SECTION 8.0 



ERRORS INTRODUCED BY PROCESSING 

 OF THE BT DATA 



The method of processing BT data primarily involves superimposing the calibrated grid 

 of the Bathythermograph over the raw slide and thereby establishing their relative positions, 

 giving not only the correct temperature changes but also the correct temperature values. 



Assigning correct temperature values involves locating the grid with relation to the slide 

 through the medium of temperature and depth set corrections. In the field and under the as- 

 sumption that no changes have occurred since calibration, the grid can be matched to the raw 

 slide by matching the edges of the grid with the edges of the slide. Whenever a set develops 

 in the Bathythermograph, the grid is then moved from its original coincident position by the 

 set correction to account for this error. 



It is necessary to establish correct temperature values for each Bathythermograph in 

 cases where it is desired to compare a series of Bathythermograms taken over a period of 

 time. In this case one is not only interested in the relative changes which occur on any one 

 Bathythermogram, but also the relative changes which occur between Bathythermograms. 

 These relative changes which occur between Bathythermograms must not include the changes 

 in the temperature and depth correction factors of the Bathythermograph. 



The change in sets of the Bathythermograph is a greater source of error than any 

 variance in its precision or reproducibility. Since sets are determinate errors, the magnitude 

 and direction can be calculated and allowed for. It is this process of establishing the determi- 

 nate error and making proper corrections which warrants careful analysis. 



8.1. Temperature Correction Technique 



The present practice employed is to correct Bathythermograph temperatures by 

 a supplemental independent surface temperature measurement. This is accomplished 

 by making simultaneous bucket temperature casts or by reading the water injec- 

 tion thermometer. 



The technique for applying the temperature corrections varies considerably, the 

 technique being dependent primarily upon the reliability of the independent surface 

 temperatures taken. These in turn depend upon the experience of the personnel and 

 the care they exercise in taking the surface temperature measurements. 



Where exceptional care is taken in obtaining bucket temperatures, individual 

 corrections are applied to each Bathythermogram. This technique is limited to special 

 scientific studies where the precision desired is justified by the experience of the 

 personnel taking the data. 



The majority of corrections are not applied individually to each corresponding 

 Bathythermogram. Instead, the discrepancy between each surface temperature and 

 the corresponding Bathythermogram are accumulated for one specific Bathythermo- 

 graph over a period of time and averaged. This average discrepancy is used as a 

 correction factor, and is applied to each Bathythermogram involved in the averages 

 when corrected photographic prints are made. 



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