Chapter 5— BATHYTHERMOGRAPH 



water. The temperature element moves the stylus 

 along the zero depth line of the surface water 

 temperature position during the 30 seconds or 

 more when the BT is towing at the surface. 

 Consequently, the top of most traces is an 

 almost horizontal line. This surface trace should 

 fall along the zero depth line of the grid when 

 the slide is viewed. If it appears more than 3 

 feet above or below, correct the depth readings 

 by the amount of this error. Read surface 

 temperature by noting the point at which the 

 trace starts downward from the surface trace. 



It's a good idea to make a comparison from 

 time to time between the surface temperature 

 as read from the slide, and the bucket surface 

 temperature as read for the same lowering. 

 On a series of slides taken from the same 

 BT, the readings from the slide and the bucket 

 thermometer should be about the same. If there 

 is a difference, and if the difference continues 

 for later lowerings, the calibration of the BT 

 probably has shifted. A shift in calibration, 

 sometimes called a shift in the zero points, 

 usually does not change the accuracy of the BT. 

 But, make a note on the BT logsheet showing 

 the slide number and the time at which a shift in 

 calibration was detected. 



If the BT accidentally strikes either the 

 ocean floor or an underwater object, the depth 

 at which it struck can be determined by reading 

 the depth of the horizontal line across the trace, 

 made when the stylus arm vibrated with the 

 shock. 



READING BT TRACE 



The bathythermograph, as already pointed out, 

 provides a continuous visual record of the tem- 

 peratures from the surface of the sea to a depth 

 to which it is lowered. The temperatures of 

 various ocean depths recorded on the slide by 

 the BT ai'e represented as a graph of tempera- 

 ture against depth when seen through the grid 

 viewer. Figure 5-12 shows the portion of a 

 BT grid immediately surrounding the trace. 



In figure 5-12, the line marked "A" x'epre- 

 sents changes in temperatare between the sea 

 surface and a depth of 250 feet. Little change 

 in temperature results until a depth of 50 feet 

 is reached. At that point, the temperature of 

 the water begins to cool quite rapidly to a 

 depth of 145 feet. Below 145 feet, the water 

 cools more slowly. 



From the temperature curve, one can dis- 

 cover the sound pattern of the sea at the time 



TEMPERATURE (°F) 



55 60 65 



< 

 a. 



3 

 o 



0- 



UJ 

 Q 



Figure 5-12. — Graphing the BT 



71.85 

 trace. 



of the bathythermograph observation. It is from 

 the sound pattern that an estimate of maximum 

 sonar range can be made. 



The variation of temperature with depth can 

 be quite complicated. Many different conditions 

 may be represented by the different bathythermo- 

 graph slides. It is convenient to consider the 

 water as layers. In each layer the temperature 

 can change from top to bottom. Create a mental 

 picture of the location of these layers from the 

 appearance of the slide. Visualize, too, how 

 tlie temperature is changing. Study the following 

 examples until each kind of layer can be identi- 

 fied easily. 



Negative Gradient 



A negative gradient condition exists when the 

 temperature of the water decreases with depth. 

 A BT, lowered through a negative gradient, 

 produces a slide with the etched trace sloping 

 to the left as depth increases. Figure 5-13 

 represents a trace showing a negative gradient. 

 This illustration depicts a particularly sharp 

 gradient, which is quite common. Negative gra- 

 dients spell trouble for sonar operators because 

 they result in short sonar ranges. 



Positive Gradient 



Sometimes, layers are found in which the 

 temperature increases with depth. The BT trace 

 slopes to the right. Although usually observed 



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