YSI Model 52, Operation Manual

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YSI, Incorporated Model 52 28
SECTION 9 DISCUSSION OF MEASUREMENT
ERRORS
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 of 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 not all types of errors discussed are 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.
9.1 TYPE 1 ERRORS – INSTRUMENT COMPONENTS
1. Instrument accuracy: ±0.1% plus 1 least significant digit.
9.2 TYPE 2 ERRORS – PROBE ACCURACY
1. Probe background current error: background factor x (1 - a/b)c
Where a is the 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