Chapter 5 — BATHYTHERMOGRAPH 



Depth Element 



The depth element, located in the body tube, 

 consists of a piston device with an accurately 

 wound steel spring. An envelope, made up of 

 three metallic bellows soldered together, sur- 

 rounds the spring and keeps the water pressure 

 off the spring and off the spring side of the 

 pistonhead. In one type of BT, one end of the 

 bellows assembly is soldered to the solid nose- 

 piece, but in another type this end is soldered 

 to the mounting base. In botli types, the movable 

 end is soldered to the spring side of the piston- 

 head. Increasing water pressure tends to collapse 

 the bellows and compress the spring, causing 

 the slide holder attached to the pistonhead to 

 move forward, that is, toward the nose. 



Body Tube 



The body tube serves as the main support 

 and protection case for the depth element and 

 thermal assembly. The movable brass sleeve 

 covers the slide ports next to the slide holder. 

 The sleeve may be moved forward easily when 

 it is necessary to get at the slide and slide 

 holder. When the BT is to be lowered, push 

 back the sleeve until it touches the forward 

 end of the tail fins, thereby enabling the auto- 

 matic stylus lifter to be activated. This action 

 covers the slide ports and also releases the 

 stylus lifter so that the stylus can write on the 

 coated glass slide inserted in the slide holder. 



Nosepiece 



A large percentage of the approximately 

 25-pound weight of the BT is concentrated in 

 the nosepiece. This weight makes the nose 

 sink first during a BT lowering, if the towing 

 cable is payed out freely. The nosepiece has 

 an attached towing fin, swivel, and shackle 

 where the towing cable may be attached easily. 



Auxiliary Nose Sleeve 



On vessels moving at high speeds, earlier 

 models of the hydro bathythermograph oftenf ailed 

 to dive to the depths from which information 

 was desired. Two types of diving attachments 

 have been added to overcome this difficulty. 



One attachment is a heavy bronze sleeve, 

 slipped over the nose of the instrument and 

 secured by tightening two locking screws. This 

 sleeve adds to the weight of the BT and increases 

 diving speed. The ship can move at higher speed 



71.114 

 Table 5-2, — Maximum Towing Speeds 



and the BT still can reach full depth. Table 5-2, 

 based on paying out 1000 feet of cable, shows 

 the maximum ship speeds that can be used with 

 and without the nose sleeve. Note, for instance, 

 that to get the BT down to 900 feet without the 

 nose sleeve, own ship's maximum speed would 

 have to be 3 knots. With the nose sleeve, the 

 speed could be as great as 6 knots. 



The other device is a towing block attached 

 well aft on the BT and used while lowering the 

 instrument. This attachment causes the BT to 

 dive at a steep angle because the towing point 

 is well aft. 



Tail Guard 



The tailpiece assembly consists of the fin 

 pieces strengthened with two tie strips. This 

 assembly protects the capillary tubing and stabi- 

 lizes the BT during the lowering, towing, and 

 raising of the instrument. 



THEORY OF OPERATION 



The BT can be operated while the ship is 

 underway at speeds up to 18 knots. It works 

 most satisfactorily, however, at speeds of 12 

 knots or less. 



The temperature element, corresponding to 

 the mercury column in a glass thermometer, 

 consists of about 45 to 50 feet of fine copper 

 tubing filled with xylene. The tubing is wound 

 around inside the tail fins of the BT and comes 

 into direct contact with the sea water. One end 

 of the tubing is fixed; the other end is attached 



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