In-Situ TROLL 9500 Operators Manual - Manual (Page 107)

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TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
11. The final screen shows the calculated sensor offset. This value
should be 0 mV ± 20 mV.
12. Select Finish to write this value to the sensor.
The ORP sensor is now calibrated and ready to use.
TIP: You can look at the calibration report right after
calibrating, or at any time. See “Calibration History” in
Section 10 for details.
TIP: The calibration solution may be flushed down the drain
with running water, or saved in a separate container and
used to rinse the next time you calibrate with the same solution.
Options for storing sensors:
s
installed. Remove the Cal Cup and rinse it and the sensors. Add
50-100 mL of tap water to the Cal Cup. Return the probe to the Cal
Cup for transport to the field site.
s
later in this chapter.
RESETTING DEFAULT COEFFICIENTS
The sensor’s calibration may be reset back to factory defaults at any
time. This is a good option if the results of a recent calibration are
suspect because the cal solution has been exposed to air or has
otherwise deteriorated
1. With a pH/ORP sensor installed, establish a connection to the
instrument in Win-Situ 4 or Pocket-Situ 4.
2. Select ORP 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 OFFSET
The offset for ORP is a traditional “zero offset,” and is typically 0 ± 20
mV. If the offset calculated during calibration is outside this range, in-
spect the sensor. If it is clean, then the fault was probably with the cal
solution (aging, exposure to air, etc.). In this case, resetting the factory
defaults for ORP can often restore the sensor performance.
UNITS AND CALCULATED MEASUREMENTS
ORP readings may be displayed in Volts or milli Volts. No calculated
measurements are available.
RECOMMENDED CALIBRATION FREQUENCY
Calibration frequency will depend on the nature of the sample and
the degree of accuracy required. In clean water samples, the sensor
could retain its accuracy for 2-6 weeks. Recalibrate the sensor—
s
NORMALIZATION TO STANDARD HYDROGEN
ELECTRODE VALUES
ORP values are dependent on solution composition, temperature, and
sensor type. ORP (Eh) values reported in the literature are often nor-
malized to the standard hydrogen electrode as a standard reference
electrode. Since the standard hydrogen electrode is extremely fragile,
it is impractical to use in the field. In-Situ’s sensor uses a silver/silver-chlo-
ride reference electrode in place of the standard hydrogen electrode.
The following equation may be used to normalize the readings re-
ported by our sensor calibrated with our solution to standard hydrogen
electrode values (±50 mV).
ORP
SHE
= ORP
observed
+ {215.81 – T
C
[0.77942 + T
C
0.001934]}
where ORP
SHE
is the sample potential relative to the standard
hydrogen electrode
ORP
observed
is the sample potential relative to the In-Situ
reference electrode
T
C
IS
ORP measured by a sensor immersed in a solution should not be
equated with thermodynamic Eh. Differences may occur due to lack
of chemical equilibrium, presence of multiple redox couples, sensor
poisoning, and other factors.
In addition, like all platinum ORP electrodes, In-Situ’s pH/ORP sensor
may give unstable readings in solutions containing chromous, vana-
dous, titanous, and other ions that are stronger reducing agents than
hydrogen or platinum.
SECTION 14: ORP
12