re 3-3. Mean density (pm) of sea water. 



r„=9.i2°. 



p„= 1.0281 at 600 m. 

 Q=0.01275. 

 Find: 



(16.17)-(9.12) 



Z=, 



(1.0281) (0.01275) 



y_ 7.05 



0.01311' 

 Z = 538 meters. 



This figure is entered in the "Thermometric 

 Depth" column of the A-sheet. 



3-11 Determining Tliermometric Depth by 

 Depth Anomaly (AZ) Graphs.— The depth 

 anomaly (AZ) graph permits more rapid appli- 

 cation of the above formula inasmuch as certain 

 values of the formula may be precalculated and 

 graphed. Thus, to find the thermometric depth 

 of reversal of a given unprotected thermometer, 

 AZ is read from its graph and added algebrai- 

 cally to 100(7'u— r„) corresponding to the 

 given Tu—Tu,. AZ is defined as the meters 

 depth by which Z differs from 100(r„-T„). 

 In other words, since pm is approximately 1.0 

 and Q is roughly 0.01, Z=100(r„-r,„) + AZ. 



Each unprotected thermometer should have 

 its own depth anomaly (AZ) graph. A sample 

 one is shown in figure 3-4. Enter the graph 

 with T^-Ty, from the Difi"erence column of the 

 A-sheet (7.05°) and determine the AZ (-167m). 

 Enter this value in the Correction column. Be 

 sure to indicate if the correction is to be added 



34 



or subtracted. Add this correction algebraically 

 to the difference times 100 and enter this value 

 (538m) in the Thermometric Depth (Z) column 

 of the A Sheet. 



3-12 Constructing a Depth Anomaly (AZ) 

 Graph. — If it is assumed that an ideal un- 

 protected thermometer will register an increase 

 of 0.01° C per meter of depth in sea water, then 

 100 times the difference between the protected 

 and unprotected thermometer readings would 

 equal the depth in meters. Actually, this is 

 not exactly the case due to minute variations 

 in the glass and other slight imperfections that 

 are impossible to avoid in manufacture. Thus, 

 the unprotected thermometer will have Q-factors 

 that are somewhat greater or less than the ideal 

 (0.01). Therefore, correction graphs can be 

 constructed using the values of Q and p„„ 

 assuming values of T^-T^,, and then solving the 

 formula, in section 3-10 above, for depth (Z). 

 The difference between the computed or thermo- 

 metric depth and the ideal or assumed depth 

 is the depth anomaly (AZ). For example, the 

 values used to construct the AZ graph shown in 

 figure 3-4 are given below: 



The values of AZ are plotted at the values of 

 Ty-T^ and a curve drawn through the points. 

 It should be noted that in the sample these are 

 negative errors, that is the thermometric depth 

 is less than the assumed or ideal depth, and that 

 the corrections obtained from this graph must 

 be subtracted from 100(7'„-7'„) to obtain the 

 correct thermometric depth. 



3-13 ACCEPTED DEPTH DETERMINA- 

 TIONS. — There are two methods in general 

 use for determining the accepted depth; e. g., 

 the best possible determination of the true 

 depth of each Nansen bottle at the time of 

 reversal. These are the depth-difference method 

 (wire length, L, minus thermometric depth, Z) 

 and ,the depth-ratio method (thermometric 

 depth, Z, divided by wire length, L). In the 

 first, a reasonably accurate picture of the true 

 wire shape during cast is reproduced graphi- 

 cally, and in the second the ratio of the thermo- 



H. O. 607 



