signal to an indicator unit. These vehicles 

 use a pressure transducer as the primary 

 depth sensor, and the strain gages are used 

 as both a backup and a check on the pressure 

 transducer. If a critical difference develops 

 between the strain gages and the transducer 

 a "difference" indicator lights to alert the 

 operator. The pressure transducer is manu- 

 factured by Electric Boat/General Dynamics 

 and has a range to 7,500 feet with an accu- 

 racy of 0.45 percent of full scale. Its readings 

 are displayed on a nixie tube (numerical 

 indicating), but can be switched to a dial 

 indicator if the nixie display fails. The trans- 

 ducer output is also differentiated to an as- 

 cent-descent rate amplifier and displayed on 

 a meter in the depth indicator unit. 



Measurement of vehicle depth (or, con- 

 versely, of height off of the bottom) as a 

 function of water pressure is accompanied by 

 a host of variables which work to reduce the 

 measurement accuracy. Whether deployed in 

 air or water, there are the inherent inaccur- 

 acies encountered in the instruments them- 

 selves, their design and materials of con- 

 struction. Their use in the ocean environ- 

 ment introduces dynamic conditions not gen- 

 erally found ashore in land-based applica- 

 tions. Hydrostatic pressure at a given depth, 

 for example, is affected by gravity, water 

 density (which varies with temperature, sal- 

 inity and compressibility — the last being a 

 factor mainly at the greater depths) and 

 atmospheric pressure variations. 



Since atmospheric pressure seldom varies 

 by more than 1 inch of mercury except under 

 extreme storm conditions, its effect on depth 

 readings is not likely to exceed 1 foot. Below 

 depths of 100 to 200 feet this variation is lost 

 in instrument error. The effects of waves are 

 not felt below a depth of about two-thirds of 

 the significant wavelength — the effective hy- 

 drostatic pressure being averaged to mean 

 sea level below that depth. The error caused 

 by variations in density is random but in 

 most submersible diving situations is small 

 in comparison to the gravity variation. Grav- 

 ity variations produce measurable humps or 

 depressions in local sea level. These may be 

 static (permanent), the result of local density 

 anomalies in the solid-earth structure, or 

 they may be cyclical, the result of tidal influ- 

 ences. In the open ocean, however, even 



these are measured in centimeters, not me- 

 ters. 



Nevertheless, density variations are a con- 

 sideration, and Woods Hole's N. P. Fofonoff 

 has constructed a standard specific volume 

 profile (the reciprocal of density) which pro- 

 vides corrections by geographic location at 

 various depths within the world's oceans. 

 Contrary to popular belief, water is com- 

 pressible — though much less so than most 

 other liquids. At the tremendous pressures 

 encountered in the deep ocean, compressibil- 

 ity (which increases local water density) can 

 produce a considerable error in depth by any 

 type of pressure transducer. 



In addition to gravity, ocean currents — 

 whether driven by the wind or Coriolis 

 force — can and do produce variations in local 

 sea level over considerable areas. Although 

 this does not affect the depth sensor's meas- 

 urement of how deep the submersible is be- 

 neath the local sea surface, unless provision 

 is made for this anomaly, calculations of how 

 far the vehicle is above the bottom will be in 

 error. Again, considering basic instrument 

 error and all the other factors which affect 

 depth readings, these differences (particu- 

 larly at the greater submersible depths) are 

 not significant — except under the most ex- 

 acting physical oceanographic requirements, 

 as in geostrophic flow and dynamic sea-sur- 

 face topography research, for example. 



From a safety standpoint the errors intro- 

 duced from the above factors do not prohibit 

 use of pressure/depth gages in submersibles, 

 because the errors are small in comparison 

 to the general safety factor of 1.5 built into 

 pressure-resistant components. An excellent 

 example of this pressure versus depth differ- 

 ence is related by J. Piccard in Seven Miles 

 Down. TRIESTE'S depth at the bottom of the 

 Challenger Deep was measured at 37,800 feet 

 on a pressure/depth gage calibrated in fresh 

 water; subsequent corrections for specific 

 volume, gravity, compressibility and temper- 

 ature reduced this to 35,800 feet. An inaccu- 

 racy of 2,000 feet in depth was, from a safety 

 standpoint, no real concern because TRI- 

 ESTE'S pressure hull had a safety factor of 

 2 which would take it to a computed collapse 

 depth of 72,000 feet. 



The greatest area for concern is evidenced 

 by the scientist or engineer who wants the 



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