In-Situ pH/ORP Sensor - Manual (Page 3)

Page 3
STABLE is displayed when the readings have stabilized.
• 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. This is also a good option for the ORP sensor if
the results of a recent calibration are suspect because
the calibration solution has deteriorated. Results for each
parameter must be set separately.
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 ideal slope is between (-54 mV/pH and -62 mV/pH). A
calculated slope greater than (-50 mV/pH or less than -66
mV/pH) is acceptable but indicates that the sensor may
soon require maintenance.
The offset for ORP is a “zero offset” and is typically (0 ± 20
mV). If the offset calculated during calibration is outside
this range, inspect the sensor and clean it if necessary.
Ensure that the calibration has been stored properly and
has not expired.
Usage Recommendations
• The ORP sensor readings vary slightly due to
temperature changes. These variations are not
easily corrected because, unlike pH, the ORP value is
dependant on the activity of many ions in solution.
The pH values are more easily corrected because
they are a result of the activity of one ion, H
+ .
• 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) may require up to 20 minutes after
calibration to stabilize and begin producing
accurate results. After calibration, condition the
sensor in a low conductivity solution.
• Like all platinum ORP electrodes, the In-Situ pH/
ORP sensor may provide unstable readings in
solutions that contain chromous, vanadous,
titanous, and other ions that are stronger reducing
agents than hydrogen or platinum.
Sensor Handling and Storage
Routine Maintenance
If the ORP platinum electrode is dull or dirty, it can be
cleaned with a swab and methanol or isopropyl alcohol.
Rub the electrode gently until it is shiny.
The pH sensor must be kept moist for the life of the sensor.
The 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.
Figure 1. pH sensor calibration graph