YSI 5000 - Manual (Page 52)

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10. Understanding Dissolved Oxygen Error Factors
The major sources of error in DO measurement are the accuracy of the instrument components,
the accuracy of the probe, and the user’s ability to calibrate the system precisely. Most errors can
be reduced substantially by calibrating at DO levels and probe temperatures as close as possible
to the expected measurement DO levels and temperatures.
In the following, individual sources of error and their ranges are listed. By calculating the root-
mean-squared sum of these individual uncertainties (usually less than half the possible error), the
user can estimate the probable error in any reading.
Note that all types of errors discussed are not necessarily present in a given situation. If salinity
compensation is not used, for example, no salinity compensation error need be considered. If
calibration is to a Winkler Titration sample, calibration errors are replaced by the Winkler
uncertainty.
Type 1 Errors: Instrument Components
a. Instrument accuracy:
±0.1% plus 1 least significant digit.
Type 2 Errors: Probe Accuracy
a. Probe background current error:
background factor x (1 - a/b)c
where a is the observed calibration value, b is the solubility of oxygen in fresh water at 760 mm
Hg and at measurement temperature, and c is the measured DO value.
a, b and c are all measured in mg/L or all are measured in % air saturation.
Use the following table to determine the background factor:
probe temp. in °C background factor (%)
0 2.3
10 1.5
20 1.0
30 0.8
40 0.6
b. Probe nonlinearity error:
±0.3% of reading