CHAPTER C 



MEASURING WATER TEMPERATURE AND DEPTH WITH A 

 BATHYTHERMOGRAPH 



C-1 The Bathythermograph or BT.— The 



BT (fig. C-1) is an instrument for ol)taiiung a 

 I'ecord of the temperature of sea water at 

 moderate depths. The BT is lowered into the sea 

 and retrieved by means of a wire rof)e. It can 

 be operated while the ship is underway at speeds 

 up to 18 knots. It works mere satisfactorily, 

 however, at speeds of 1*2 knots or less. 



C-2 How a BT Works.— The thermal element 

 of the BT, corresponding to the mercury' column 

 in a glass thermometer, consists of about 4,5 to 50 

 feet of fine copper tul)iiig IHI'.mI with xylene (fig. 

 C-2). As the xylene expands or contracts with 

 the changing water temperature, the pressure 

 inside the tubine increases and decreases. This 



pressure change is transmitted to a Bourdon 

 tube, a hollow brass coil spring, which carries a 

 stylus at its free end. The stylus records, on a 

 coated glass slide, tlie movements of the 

 Bourdon tube as it expands or contracts with 

 changes of temperature. The slide is held rigidly 

 by the depth element assembly which is on the 

 end of a coil spring enclosed in a copper bellows 

 or sylphon. The temperature range of the BT 

 is 28° to 90° F. 



Water pressure, which increases with depth, 

 compresses the sylphon as the BT sinks. This 

 pulls the slide toward the nose of the BT, at 

 right angles to the direction in which the stylus 

 moves; tlius, the trace scratched on the coated 

 surface of the glass slide is a combined record 



Figure C-1. The Bathythermograph (BT). 



FUjiirc C-2. BT thermal element, depth element, and styllus assemblies. 



Change 1—1970 



C-1 



355-756 O - 70 - 2 



