Temperature-Related Error 



7. It has been determined that for some unknown number of Waveriders 

 the sensitivity is drifting downward, possibly since manufacture, on the aver- 

 age of about 1 percent per year. Sensitivity loss from some unknown chemical 

 reaction is related to increases in electrical conductivity of the fluid sur- 

 rounding the accelerometer. This drift is identified from successive calibra- 

 tions over a period of years. 



8. Recently, Datawell has introduced an improved modulator printed- 

 circuit board for bringing calibrations within specification and for prevent- 

 ing further decreases in sensitivity. This modification has been made for 

 buoy 66968, so the temperature-related error correction need not be applied. 

 For all the other buoys — e.g., 66967, 66969, and 66977 — it is recommended that 

 the correction be used. Datawell has provided curves for correction of cali- 

 bration and buoy temperature when the buoy is measuring waves in the ocean. 

 The NOAA Engineering Support Office has, in turn, developed a table based on 

 the Datawell curve which can be entered with the uncorrected difference error 

 value d and the temperature of the water during the time of buoy operation 

 to determine the difference error correction (Table AlO) . The difference 

 error correction is added to d to obtain the corrected difference error, 



D . For temperatures during buoy operation greater than the buoy temperature 

 during calibration (i.e., 22.4° C) , no correction is necessary. Water tempera- 

 ture values may best be determined from Table 9 in the section entitled Water 

 Characteristics in the main text of this report or from the FRF Monthly Pre- 

 liminary Data Summaries (see References). 



9. Since these error corrections are oscillation period dependent, 

 their application requires that the wave data be decomposed into amplitude 

 coefficients or variance-spectrum coefficients for each frequency or period. 

 A less accurate but also less complicated procedure would be to apply a single 



correction to the wave height H based on the peak spectral wave period 



o 

 and an average water temperature estimate. For correction of amplitudes or 



derived parameters linearly related to amplitude, a correction factor F(T) 



can be obtained from the sum of the Datawell DW and temperature corrected 



difference error D using 



^(T) = 1 . (DW -H D) (2^ 



A24 



