several matched sets of 50 each are selected to be used 

 with the oceanographic chain. The groups of 50 are en- 

 capsulated with epoxy resin in a Lucite form of the proper 

 shape to fit the mounting holes in the thermistor chain 

 fairings. 



The completed probes are then recalibrated at 

 0°, 3°, 10°, 15°, 20°, 25°, and 30°C. The information is 

 used to determine the resistance padding to be used in the 

 bridge circuit of the chain recorder so as to bring all 

 circuits to the same resistance at a given temperature. 



Finally, the time constant of each probe is 

 determined. The lag is built in during the potting process 

 by allowing the compound to cover the end of each bead to 

 a uniform thickness (0. 010 inch). The time constants are 

 measured by using two baths, first stabilizing the beads in 

 a low -temperature bath, then plunging them into a high- 

 temperature bath. The lag time is defined as the time re- 

 quired to reach 63 percent of the temperature change. 



SOUND- VELOCITY METERS 



The calibration tank must be specially prepared 

 for the calibration of sound-velocity meters. It is first 

 drained of the alcohol-water mixture, scrubbed, and rinsed. 

 The hot and cold tanks are similarly purged. The tank 

 system is then filled with distilled water. The water is 

 circulated for several hours and kept in the tank overnight. 

 The system is again drained to flush any possible contami- 

 nant and refilled with distilled water. After these prepara- 

 tions, salt contamination never exceeds 15 parts per million, 

 which is well below the concentration that could affect sound 

 velocity values. 



The sound-velocity meters are calibrated, up to 

 three at a time, after their cables and brackets have been 

 scrubbed and rinsed in distilled water. When distilled 

 water is being circulated and cooled by the thermal system, 

 its lower temperature limit has to be set just above C to 

 prevent icing of the refrigeration coils. With a maximum 

 flow from the cold tank, a minimum temperature of 0. 8 C 

 can be maintained in the calibration tank. The calibration 

 begins at that point and continues from 1°C through 30. C 



27 



