Appendix D 
Current Meter Calibrations 
Two types of electromagnetic current meters were used in the DELILAH 
array, Marsh-McBirney 551's (Figure 4 in the main text) and Scripps 
Institution of Oceanography “open frame” sensors (Figure D1). All of the 
current meters were calibrated in a constant flow tank by the Naval 
Oceanographic Office in Bay St. Louis, MS. Each instrument was calibrated 
at five speeds ranging from 0.0 to 1.91 m/s and four probe orientations 
(+ X, +Y). All instruments deployed during the experiment were also 
postcalibrated. Because of the biological growth that developed on the open 
frame current meters, they were post-calibrated with and without the growth. 
Although the growth had decayed somewhat by the time the instruments were 
recalibrated, the results give some indication of their performance under fouled 
conditions. The calibration results, including pre- and postcalibration gains 
and bias values, and computed changes between the calibrations, are listed in 
Tables D1 and D2. 
Pre- and postcalibrations were used to adjust the collected time series data. 
In general, the precalibration was used unless the postcalibration was 
significantly different. Then, either the postcalibration was used or the pre- 
and postcalibrations were averaged. The decision to use only the 
postcalibration was based primarily on a comparison with nearby gauges 
during a time of high flow over linear bathymetry. In most cases, the current 
meter offsets were determined using the calibration curve for each channel of a 
given gauge and applied as a bias in volts (Figures D2 through D33). After 
plotting the time series in engineering units, however, further offsets of the 
velocities from zero were obvious for open frame channels 2711, 2731, and 
2741 in the crest subarray, and for Marsh-McBirney channels 2701 and 2901 
in the primary cross-shore subarray. These channels all measured cross-shore 
currents. The offsets appeared to be systematic throughout the duration of 
each time series, suggesting that a constant value applied to these data would 
be a satisfactory adjustment. To correct for these offsets, current velocities 
taken from these channels during a calm period (6-10 October) were correlated 
against data from channels which did not appear to require further offsets. The 
y-intercept of the regression curve for each channel was the offset applied in 
meters per second to the current velocities. Correlation coefficient (r°) values 
Appendix D Current Meter Calibrations 
D1 
