YSI 3200 - Manual (Page 54)

Principles of Operation Section 8
8.3 SALINITY
Salinity is determined automatically from the Model 3200 conductivity and temperature 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.
8.4 TOTAL DISSOLVED SOLIDS
Total dissolved solids (TDS) is determined automatically by the Model 3200 by multiplying
conductivity by an empirical factor (Standard Methods for the Examination of Water and
Wastewater ed. 1995). The factor may vary from 0.55 to 0.9, depending on the composition of the
water and the temperature.
8.5 TEMPERATURE
The Model 3200 system utilizes a thermistor of sintered metallic oxide that changes predictably
in resistance with temperature variation. The algorithm for conversion of resistance to
temperature is built-in to the Model 3200 software, and accurate temperature readings in degrees
Celsius or Fahrenheit are provided automatically. No calibration or maintenance of the
temperature sensor is required.
8.6 SOURCES OF ERRORS
CELL CONTAMINATION
This error is usually due to contamination of the cell by some previous solution. Normally this is
in the form of an organic film that reduces the solution-electrode interface conductance. Follow
the cleaning instructions carefully.
An entirely different form of contamination sometimes occurs when cells are stored for long
periods of time wet; alga and other life forms grow on the electrodes. While rare, such deposits
have, on occasion, markedly reduced the effectiveness of the cell by reducing the solution-
electrode interface conductance.
CELL PLATINIZING
Cells that have begun to lose their electrode coating of platinum black when measuring solutions
having high conductivity values can introduce errors. The effect of poor platinization is a loss of
linearity and a noticeably large change in conductance from range to range on the instrument.
When you suspect a problem with the cell platinization, follow the instructions for electrode
inspection and replatinization carefully before attempting any critical measurements.
YSI Incorporated Model 3200 50