57 



mograph considerably by adapting the Spillhaus instrument to be used as a diving 

 instrument. The concept of towing a recorder at various depths was abandoned 

 because of the heavy weight, cable and winch required. The emphasis was 

 placed on redesigning the BT so that you dropped it overboard, paid out cable 

 faster than the ship was going and let it fall free. The winch and cable were to 

 retrieve the BT but were not designed to drag it at any appreciable depth and 

 speed. This technique permitted measurements down to 400 feet at 12 to 15 

 knots on l/l6 inch steel cable, using a winch which only weighed about 300 

 pounds. It required that the BT be made to dive rapidly and that obtaining high 

 speed of thermal response become the predominate design feature. The addi- 

 tional requirements of simplicity and freedom from vibration decided the choice 

 of a liquid-filled bourdon temperature assembly, using some 50 feet of .020 inch 

 bore by .050 inch O.D. Copper tubing for the thermal sensitive unit. The tem- 

 perature bourdon was case-compensated with a bimetalic strip to reduce the 

 thermal hystersis of the instrument. With such a system the time for 90% re- 

 sponse was decreased to about 4/l0 seconds, or about 2/10 seconds to l/e. 

 Even this is none too good when the BT is dropping at 10 feet per second through 

 a sharp thermocline. 



The precision of reading the BT is from . 1 to . 3°F depending upon how 

 fast the ship is going and on the adjustments of the pen. The depth accuracy 

 is about 1% of full scale. The record is made on a 1 x 1 3/4 inch smoked 

 microscope slide. While the record is small it is capable of being read as ac- 

 curately as the data on it and it can readily be examined against a calibrated 

 grid with either a 5 power eye piece or a projector. Figure 1 is a schematic 

 of this instrument essentially as it is available on the commercial market. 



EXTERNAL VIEW 



^BOUROOW TUBE 



INTERNAL VIEW 



Fig. 1 . The Bathythermograph 



