increasing pressure no change in the oscillator was observed. 



Thermistors : Thermistors, used for measuring temperature, 

 should be treated with a great degree of caution at high pressures. 

 Here they may be erratic; they may be in error by as much as 

 one-half degree centigrade per thousand p. s.i. in a temperature 

 bridge. 



Filling fluids : The temperature coefficient of expansion of fil- 

 ling fluids is very important. Many of the convenient hydrocar- 

 bons which nnay be used have a temperature coefficient of expan- 

 sion such that their volume changes about one cubic centimeter 

 per liter per degree centigrade. A word of caution here -- these 

 instruments should be designed to withstand not only the cold 

 ocean environment, but also the hot sunshine of the deck. The 

 bulk modulus (the unit volume change with unit pressure) of the 

 fluid is very important. Many of the fluids change only about 6 to 8 

 percent in volume when the pressure goes from 15 to 20, 000 p. s. i. 

 However, silicones, which are quite desirable electronically, 

 change about 10 percent under these conditions and should be 

 treated with caution or avoided if possible. Also, remember, 

 if you are considering silicones, to be careful because many peo- 

 ple are allergic to them. Fluid filling has a decided advantage 

 with respect to a heat exchange. The fluid makes possible the 

 ready conduction of heat by convection from the components 

 to the world's best heat sink, the ocean. 



No talk of this sort would be complete without mention of one 

 of the early oceanographic instruments. This is the well-known 

 BATHYTHERMOGRAPH or BT (fig. 6.4). This instrximent has 

 performed very well on the whole. Its record is made on a small 

 smoked slide which you can see being stuck into the instrument 

 at the side. Benjamin Franklin's thermometer was dumb and 

 could not write; the bathythernnograph, we may say, has learned 

 to write, albeit it is still dumb. 



The NIGHT SECCHI DISC (fig. 6. 5) is a technique or device 

 for measuring the trajisparency of sea water at night. This is not 

 normally an easy thing to do, but this technique has proved to be 

 quite accurate, very sinnple, and inexpensive, costing, I suspect, 

 probably not more than $. 50 including labor and overhead. It is 

 a single flashlight cell with a bulb soldered on top. In use it is 



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