YSI 30 - Manual (Page 26)

Principles of Operation Section 7
YSI, Incorporated Model 30/30M 22
Salinity is determined automatically from the Model 30 conductivity readings according to
algorithms found in Standard Methods for the Examination of Water and Wastewater (ed. 1995).
The use of the Practical Salinity Scale 1978 results in values which 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.
For further information on conductivity and the above standard information, refer to the ASTM
document, Standard Methods of Test for Electrical Conductivity of Water and Industrial
Wastewater, ASTM Designation D1125-82, and OIML Recommendation Number 56. ASTM
symbols for conductivity, cell constant, and path length differ from those preferred in the general
literature and also from those used in this manual.
7.2 DISCUSSION OF MEASUREMENT ERRORS
System accuracy for conductivity measurements is equal to the sum of the errors contributed by the
environment and the various components of the measurement setup. These include:
⋅ Instrument accuracy
⋅ Cell-constant error
⋅ Solution temperature offset
⋅ Cell contamination (including air bubbles)
⋅ Electrical noise
⋅ Galvanic effects
Only the first three are of major concern for typical measurements, although the user should also be
careful to see that cells are clean and maintained in good condition at all times.
Instrument Accuracy = ± .5% maximum
The accuracy specified for the range being used is the worst case instrument error.
Cell-Constant Error = ± .5% maximum
Although YSI cells are warranted to be accurate to within one percent, you should still determine the
exact cell constant of your particular cell. Contamination or physical damage to the cell can alter the
cell constant. Performing a calibration will eliminate any error which might arise because of cell
constant change.
YSI cells are calibrated to within one percent of the stated cell constant at a single point. We
consider these products to be usefully linear over most instrument ranges. The cell constant can be
calibrated to ±0.35% accuracy with YSI conductivity calibrator solutions.
Temperature Error = ± 1% maximum
The solution temperature error is the product of the temperature coefficient and the temperature
offset from 25
°C, expressed as a percentage of the reading that would have been obtained at 25°C.