YSI 600DW-B Sonde - Manual (Page 216)

Principles of Operation Section 5
5.4 OXIDATION REDUCTION POTENTIAL (ORP)
The sondes determine the Oxidation Reduction Potential (ORP) of the media by measuring the difference
in potential between an electrode which is relatively chemically inert and a reference electrode. To
measure ORP with a sonde, a combination pH/ORP probe must be in place in the sonde bulkhead and ORP
must be accessed via the ISE2 channel of the sonde. The ORP sensor consists of a platinum button found
on the tip of the probe. The potential associated with this metal is read versus the Ag/AgCl reference
electrode of the combination sensor that utilizes gelled electrolyte. ORP values are presented in millivolts
and are not compensated for temperature.
CALIBRATION AND EFFECT OF TEMPERATURE
Calibration may not be required for the ORP sensor of the sonde when it is new. However, older probes
that have been deployed extensively may show some deviation from the theoretical ORP value. This
deviation is usually due to a change in the concentration of the KCl in the reference electrode gel. To
determine whether the sensor is functioning correctly, place the ORP probe in 3682 Zobell solution and
monitor the millivolt reading. If the probe is functioning within specifications, the ORP reading should be
within the range of 221-241 at normal ambient temperature. If the reading is outside of this range, the
probe can be calibrated to the correct value (231 mV at 25°C) using the calibration procedure outlined in
Section 2.6.1, Calibration.
ORP readings for the same solution can vary up to 100 mv depending on the temperature. However, no
standard compensation algorithms exist for this parameter. Be sure to take this factor into account when
reporting ORP values and checking sensor calibration. For Zobell solution, consult the following chart:
TEMPERATURE,
CELSIUS
ZOBELL SOLUTION VALUE,
MV
-5 270.0
0 263.5
5 257.0
10 250.5
15 244.0
20 237.5
25 231.0
30 224.5
35 218.0
40 211.5
45 205.0
50 198.5
MEASUREMENT AND CALIBRATION PRECAUTIONS
1 - Instructions for preparation of ORP calibrator solutions (including Zobell reagent) can be found in
Section 2580 A. of Standard Methods for the Examination of Water and Wastewater. Alternatively, the
Zobell solution is available from YSI.
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-3