Most of the metal parts are 6061 T6 alumi- 

 num. The drum and spacer rings are plastic. 

 The clock used is from a kitchen timer and 

 runs for 55 minutes. 



The clock can be calibrated by reference to 

 a known standard. 



ACCURACY 



OPERATION 



The clock is wound by turning its housing 

 clockwise 350°. The recording drum is ad- 

 justed by loosening its setscrew (9 in fig. 3) 

 until its notched rim engages the lower rim 

 of the clock housing and the paper clamp bar 

 is in the position shown in figure 2. The set- 

 screw is then tightened, the case screwed on, 

 and the assembled BKG, padded as necessary, 

 is tied to the net by means of a line through 

 the ring welded to the case. As the BKG enters 

 the water, the water pressure moved the "O" 

 ring sealed piston in its housing compressing 

 the spring. This movement carries the lower 

 rim of the clock housing away from the upper 

 rim of the recording drum and allows the 

 clock to start. This rotates the clock housing 

 counterclockwise about its shaft. As it rotates, 

 the clock housing carries the stylus, which 

 provides a time mark. The excursion of the 

 piston in the piston housing moves the clock 

 housing and stylus, making a trace on the 

 depth-time scale of the recording paper. 

 A typical trace is shown in figure 4. 



10 



15 



Time (Minutes) 



20 25 



7/30/61 



Figure 4.--Typical trace obtained from a purse seine. 

 The abscissa is in minutes and the ordinate is depth 

 in feet. (Hester, 1961.) 



The strength of the spring and the diameter 

 of the piston were chosen to give a full-scale 

 deflection equivalent to approximately 100 

 meters in sea water. Combined errors give 

 a usable accuracy of ± 2 percent of this full- 

 scale deflection. These errors arise from 

 friction losses between the piston and the 

 walls of its housing, internal air temperature 

 changes, changes in water density, and free- 

 play losses including that incurred through the 

 necessity of unseating the clock housing from 

 its notch on the recording drum. The latter 

 error can be reduced by using knurling instead 

 of notches. The friction loss is minimized by 

 maintaining a high polish on the piston housing 

 bore and lubricating the "O" ring with a good 

 grade of silicon grease. A single "O" ring on 

 the piston was found to give a satisfactory 

 seal. 



COST AND CONSTRUCTION NOTES 



The dimensions of the device will vary with 

 the application. The model described can be 

 constructed for about $130. The cost of mate- 

 rials is low per unit. The total cost varies 

 more with labor than with construction mate- 

 rials. Corrosion resistant aluminum is satis- 

 factory for most of the parts. Machining is 

 minimized by having the case constructed of 

 pipe or tubing. The piston housing may be 

 cast, but this can result in a porous bore 

 which requires sleeving. 



The diameter of the piston and the strength 

 of the spring depend upon depth requirements. 

 Tiie spring should compress linearly with 

 depth over its useful range to facilitate cali- 

 bration. 



CALIBRATION 



Calibration of the BKG can easily be ac- 

 complished by replacing the protective cap 

 by a calibration cap and applying fluid under 

 measured pressure to the face of the piston. 



The clock is from a standard kitchen timer. 

 Timers are usually shrouded in some sort 

 of decorative shell which, when removed, 

 leaves the clock mechanism mounted by bolts 

 in a metal case. The clock housing can be 

 constructed by attaching the stylus to this 



