Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 177)

<15. OPERATION OF SENCOM pH/ORP>
15-8
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
l Impedance 2
Impedance shows the electrical resistance of the reference electrode liquid junction. The liquid
junction forms the electrolytic contact between the reference electrode and the measuring
electrode, so it must be kept clean and filled with conductive electrolyte. Otherwise the
measurement will suffer from instability, drift and measuring errors. The electrical impedance is
one of the most important.
In case of “Input Impedance setting” is “High” and the measured input-2 impedance value is
higher than 100 kΩ,the display shows “MΩ RANGE”. The measured input-2 impedance value is
lower than 100 kΩ, display shows “BAD”.
If both impedance measurements are disabled (Error setting: Off), the display shows “- - - - (bar)”.
If either impedance measurement 1 or 2 is enabled, the display shows both the impedance
values.
n Sensor wellness
At the Sensor wellness window, the soundness of a module is displayed. A larger number of n
in each gauge indicates that the parameter concerned is sound. A gauge is indicated for only
those parameters whose sensor wellness setting is “enabled,” while a bar (----) is displayed if the
sensor wellness setting is “disabled.”
Sensor wellness setup can be made in Commissioning → Measurement setup → Sensor diag.
settings. For details, see section 16.2.7.
The “Reset wellness data” button can reset wellness data.
When a sensor or electrode is exchanged or replaced, the warning “SENCOM changed” is
displayed depending on the settings (see Section 16.4). Reset the wellness data of the sensor.
NOTE
When a sensor is replaced, the replacement can be recorded manually into a logbook. (Refer to
the figure 15.9.)
n Last calibrated
= date on which the last sensor calibration was performed. The displayed value of the Zero is the
result of this calibration. The displayed value of Slope was calibrated on this date only if the last
calibration was a 2-point calibration.
n Calibration due
= the date when the calibration must be done next according to the settings of the calibration
interval. The calibration intervals are set in Commissioning → Measurement setup → Calibration
settings → Limits and timing.
n Projected maintenance
The projected maintenance function predicts the date when the sensor unit will need recalibrating
for maintaining measurement accuracy. The function checks the input-2 impedance (reference
impedance) every 24 hours.
The function predicts the date when the input-2 impedance will cross the upper or lower limits,
and indicates the date and its status (the status is displayed in parentheses).
As shown in Figure 15.7, the date is predicted based on the intersection point of the upper or
lower limits and the extrapolated line of the values obtained by the least squares method.