Water samples from different depths are often required both by physical oceanographers and by biological oceanographers. Information from the analysis of such samples, along with temperature and salinity measurements, not only provides direct information about a water body, but has also been used to obtain indirect measurements of deep water circulation. Such ocean currents are difficult to measure directly, especially the slow movements of water at deep levels. The waters of different parts of the oceans are to some extent distinguishable by virtue of their physical and chemical characteristics, in particular their temperature, salinity, and content of oxygen, nitrate or phosphate. By studying the distribution of these quantities throughout the oceans, the movements of the water can be inferred. Temperature measurements and the analysis of water samples therefore provide much of the basic data of oceanography (Strickland and Parsons, 1968). The foundations of this science were laid during the voyage of HMS Challenger, 1872-76, when a large amount of this information was first collected from a series of depths at each of some 360 stations spread over the major oceans (Linklater, 1972).
CTD instrument packages
Temperature readings and water samples are usually taken together. Most water sampling from research vessels now utilizes modern electronic instruments known as CTD (conductivity-temperature-depth) probes developed in the 1970s. These are usually mounted on rosette water samplers. Temperature is measured through electrical resistance with an accuracy greater than that of mercury thermometers (0.002°C). Development of electrical resistance thermometers was made possible with the advent of the transistor and integrated circuit and such thermometers were first used in the early 1960s, although at this time they were not very accurate.
Salinity, to an accuracy of 0.005 units, is calculated from measurements of the electrical conductivity and temperature of the water using a salinometer. The data return to the ship as an electronic signal and are interpreted by on-board computers or connected to graphic or digital display units. However, the conductivity measurements need to be calibrated against water samples which is why CTDs are usually used as part of an instrument package which includes a water sampler. Rosette multisamplers can have as many as 24 bottles of 2, 10 or even 20 litre capacity which collect water at the required depths, when triggered electronically from the surface. Water samples are also necessary for chemical analysis of water. Small, hand-held salinometers on long cables are also available for use in shallow water from small boats.
CTDs can be lowered from stationary ships to provide continuous data from the surface down to about 6000 m. They can also be left in place if attached to a fixed buoy, to record changes in salinity and temperature over time. The data can be stored in the instrument or transmitted to a base via a satellite. Fluorometers (chlorophyll content) and oxygen electrodes are also routinely attached to the instrument package.
Oceanographic work requires extremely accurate thermometers because small variations in temperature produce considerable changes in water density. Modern CTD instruments have greatly increased the precision, speed and accuracy of measurements. As already mentioned, water samples are still needed to calibrate salinity measurements from these instruments. Precision thermometers are also used to check the accuracy of the CTD instruments. These usually have platimum resistance sensors and an LCD display rather than mercury in glass.
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