In-Situ pH Sensor - Manual (Page 3)

Page 3
STABLE is displayed when the readings have stabilized.
The calibration automatically proceeds to the next screen.
• Temperature and barometric pressure may be
displayed for some calibrations.
• Sensor Reading: The current sensor response in
indicated units.
• Sensor Deviation: Change in sensor response
between the last two readings.
Resetting Default Coefficients
Sensor calibration may be reset to factory defaults at
any time. This option is most effective when the sensor
is new.
1. With a pH sensor installed, establish a connection to
the instrument in Win-Situ 4 or Pocket-Situ 4.
2. Select pH in the Parameters list and click Calibrate.
3. In the first screen, select Use Default Coefficients,
then Next.
4. In the final screen, click Finish to send the values to
the sensor.
Sensor Slope and Offset
The pH calibration curve pivots around pH 7 (0 mV
response). The offset calculated by the software when
calibrating at pH 7 will typically be between 350-450 mV.
If the offset falls outside these limits, replace the filling
solution or the junction.
The slope should fall between -54 mV/pH and -62 mV/pH.
A calculated slope greater than -50 mV/pH or less than -66
mV/pH indicates that the sensor requires maintenance.
Usage Recommendations
• Temperature compensation is provided in the
software to account for measurements taken
at temperatures different from the calibration
temperature. For most accurate results, try to
calibrate at the same temperature as the expected
sample temperature.
• After an initial 1-2 week settling period, during
which the sensor should be calibrated weekly,
calibration frequency will depend on the nature
of the sample and degree of accuracy required. In
clean water samples, the pH sensor should retain
its accuracy specification for 2-6 weeks before
requiring calibration.
• A small error in pH will occur in basic solutions
(>pH 10) that contain high levels of sodium salts
(>0.01M) due to sodium interference.
• pH readings in pure water samples (<100µS/cm
conductivity—also known as “low ionic strength”
samples) require up to 20 minutes after calibration
to stabilize and begin producing accurate results.
After calibration, condition the sensor in a low
conductivity solution.
Sensor Handling and Storage
Routine Maintenance
After the pH sensor is moistened, it must be kept moist
for the life of the sensor.
The pH sensor fill solution has a shelf life of 2 years.
Replace the fill solution every 5 to 6 months or when:
• The sensor fails to calibrate within the acceptable
slope and offset range.
• Sensor readings vary.
• Readings during calibration at pH 7 are greater than
+30 mV or less than -30 mV.
• Sensor is slow to respond.
Note: If the sensor fails to calibrate after you replace the fill
solution, replace the reference junction.
Replacing the Filling Solution
1. Use the sensor removal tool to pry the sensor up
and out of the instrument.
2. Unscrew the reference junction. See Figure 4.
3. Protect the connector end of the sensor with
Figure 3. pH sensor calibration graph
Figure 4. pH sensor with reference junction removed