudler type 18 sensor, and the other functions of the
EXA analyzer will need to be set in the normal way to suit the use to which the loop is being put.
Output ranges, control functions and alarms should all be set as described elsewhere in this manual.
10-5-2. Calibration set up
4. The alternative Zero point (calibration and display) according to IEC 60746-2 may be enabled in
service code 27, and set in the MAN.CAL routine. A value of 10.5 pH is a good starting point for the
Pfaudler 18 sensor.
5. Where lab test data are available for the sensor, service code 23 can be used to set values for ITP &
Slope (and As pot for parameter 2 when enabled).
(This method can be useful for the type 18 sensor, as it is not usual to perform regular calibrations on
this system as with normal sensors. This is because the system may well respond differently, to ordinary
buffers, than with the process solutions. The procedure is to determine the temperature response (ITP)
and the sensitivity (Slope) of the sensor, and enter these values in code 23.)
Because this is a rather complex procedure, it is recommended instead to use the default settings of ITP
= 7.00, and Slope = 100 %, and make a single point (MAN.CAL) calibration in the process at the work-
ing temperature, and at the normal operating (control setpoint) pH. This ensures that the desired control
point will be measured accurately, even if there may be small deviations when there is a big deviation
from the setpoint. This of course has no effect on the accuracy of a control loop. The special construc-
tion of the Pfaudler sensor ensures that there is practically no drift in the calibration. All that is necessary
is to keep the sensor membranes clean. This is best done by cleaning with low pressure steam, which
restores the original condition of the sensor, including the original calibration values.