Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 62)

<4. COMMISSIONING OF pH/ORP>
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IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
4.2.5 Impedance settings
This screen is used to set the impedance relating to an input impedance check.
Input 1 impedance represents the “glass membrane impedance” of a pH sensor. In case of an
ORP sensor, it represents “metal electrode impedance.” Input 2 impedance stands for “reference
impedance.”
If you select “Input impedance: Low,” the display moves to the Input impedance screen, enabling
you to set the high and low limits.
The FLXA21 has an impedance check, which is capable of monitoring the impedance of various
sensor systems. In order to “fine tune” this diagnostic tool, it is necessary to set it up to match the
sensors used.
The system can be set to measure the impedances of glass (input 1 impedance: high) and
reference (input 2 impedance: low) electrodes. In applications that tend to leave deposits on the
electrodes or to clog the reference sensor junction, the impedance check (set error configuration)
on the reference sensor can be used to initiate an alarm, or to initiate the wash cleaning process,
if one of the limits is exceeded. Sensors with a liquid earth use this liquid earth in impedance
measurement. Sensors with a liquid earth that Yokogawa can provide include PH8EFP, PH8ERP,
PH8EHP, OR8ERG, OR8EFG, etc. In pure water measurement using PH8EHP, the impedance
of the solution to be measured is high; settings on the Error configuration screen should be set to
Off so that the “Impedance 2 too high” error is not generated.
A sensor without a liquid earth cannot measure impedance. All settings on the Error configuration
screen should be set to Off so that no “Impedance 1 too high” or “Impedance 2 too high” error is
generated.
Sensors not equipped with a liquid earth that Yokogawa can provide include HA405, HA406,
DPA406, DPA405, DPAS405, HF405, HA485, DPA485, etc.
This liquid earth refers to a metal pole built into a sensor or connected externally that contacts the
solution to be measured and transmits the potential of this solution to the converter.
4.2.6 Concentration
Generally pH values are not converted into concentrations. Following linear conversion is not
suitable for a lot of measurements.
The FLXA21 is capable of converting a measured pH value into concentration for display.
Parameters required for concentration calculation can be arbitrarily set by the user. Those
parameters are as follows:
Unit: The default value is %. The unit can be selected from among mg/L, g/L, and
ppm.
pH: The default values are 0 and 14 pH (0 to 14). They are the pH zero and span
required for conversion to concentration.
Concentration: The default value are 0 and 100%. Concentration for each pH can be set.
Concentration can be assigned to the pH value zero and span individually.