Before the advent of CTD probes and rosette samplers, most water sampling was done using simple water bottles such as the Nansen-Pettersson (Figure 3.5). Most such bottles take the form of an open-ended cylinder with spring-loaded valves for closing the ends. The sampler is lowered with the ends open so that water flows freely through it. When the required depth is reached, a release mechanism is operated by sending a slotted weight known as a messenger down the suspending wire, which causes the valves to snap shut closing the ends of the cylinder. The water bottle can then be hauled to the surface. Modern versions of such bottles are sometimes still used today and may be operated acoustically or electronically.
Before electrical resistance thermometers came into use, water samples and temperature measurements at deeper levels were often taken, using reversing bottles in conjunction with reversing thermometers. The reversing water-bottle is
Figure 3.5 The Nansen-Pettersson.
a cylindrical container attached to the lowering wire by a hinged frame, the cylinder lying above the hinge when the bottle is in the open position. The bottle is closed when a messenger strikes a release mechanism which causes the cylinder to swing downwards through 180 degrees until it lies below the hinge. As this occurs, valves automatically close the ends of the cylinder (Figure 3.5). The purpose of this reversing mechanism is to allow the use of a reversing thermometer mounted on the side of the bottle. Such thermometers are rarely used today and are not described further here. Detailed descriptions are given in earlier editions of this book.
Continuous records of temperature, depth and salinity can be obtained from moving ships throughout a range of depths from the surface to about 500 m by mounting electrical sensors such as CTDs on a towed undulator. These torpedo-like devices follow an undulant course controlled by movable vanes operated electrically from the ship. This saves considerable ship time since the ship no longer needs to stop and lower instruments. A popular model is the UK SeaSoar (Figure 3.6) which can be towed at 8 knots and can undulate from the surface to 500 m. As well as CTDs, the instrument package often includes pH, dissolved oxygen, and sound velocity meters. It can also be fitted with optical and acoustic sensors that measure colour and pigments in the water (indications of phytoplankton productivity), and zooplankton. Such packages can also be used
Moveable vanes controlling depth
Moveable vanes controlling depth
from stationary ships or attached to data-recording buoys (see below). Many smaller towed instruments have also been developed, some of which can be operated by non-specialists from merchant ships. One such is the Undulating Oceanographic Recorder which can operate down to 200 m.
A relatively inexpensive method of data collection over a range of depths from the surface is the use of expendable probes. First developed as a means of quickly and cheaply plotting temperature profiles from a ship moving at full speed, these probes can now also be fitted with conductivity sensors for simultaneous salinity profiles.
An expendable probe is a small, bomb-shaped object carrying the sensors in the nose connected to a spool of fine electric cable contained within the tail. This cable is joined to a similar spool of wire in a launching tube which may be hand-held or deck-mounted on the ship. The ship end of the cable is connected to a signal-processing unit which energizes the probe and interprets and records the signal. Without the vessel needing to reduce speed the probe is allowed to drop from the launching tube into the water. The two spools of wire simultaneously run out freely. This double system of unspooling allows the probe to fall vertically through the water, virtually unaffected by the movement of the ship (Figure 3.7). As the probe descends it transmits electrical signals to the processing and recording unit on the ship, giving continuous readings of temperature and salinity until the probe falls away when the cable has been fully extended. As the rate of descent is known, the profiles can be directly related to depth on the vertical scale of a chart recorder. These devices are usually constructed to work to depths of 750 m but are obtainable for depths to 2000 m.
The development of apparatus for data recording and telemetry has greatly extended the possibilities of long-term surveys by unattended instruments
-------Wire unspooling from probe as it descends
Figure 3.7 Expendable probe for obtaining temperature and salinity profiles from a moving ship.
suspended from moored buoys. This is much less expensive than the use of ships. Instruments can be attached at several depths along a cable below a buoy, including sensors for wave height, sea temperature, current speed and direction, salinity, pressure, pH and oxygen concentration. The surface float can be fitted with instruments for measuring atmospheric conditions such as air temperature, barometric pressure, wind speed and direction. All the collected measurements are recorded in a memory store within the buoy and can be transmitted periodically or on command by radio to shore stations.
Moored buoys floating on the surface have certain disadvantages for attachment of hydrographic instruments. Wave motion may cause misleading measurements for certain parameters, especially current speed. Heavy seas may lead to loss of the buoy and its instruments by breakage of the cable. Valuable equipment attached to a buoy may be stolen. These problems have prompted the development of 'pop-up' arrangements where the float is completely submerged, anchored below the surface to a weight on the bottom by a cable containing a release link. On receipt of a command signal, the release detaches from the anchor weight, allowing the float and attached instruments to float to the surface. Command releases operate on receiving a particular coded acoustic signal transmitted from the surface. The float is equipped with a radio beacon and light for easy retrieval.
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