YSI 85, Operation Manual

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Discussion of Measurement Errors Section 11
YSI, Incorporated Model 85 29
11.2 CONDUCTIVITY 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 that 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.
The error is not necessarily a linear function of temperature. The statement of error is derived from a
25 C temperature offset and a 3%/ C temperature coefficient.
Total Error
Considering only the above three factors, system accuracy under worst case conditions will be ±2%,
although the actual error will be considerably less if recommended and properly calibrated cells and
instrument ranges are used. Additional errors, which can essentially be eliminated with proper
handling, are described below.
Cell Contamination
This error is usually due to contamination of the solution being measured, which occurs when
solution is carried-over from the last solution measured. Thus, the instrument might be correctly
reporting the conductivity seen, but the reading does not accurately represent the value of the bulk