through is 



R^ + R B 



(11) 



where Rg is the bridge resistance seen "by the 

 detector and R,j the detector impedance. An addi- 

 tional nonlinearity accrues from the unbalanced 

 bridge which is minimized by keeping the per- 

 centage of resistance change in the bridge rela- 

 tively low and the amplifier impedance high. 



Bridge output changes proportionally in the 

 unbalanced bridge with bridge voltage supply. 

 However, the load is relatively fixed and tem- 

 perature changes are small so there is little 

 difficulty in zener regulating the bridge supply 

 to 0.015$. Manganin bridge resistors are used, 

 suitably aged. Care must be exercised in avoiding 

 thermocouples at junctions. (Manganin has an 

 output of 3 microvolts/°C with respect to copper.) 



The bridge is mounted in a cylindrical pres- 

 sure housing approximately 1.75 inches in diameter 

 by 3 inches long, made of steel or high strength 

 bronze. One end houses a conventional ^-conduc- 

 tor underwater electrical connector. Various 

 mounting arrangements are furnished for attaching 

 the probes to cables or structures . Sealing is 

 accomplished with "0" rings. A mechanical shield 

 for the sensitive element is insulated from the 

 housing. 



FOULING 



Where thermal probes are to be immersed for 

 long periods in relatively shallow water, marine 

 fouling of the element will increase the time 

 response sharply. As an example, a coat of paint 

 on an element of the type described delays 

 response time about 25$. We have noted weed for- 

 mations several inches long and barnacles 3/8 

 inch in diameter after 3 weeks of unprotected 

 immersion in tropical waters. The element shield 

 therefore is made of nonfouling sintered material 

 impregnated with anti-fouling agents. We find 

 that this arrangement, with the relatively high 

 and constant toxic dispersion mechanism, holds an 

 area several inches adjacent to it clear of marine 

 life except for some of the bacterial slimes . 

 These vary in thickness and tenacity and the anti- 

 fouling agents have some effect on them but at 

 this time we have no definitive information as 

 to their thermal conductivity. It is believed 

 that slime acts as an additional boundary layer. 



SIGNAL CONDITIONING 



To convert the relatively low output of the 

 probe to a signal suitable for feeding a data 

 system a multiple stage second harmonic magnetic 

 amplifier (Fig. 5) has been designed. This has 

 the advantage of being hermetically sealed and 

 not subject to maintenance so that its precision 



Fig. 5- Amplifier - power supply assemblies. 



is not a matter of careful adjustment . These 

 second harmonic amplifiers have a null shift into 

 the bridge impedance of the order of 10 micro- 

 volts referred to the input over a k0° to l i 4-0°F 

 temperature span and a -10$ supply voltage varia- 

 tion. A feedback factor of several hundred sta- 

 bilizes the gain and linearizes the output, 

 usually to 5 volts DC. The same amplifier 

 housing furnishes an individual DC bridge supply, 

 an on-off switch and circuit protection. 



A SPECIAL APPLICATION 



An interesting application of the probe sug- 

 gests itself for dynamic height measurements . If 

 one probe uses the pressure protective sheath 

 and another a compliant sheath, we have the equiv- 

 alent of protected and unprotected reversing ther- 

 mometers of the same or perhaps improved accuracy. 

 It is practical, in this instance, to use a 2- 

 eonductor cable carrying DC power to the assembly 

 and 2 frequencies proportional to temperature to 

 the surface . All of the hardware is available 

 and could be packaged in a cavity about 2 inches 

 in diameter and k- inches long. The addition of 

 an in situ salinometer to this package would take 

 over some Nansen bottle functions and the com- 

 bination could make rather rapid deep stations 

 possible. 



ke 



