Owner manual

20
IM 12B07C05-01E
ofthesensor(default7.00pH).TheZERO
value indicates the condition of the sensor. If
the value exceeds +/- 120 mV (or user defined
limits) an error message is displayed after
calibration and the calibration is rejected. The
trendofZEROdriftduringthelifetimeofthe
sensor is used to predict the lifetime of the
sensor.
ZEROcanalsobedisplayedinpHunitsand
then it represents the pH value where the
sensor output is 0 mV at 25 ºC. Go to: Com-
missioning >> Measurement >> Calibration
Settings >> Zero and Slope Units
4-3-4. Slope
=calibratedefficiencyofthesensorunitin
percentage of theoretical slope of the sensor
unit.ThetheoreticalslopefollowstheNERNST
equationandis59.16mV/pH.TheSLOPEcan
be calibrated only after a two-point calibration
in buffer solutions with a different pH value. A
low slope indicates that the sensor is not clean
or it indicates a bad sensor. If the calibrated
slopeexceedstherange70-110%(oruser
defined limits) then the calibration is rejected
and a error message is shown.
TheSLOPEcanalsobedisplayedasmV/pH
value at 25 ºC if the user has defined this
variable as mV/pH in Commissioning >>
Measurement >> Calibration Settings >>
Zero and Slope Units
4-3-5. Sensor mV
=theoutputofthesensorunitpriortocalibra-
tion and temperature compensation. This value
is important for trouble shooting.
4-3-6. Reference impedance
=theelectricalresistanceoftheliquidjunc-
tion. The liquid junction forms the electolytical
contact between the reference element 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 elec-
trical impedance is one of the most important
diagnostic tools for keeping the measurement
in good condition. If the value exceeds a user
definedlimit(1000Ω-1000kΩ)anerrormes-
sage will be displayed.
4-3-7. Last calibrated
=thedateonwhichthelastsensorcalibra-
tionisdone.ThedisplayedvalueoftheZERO
is the result of this calibration. The displayed
valueofSlopeisnotnecessarilycalibrated
on this date: only if the last calibration was a
2-point calibration.
4-3-8. Calibration due
=thedatewhenthecalibrationmustbedone
according to the settings of the maintenance
timer. This is based on scheduled maintenance
procedures. The maintenance intervals are
set in menu: setup>> Commissioning>>
measurement setup>> calibration settings
>> limits and timing
4-3-9. Projected calibration
=thedatewhenthepredictivemaintenance
function expects that recalibration of the sen-
sor unit is necessary for good measurement
accuracy. The converter checks the reference
impedance every hour. The user is notified
when maintenance should take place. Prior to
calibration the sensor should be well cleaned
and rinsed.
4-3-10. Projected replacement
=thedatewhenthepredictivemaintenance
function expects that replacement of the
sensor is necessary for good measurement
accuracy. After each calibration the slope, zero
and reference impedance are logged. Aging
of the sensor can be detected from this data.
The observed trend is extrapolated and the
trend predicts when max deviations will be
exceeded. Good predictions are only achieved
with good calibration data. Prior to calibration
the sensor should always be well cleaned and
rinsed and the calibration procedures strictly
observed.