Page 40 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

28 CHAPTER 2 The Ocean Environment
Fresh water has a maximum density at approximately 4C (Figure 2.1) yet ocean water has no maximum density above the freezing point. As a result, lakes and rivers behave differently at the freezing point than ocean water. As the weather cools with the approach of winter, the surface water of a fresh water lake is cooled and its density is increased. Surface water sinks and displaces bottom water upward to be cooled in turn. This convection process is called “overturning.” This overturning process continues until the maximum density is achieved, thus stopping the convection churning process at 4C. As the lake continues to cool, the crystal structure in the water forms, thus allowing the cooler water at the surface to decrease in density, driving still further the cooler (less dense) surface water upward, allowing ice to form at the surface.
2.2.2 Pressure
The SI unit for pressure is the kilopascal (expressed as kPa). One pascal is equal to one newton per square meter. However, oceanographers normally use ocean pressure with reference to sea-level atmospheric pressure. The imperial unit is one atmosphere. The SI unit is the bar. The decibar is a useful measure of water pressure and is equal to 1/10 bar. Seawater generally increases by one atmosphere of pressure for every 33 ft of depth (approximately equaling 10 m). Therefore, one deci- bar is approximately equal to one meter of depth in seawater (Figure 2.3).
As an ROV pilot, seawater pressure directly affects all aspects of submersible operation. The design of the submersible’s air-filled components must withstand the pressures of depth, the flota- tion must stand up to the pressure without significant deformation (thus losing buoyancy and sink- ing the vehicle), and tethers must be sturdy enough to withstand the depth while maintaining their neutral buoyancy.
2.2.3 Compressibility
For the purposes of ROV operations, seawater is essentially incompressible. There is a slight com- pressibility factor, however, that does directly affect the propagation of sound through water. This compressibility factor will affect the sonic velocity computations at varying depths (see Section 2.2.8 later in this chapter).
2.2.4 Conductivity
Conductivity is the measure of electrical current flow potential through a solution. In addition, because conductivity and ion concentration are highly correlated, conductivity measurements are used to calculate ion concentration in solutions. Commercial and military operators observe con- ductivity for gauging water density (for vehicle ballasting and such) and for determining sonic velocity profiles (for acoustic positioning and sonar use). Water quality researchers take conductiv- ity readings to determine the purity of water, to watch for sudden changes in natural or wastewater, and to determine how the water sample will react in other chemical analyses. In each of these applications, water quality researchers count on conductivity sensors and computer software to sense environmental waters, log and analyze data, and present this data.