V. PRESSURE (DEPTH) MEASUREMENTS 

 Murray H. Schefer 



A. INTRODUCTION 



All oceanographic measurements must be positioned along three 

 coordinates: latitude, longitude, and depth. The latter, the measure- 

 ment of the distance to equipment located beneath the sea surface, 

 is discussed in this section. Measurement of the total depth of water 

 is discussed in Section XI. 



Although depth is a distance measurement, it has been found more 

 feasible to take advantage of the direct relationship between pressure 

 and depth by measuring pressure and converting it to a depth value. 

 The problem, therefore, is fundamentally one of pressure measure- 

 ment. Sea water pressure varies between 0.44 to 0.46 p.s.i. per foot 

 of depth, or roughly 0.5 p.s.i. per foot of depth. Much oceanographic 

 survey work is conducted in the range of zero to 5,000 p.s.i., or from 

 the surface to a depth of about 10,000 feet, but measurements to the 

 bottoms of the ocean deeps require equipment with a range to 15,000 

 p.s.i. Depth measuring equipment should have an accuracy of ± 2 

 percent. 



B. INSTRUMENTS 



The measurement of pressure, like temperature, is one of the most 

 fundamental in science. As a result, many diverse methods have been 

 devised to accomplish it. Pressure measuring instruments can be 

 classified in several ways. One system of classification describes 

 the instruments as either primary or secondary and depends upon the 

 principle employed. 



1. Primary pressure instruments 



These instruments are manometers and gauges which can be 

 calibrated without reference to another pressure measuring instru- 

 ment. The mercury barometer and dead weight scale are examples 

 of such primary pressure instruments. However, these types of 

 instruments generally do not lend themselves to incorporation into 

 over- the- side oceanographic equipment. 



2. Secondary pressure instruments 



All other pressure measuring instruments are included in this 



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