INTRODUCTION TO SONAR 



71.83.2 

 Figure 5-10, — Locking the BT slide in the 

 grid mount assembly. 



in the viewer, make sure that the slide is kept 

 tight against both the stop pin and the groove 

 opposite the spring. Otherwise, temperature and 

 depth readings will be incorrect. 



If the grid becomes loose in the holder, 

 apply cement to the face of the grid. Check to 

 see that the narrow edge of the grid is tight 

 against the stop pin, and that the long edge 

 (the zero pressure edge) is tight against the 

 ec^e of the holder opposite the spring. The 

 spring is narrow and is placed in such a posi- 

 tion that the grid fits behind it. As a result, the 

 spring bears on the slide and holds it snugly 

 without pressing on the grid. 



Reading Accuracy 



The trace scratched by the stylus is a tem- 

 perature-depth curve. Each point on the trace 

 represents the temperature at a given depth. 

 Temperature and depth values are read from the 

 curve on the slide. Temperatures are read 

 horizontally and should be read as close as 

 possible to the nearest tenth (0.1) of a degree. 

 Depths are read vertically and should be read 

 to within 10 feet or better on the 900-foot BT, 

 5 feet or better on the 450-foot BT, and 2 feet 

 on the 200-foot models. 



When a BT slide is seen through the viewer, 

 the trace is superimposed upon the grid for the 

 particular BT that made the temperature record- 

 ing. Figure 5-11 shows what a Sonar Technician 

 sees as he views a BT slide. 



Place the slide back of the transparent glass 

 grid, which is ruled off in temperature and depth 

 scales. The slide should be held back of the 

 grid in the same position it had in the BT. The 

 viewer's eyepiece may be adjusted to bring the 



30 36 40 45 50 55 60 65 70 75 80 85 



71,84 

 Figure 5-11. — BT slide as seen through the 

 viewer. 



trace and scales into proper focus. Handle the 

 grid carefully to avoid scratching the grid or 

 the slide. Each BT has three identical grids. 

 Two are sent along with the BT, one of them 

 to be used as a spare. The third grid is retained 

 at the National Oceanographic Data Center. 



HYSTERESIS EFFECTS 



The stylus etches information while the BT 

 sinks and also as it rises. If water conditions 

 where it rises differ greatly from where it 

 sinks, two distinctly different traces will be 

 seen. Usually, however, differences in water 

 conditions are negligible, so that, if two traces 

 appear, they may be caused by hysteresis. This 

 condition is a lag in the movement of one of the 

 elements. 



If it is suspected that hysteresis exists, 

 insert a new slide in the BT and make a second 

 lowering. A brief inspection of the slides will 

 reveal if water conditions changed during the 

 two lowe rings. If water conditions are the same, 

 and the BT is affected by hysteresis, each of 

 the two slides will show two separate traces 

 following each other. If the traces are essentially 

 similar, a slight separation is negligible. 



READING SURFACE TEMPERATURE 



When the BT is on the deck, it usually has 

 a different temperature than when put into the 



72 



