The reason for taking averages is twofold : first, it facilitates the photographic 

 process when only a single correction setting has to be made. Secondly, the inde- 

 pendent surface temperature measurements are themselves subject to considerable 

 inaccuracies. These inaccuracies are ironed out, or so it is believed, when a series of 

 measurements are averaged. 



The length of time over which these averages are taken varies considerably. 

 These periods end whenever there seems to be some sharp break in the discrepan- 

 cies (difference between surface value of the Bathythermograph and the independ- 

 ent surface measurements) noted for a particular instrument. 



Sometimes no correction factor is applied. This occurs when considerable doubt 

 is cast upon the validity of the independent surface thermometer readings (bucket 

 temperature or injection temperatures) . One check for inconsistencies is accom- 

 plished by comparison with a chart which shows average surface temperatures for 

 various times of the year for various ocean areas. Another check is gotten by look- 

 ing for unwarranted variations in the reported surface temperature values while 

 the Bathythermogram shows a steady repetition. 



8.2. Depth Correction Technique 



Each Bathythermogram is subject to errors along its depth scale which are 

 noted by comparing the zero depth grid line to the horizontal trace usually found 

 on all Bathythermograph slides at the true zero depth level, caused by the rapid 

 change in temperature from air to water. 



Individual corrections are seldom, if ever, applied. Again an average discrep- 

 ancy is recorded for a series of Bathythermograms, and an accumulated average of 

 varying periods taken. Interruptions of these averages are effected whenever a sharp 

 change in the values is noted. A single setting facilitates the photographic process, 

 as was the case in temperature corrections. 



8.3. Errors Involved in Correction Techniques 



The methods described above to establish temperature and depth correction 

 factors involve four basic weaknesses which are possible sources of error. These are 

 summarized as follows : 



1. Correction of Bathythermograph to poor secondary information. 



2. No allowance for variations in set unless there is a sharp discontinuity. 



3. Lack of reliability of the zero depth value of the Bathythermograph. 



4. Applying the corrections at the most unstable point of the ocean, the sur- 

 face. 



8.3.1. Poor Secondary Information 



The ability to correct the Bathythermograph to read correct temperatures is 

 limited by the extent to which the independent measurements are reliable. Here is 

 where the human element plays a very important part. The independent tempera- 

 ture measurements, whether they are made by a bucket thermometer or injection 

 thermometer, must be correctly read and correctly recorded. As has been previously 

 pointed out under Section 7.3, the operating personnel often record erroneous data 

 either accidentally or otherwise. This is especially true where injection thermometers 

 are employed for the independent measurement. The thermometers are not read cor- 

 rectly and in many cases are not read at all, but the last reading is merely repeatedly 

 recorded. 



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