YSI 600LS - User Manual - Manual (Page 241)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-3
generally speaking, TDS cannot be calculated accurately from specific conductance unless the make-up of
the chemical species in the water remains constant.
NOTE: EcoWatch for Windows contains a conversion factor of 0.65 for conversion of specific
conductance in mS/cm to TDS in g/L. However, unlike the sonde software, this constant cannot be varied
in EcoWatch. Thus, to calculate TDS from data resident in an EcoWatch file, the user should export the
specific conductance readings to a spreadsheet and carry out the calculation there using the formula:
TDS in g/L = (Sp. Cond in mS/cm) x (User-Derived Constant)
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