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MECHANICAL AND ACOUSTICAL SENSES 



two-stage units, having one thin-walled bellows for the shallow depths 

 (0-20 m) and one thick-walled bellows for the greater depths (0-200 m). These 

 may be operated on separate transmitter channels for maximum resolution 

 or connected in series for one-channel operation (Figure 13). 



Swimming Speed— Swimming speed is sensed by measuring the rate 

 of water flow past the externally attached transmitter. Water force against a 

 wand or drogue is converted to resistance change by a vane-photocell ar- 

 rangement (Figures 12 and 14). One low-friction version operates by mag- 

 netic coupling of the external wand to an internal vane on a jewel-mounted 

 pin. The wand-type sensor is mounted forward on the transmitter housing so 

 that the wand encounters relatively smooth, turbulence-free water flow. 



An indirect way to measure swimming speed is to relate it to tail-beat 

 frequency. This can be measured by sensing lateral accelerations, such as in 

 the tail-beat sensor described by Holland et al. (1974). 



Swimming Direction— The instantaneous compass direction (azi- 

 muth) of the fish's axis at any moment can be sensed by a miniature 

 magnetic compass sensor, one version of which is shown in Figure 12. A 

 more recent version (Figure 15) uses a circular transparency gradient at- 

 tached to the compass magnet and gives unambiguous azimuth readings by 

 comparison of the resistances of two photocells mounted 90° apart, each 



END VIEWS 



Figure 13 Depth sensors incorporating two partially oil- 

 filled metal bellows. Designed to be impervious to over- 

 pressure and to have relatively greater resolution at shallower 

 than at deeper depths: (A) Twin-sensor unit for use on two 

 separate transmitter channels. (B) Single-sensor unit for one 

 transmitter channel. 



a. Thin-walled bellows (e.g., 90% compressed at 15 m) 



b. Thick-walled bellows (e.g., 90% compressed at 150 m) 



c. Opaque vane 



d. Photoresistive cell 



e. Light source (LED, betalight) 



f. Oil-filled chamber at ambient pressure (capped with 

 opaque, compliant diaphragm) 



g. Thin-walled tube (not pressure resistant). 



