Page 38 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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CHAPTER 2 The Ocean Environment
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Salts dissolved in water change the density of the resultant seawater for these reasons:
The ions and molecules of the dissolved substances are of a higher density than water. Dissolved substances inhibit the clustering of water molecules (particularly near the freezing point), thus increasing the density and lowering the freezing point.
Unlike fresh water, ocean water continues to increase in density up to its freezing point of approximately 22C. At 0 PSU (i.e., fresh water), maximum density is approximately 4C with a freezing point of 0C. At 24.7 PSU and above, ocean water has a freezing point of its maximum density; therefore, there is no maximum density temperature above the freezing point. The maxi- mum density point scales in a linear fashion between 0 and 24.7 PSU (Figure 2.1).
Thus, ocean water continues to increase in density as it cools and sinks in the open ocean. As a result, the deepwaters of the world’s oceans are uniformly cold.
Comparisons of salinity and temperature effects upon water density yield the following:
• At a constant temperature, variation of the salinity from 0 to 40 PSU changes the density by about 0.035 specific gravity (or about 3.5% of the density for 0 PSU water at 4C).
• At a constant salinity (i.e., 0 PSU), raising the temperature from 4C (maximum density) to 30C (highest temperature generally found in surface water) yields a decrease in density of 0.0043 (1.0000.9957) for a change of 0.4%.
Clearly, salinity has a much higher effect upon water density than does temperature.
As a practical example, suppose a 220 pound (100 kg) ROV is ballasted for 4C fresh water at exactly neutral buoyancy. If that same submersible were to be transferred to salt water, an addi- tional ballasting weight of approximately 7.7 pounds (3.5 kg) would be required to maintain that
vehicle at neutral buoyancy.
4 3 2 1 0
–1 –2 –3 –4
FIGURE 2.1
Liquid water
Freezing point
Ice
Liquid water
Maximum density temperature
0 10 20 Salinity (PSU)
303435 40
Crossover point (24.7 PSU)
Average ocean water salinity
Salt water density, salinity versus temperature.
Temperature (°C)
