206 



TELEMETERING UNDERWATER 



Fig. 79. Basic telemetry systems 



-SEA LAB ) 



( BERKONE AND ) 



PGH 



Fig. 80. Basic electrode construction; A - silver- 

 silver chloride (Beckman) electrode, B - tin (Sem- 

 Jacobsen) electrode 



Both types of electrodes were attached to the chest with waterproof tape; the silver-silver 

 chloride also employed double-backed tape washers and "Stomaseal Disks" to achieve water 

 tightness. The tin leads (developed by Dr. Sem-Jacobsen) were simply taped to the chest or 

 were glued on prior to going down into the Sealab habitat. Electrodes were placed as shown 

 in Fig. 81. 



Transmitter Details (Electrical) 



The basic block diagram of the transmitter is shown 

 in Fig. 82. Electrocardiographic signals from the elec- 

 trodes drive the input preamplifier, which provides 

 differential amplification of the signals from the two 

 chest leads. Variations in signal levels induced by 

 changing impedance of the skin at the electrode contacts 

 is minimized by the 3.3-megohm input impedance. In 

 addition, the preamplifier has a high value of common 

 mode rejection and a large common mode voltage- 

 handling capability to minimize requirements for good 

 contact of the ground electrode. 



Output from the preamplifier is capacitively coupled 

 to a single- ended amplifier with a gain of 50. The am- 

 plifier output, in turn, is capacitively coupled to the 

 base junctions of a multivibrator. Variations of the 

 base voltages shift the frequency of oscillation of the 

 multivibrator from its natural frequency of 25.0 kc/sec. 

 Frequency shifts of approximately 250 cps per millivolt 

 of EKG voltage at the electrodes are obtained. Output 

 from the multivibrator drives apush-pull amplifier which 

 drives the transducer. The resultant ultrasonic energy containing electrocardiographic infor- 

 mation propagates through the water to the receiver. 



Fig. 81. Placement of elec- 

 trodes on subject 



