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Example of Post-Processing Data Correction
Sometimes, in-field validation is not possible. This restricts correction of possibly fouled data to post-processing,
and requires a reference validation sample be taken at the end of the deployment or soon after the instrument is
recovered. The reference validation sample could be a water sample taken in the field prior to recovery or a factory
calibration.
A simple method for data correction in post-processing is to assume a linear fouling adjustment per day (or week, or
month) for the entire period or between field validation data collection periods. The resultant rate of change in the
sensor measurement can then be programmed into a simple algorithm to calculate corrected DO data with time.
Post-Processing Correction Example 1:
The fouling drift in sensor data shown in Figure 8 is linear with time. To apply a correction to the data, simply
compute the rate of change per day in the Soc value from the beginning of the deployment to May 13, when an
in situ reference sample was made (50 days).
SOCrate = (NewSOC – previousSOC) / # of days
0.0012e-04 per day = (1.3866e-04 – 1.3256e-04) / 50
Post-Processing Correction Example 2:
For this example, pre- and post-calibration data from sensor 1114 are used (Table 2). Sensor 1114 was deployed in a
high-fouling coastal lagoon for 2 months with no interim servicing. After recovery, the sensor was returned to the
factory for calibration and shown to have fouled ~1% (Table 2). The post-calibration SOC value prior to any
cleaning, and the rate of change in SOC, are computed as follows:
NewSOC = previousSOC * ([Winkler] / [SBE 43])
0.3837 = 0.3810 * 1.007
SOCrate = (NewSOC – previousSOC) / # of days
4.445e-05 per day = (0.3837 – 0.3810) / 60
During the 2-month deployment period, the sensor experienced an SOC change rate of about 0.000045/day. In
reality, a sensor may not exhibit fouling for several weeks to months (as shown here, fouling was minimal), so the
user will need to decide how best to determine the time span and data correction criteria of fouled data. Interim
water samples or reference checks (using a CTD) between service intervals can provide valuable information on
fouling. Variations on the methods used for post-correcting data are also possible.
Data corrections are only as good as the reference samples used to correct the data. We therefore recommend
replicate water sample collection, or validation methods that employ a sensor that is as accurate as the moored
sensor being tested.
References
Owens, W. B., and R. C. Millard Jr., 1985: A new algorithm for CTD oxygen calibration. J. Physical
Oceanography., 15, 621-631.
Garcia and Gordon (1992) "Oxygen solubility in seawater: Better fitting equations", Limnology & Oceanography,
vol 37(6), p1307-1312.
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