INTRODUCTION TO SONAR 





TEMPERATURE ELEMENT 



PRESSURE ELEMENT 



STYLUS 



GLASS 



XYLENE FILL 

 TUBING 



BELLOWS 



HELICAL 

 ISTON SPRING 



HEAD 



71.73 



Figure 5-4. — BT temperature and pressure elements. 



to a Bourdon tube. As the xylene expands or 

 contracts with the changing water temperature, 

 the tubing expands or contracts, causing the 

 Bourdon to move. A stylus is attached to the 

 free end of the tube. The stylus records the 

 movements of the Bourdon, as it expands or 

 contracts with changes of temperature, on a 

 gold-colored, metallic-coated glass slide. The 

 temperature range is from 28° to 90°F. The 

 slide is held rigidly on the end of a coil spring 

 enclosed in the copper bellows. Water pressure, 

 which increases in proportion to water depth, 

 compresses the bellows as the BT sinlcs. 



Figure 5-4 illustrates the BT temperature 

 and pressure elements. The dotted line drawings 

 in the lower portion of the illustration show the 

 action of the stylus moving left on the slide, 

 with a decrease in temperature, and the bellows 

 being compressed to the right (note arrow) as 

 depth increases. Increase in depth pulls the 

 slide toward the nose of the BT, at right angles 

 to the direction in which the stylus moves to 

 record temperature. When the BT is raised 



toward the surface, the spring expands the 

 bellows to its original shape. Thus, the trace 

 scratched on the plated surface of the slide is 

 a combined record of temperature and pressure. 

 Pressure is directly proportional to depth. 



Each instrument must be calibrated carefully 

 by the manufacturer because external pressure 

 slightly affects the internal pressure of the 

 xylene in the Bourdon, and because temperature 

 changes also influence the movement of the 

 bellows. 



A special grid is supplied with each instru- 

 ment for converting the stylus trace to tempera- 

 ture and depth readings. These grids are not 

 interchangeable. For correct readings the grid 

 is constructed so that the temperature lines may 

 not always be exactly straight and vertical, but 

 vary slightly with increasing depth. The depth 

 lines, likewise, are not exactly arcs of circles, 

 with radius equal to the length of the stylus. 

 Instead, they are calibrated to allow for thermal 

 expansion of the bellows. 



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