INTRODUCTION 



Nearly 800,000 bathythermograph (BT) observations have been ob- 

 tained by various agencies. These BT's, taken over a period of about 

 20 yeejTB, are extremely valuable for oceanographic research. 



BT data were originally used in qualitative studies of tempera- 

 ture-depth relationships. Emphasis for the use of BT data has gradu- 

 ally shifted from qualitative contributions to more demanding and 

 exacting quantitative requirements. Several types of errors which 

 influence quantitative statistical analyses became apparent during 

 a study of BT data taken at ocean weather station ECHO (reference 1). 



The most comprehensive study of BT errors is that of Bralove and 

 Williams (reference 2). Their report defines various types of errors 

 associated with the BT, suggests ways of measuring the magnitude of 

 some of these errors, and concludes with recommendations to improve 

 the quality of BT data. However, quantitative analysis determining 

 magnitude of the errors is not given. One of the statistical euialyses 

 presented in this report was suggested by the Bralove and Williams 

 study. 



Stewart (reference 3) performed laboratory tests on six newly 

 calibrated mechanical BT's in an attempt to evaluate their measuring 

 capabilities. These tests showed that it was not possible to attain 

 the specified full depth range accuracy of 1 percent. A more rea- 

 sonable accuracy is 5 percent of full depth range. Also, the speci- 

 fied accuracy of +0.1°F was found to be more like +0.5°F. 



Winterfeld (reference k) , in a study of correcting instrument 

 bias by means of a reference temperature, states that most of the 

 BT data on file at the National Oceanographic Data Center have been 

 adjusted by using reference temperatures. The references were either 

 injection temperatures obtained during the cruise or bucket tempera- 

 ture readings obtained specifically for bias adjustment. Winterfeld 

 also indicates that differences between simultaneously recorded BT 

 and injection sea surface temperatures are extremely variable, and 

 that the calculated temperature correction (TCS, temperature correc- 

 tion slide) for BT bias "could not be assumed to be a true instrument 

 bias J i.e., it could not be assumed that the reference temperatures 

 are plausible. In fact, for many cruises the injection temperature 

 records must have a bias of greater magnitude than the bias of the 

 BT instrument. Thus, it must be concluded that a certain percent- 

 age of processed BT cruises in the NODC archives have, despite the 

 TCS adjustment, a considerable bias in the adjusted BT temperatures. 

 In fact, in many cases the adjustment must have introduced into the 

 BT observation a bias of greater magnitude than the bias of the BT 

 instrument." 



