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EQUIPMENT AND INSTRUMENTS 



4731 



4731. Bimetallic-Reed Type Bathythermograph 



The first model of the bathythermograph contained a bimetaUic-reed temperature element enclosed 

 in a metal tube 2)^ inches in diameter and 27 inches long. The upper part of the metal tube had 

 perforated sides to permit the free passage of sea water. The reed was attached through heat-insu- 

 lating material to a pressure element in the lower watertight compartment of the tube. The pressure 

 element consisted of a series of metallic bellows that compressed with an increase in pressure, moving 

 the bimetallic reed longitudinally in the tube. Variations in temperature bent the bimetallic reed 

 to move a stylus attached to its upper end which was in contact with a stationary glass slide. The 

 contact side of the slide was smoked by a special process, and on it a curve of temperature correlated 

 with depth was traced by the action of the bending of the bimetallic reed and the compression of the 

 pressure element. 



The bimetallic-reed type of bathythermograph makes a record of the temperature during its 

 descent through the water and another during its ascent. The trace on the smoked glass is likely to 

 be rather wide when used during rough weather, because of excessive vibration in the bimetallic reed. 

 This model can be used to a depth of 82 fathoms but the vessel must be stationary in the water and, 

 because of instrumental lag, the speed of lowering and raising must not exceed 10 fathoms per minute 

 in order to avoid a double trace. 



4732. Bourdon Type Bathytherm,ograph 



The new design of the bathythermograph (fig. 88) incorporates a Bourdon type of temperature 

 element, eliminating many of the objectionable features of the bimetallic-reed type. The size is about 

 the same and the pressure element is essentially the same but is located in the upper part of the metal 

 tube. The glass slide, on which the record is traced, is attached to the lower end of the pressure element 



c 



Figure 88.— Bourdon type bathythermograph. A. Graduated grid. B. Scanning device. C. Smoked slide. 



and is moved longitudinally along the axis of the instrument by changes in pressure. The spiral of the 

 temperature element is mounted inside the lower end of the tube in a square housing, only the bulb of 

 the temperature-responsive system being outside in full contact with the surrounding water. A stylus 

 arm is attached to the Bourdon spiral, the stylus marking the temperature-depth curve on the 

 smoked slide. 



The Bourdon type of bathythermograph is more sensitive to temperature changes and less sensi- 

 tive to vibrations, and it may be lowered and raised faster than the bimetallic-reed type. Its use is 

 limited to depths less than 75 fathoms. With it a temperature trace may be obtained from a vessel 

 while underway and satisfactory results have been obtained at a speed of 15 knots. For use while 

 underway, a considerable length of wire and an additional weight are necessary to permit the instru- 

 ment to reach the maximum depth. When the bathythermograph is used from a vessel stationary 

 in the water, the record of both traces is valuable as a check and to ensure that the instrument is work- 

 ing correctly. However, when the instrument is used while underway the two curves may not coin- 



