YSI 63 - Manual (Page 26)

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7. Discussion of Measurement Errors
7.1 pH Errors
There are two basic types of pH errors. The first type are errors related to limitations of
instrument design and tolerances of components. The second type are errors due to basic sensor
accuracy tolerances, mainly background signal, linearity, and variations in temperature
coefficient. It is unlikely that the actual error in any measurement will be the maximum possible
error.
Errors
• Component and circuitry error: ±0.03 pH
• pH error caused by sensor accuracy and temperature compensation:
±0.1 pH for measurements at 10°C from calibration temperature
±0.2 pH for measurements at 20°C from calibration temperature
7.2 Conductivity 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.