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 obtaining a 

 record of the temperature of sea water at 

 moderate deptlis. The BT is lowered into the sea 

 and retrieved Iiy means of a wire rope. It can 

 be operated while the ship is underway at speeds 

 up to 18 knots. It works more satisfactorily, 

 however, at speeds of 12 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 45 to 50 

 feet of fine copper tubing filled with xylene (fig. 

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

 the changing water temperature, tlie pressure 

 inside the tubing 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, the 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 tlie 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). 



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



C-1 



