Xylem 6-Series Multiparameter - Manual (Page 240)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-2
4 - Perform sensor calibration at a temperature as close to 25 o C as possible. This will minimize any
temperature compensation error.
5- The 6560 YSI 6-series conductivity system is extremely linear and therefore the sensor will be accurate
within its 0.5% accuracy specification when calibrated anywhere in the range. Thus, there is normally no
need to calibrate with low conductivity standards when making measurements in fresh water or high
conductivity standards when making measurements in brackish or sea water. Low conductivity standards
are very susceptible to contamination and their use is not recommended unless extra care is taken to rinse
the calibration vessel and probe compartment with the standard prior to actually calibrating. For most
applications, YSI recommends calibration using a mid-range calibration standard of approximately 10,000
uS/cm such as YSI 3163 Calibrator Solution which is available in quart bottles. For special applications or
for users who insist on calibrating at values near that of the water to be monitored, YSI does offer standards
of higher and lower conductivities – see Appendix C Accessories and Calibration Reagents.
5.2 SALINITY
Salinity is determined automatically from the sonde conductivity and temperature readings according to
algorithms found in Standard Methods for the Examination of Water and Wastewater (ed. 1989). The use
of the Practical Salinity Scale results in values that are unitless, since the measurements are carried out in
reference to the conductivity of standard seawater at 15 °C. However, the unitless salinity values are very
close to those determined by the previously used method where the mass of dissolved salts in a given mass
of water (parts per thousand) was reported. Hence, the designation “ppt” is reported by the instrument to
provide a more conventional output.
5.3 TOTAL DISSOLVED SOLIDS (TDS)
The electrical conductivity of environmental water is due to the presence of dissolved ionic species. Thus,
the magnitude of the conductivity (or specific conductance) value can be used as a rough estimate of
amount (in g/L) of these ionic compounds which are present. The 6-series software provides a conversion
from specific conductance to total dissolved solids (TDS) by the use of a simple multiplier. However, this
multiplier is highly dependent on the nature of the ionic species present. To be assured of even moderate
accuracy for the conversion, the user must determine this multiplier for the water at the site of interest. Use
the following protocol to determine the conversion factor:
1. Determine the specific conductance of a water sample from the site;
2. Filter a portion of water from the site;
3. Completely evaporate the water from a carefully measured volume of the filtered sample to yield a dry
solid;
4. Accurately weigh the remaining solid;
5. Divide the weight of the solid (in grams) by the volume of water used (in liters) to yield the TDS value
in g/L for this site; Divide the TDS value in g/L by the specific conductance of the water in mS/cm to
yield the conversion multiplier. Be certain to use the correct units.
6. Enter the determined constant into the sonde software from the Advanced|Sensor menu to view the
correct TDS values from a computer/sonde interface.
CAUTION: The default value (0.65) for conversion of specific conductance in mS/cm to TDS in g/L
which is resident in the Advanced|Sensor menu of the software is only useful for a gross estimate of the
TDS. As noted above, to attain any degree of accuracy for TDS, the user must determine the conversion
factor empirically. Even then, if the nature of the ionic species at the site changes during an unattended
study or between sampling studies, the TDS values will be in error. It is important to recognize that,