CTD rosette

Up to 1974, hydrographic data were collected with reversing water bottles and thermometers, and titration of chlorinity for salinity, using certified standard seawater as the reference. From 1974, temperature and salinity profiles were measured with CTD, but up to 1988, nutrient samples were still collected with reversing water bottles. From 1988, CTD-Rosette samplers have been used. An overview of the NERI devices, methods, and uncertainties is given in Table 11.8. 

Highest quality in CTD salinity can be obtained only if the CTD is operated together with a rosette sampler (Fig. 3-5) for in situ calibration (see Section 3.6.3 for the calibration procedure and Section 3.6.4 for data processing). In these cases, a one-conductor cable serves to transfer CTD data from the underwater unit to the deck unit and a personal computer, and to control bottle closures by the operator. Recently, some manufacturers started to offer combined CTD-rosette sampler systems that have internal CTD data storage and that close bottles at prescribed pressure levels. Closing the bottles is controlled using the CTD s pressure sensor signal. 

The CTD-rosette system is best operated from amidships to minimize effects of pitch and roll on the data. When all electrical connections work properly, the CTD should not be switched off throughout the cruise. 

To make near-bottom measurements one must know the distance to the bottom when the CTD-rosette approaehes the bottom, to avoid damage. Most frequently, the distance is measured independently with an acoustic pinger that is attached to the rosette frame. In... 

The logsheet is prepared. It contains a check list for preparation of the CTD-rosette that follows the manufacturer s recommendations and the operator s experience with individual systems. Make sure by intensive rinsing with fresh water that all mechanical parts of the rosette work properly. Data acquisition starts before the CTD is lowered into the sea as inair data are needed for later processing. 

Figure 6.14. Fluorescence sensor deployment platforms. (A) CTD Rosette vertical profiler. Water was pumped in series through a fluorometer and other environmental sensors, where instruments were mounted at the bottom of the Rosette and CDOM fluorometer. (Courtesy of R. Conmy.) (B) Towed vehicle with various optical and chemical sensors. (Courtesy of R.F. Chen.) (C) Minishuttle tow-yo vehicle deployed with chlorophyll and NOM fluorometers, dissolved oxygen sensor, and CTD. (Courtesy of R.F. Chen.) (D, E) Buoys, moorings, and gliders are also platforms for optical and environmental sensors. (Courtesy of Cefas.) (See Plate 10.)...

During the cruises in the 1950s and 1960s, oceanographic measurements were performed by means of reversing thermometers (Wolf, 1959) and Nansen bottles. Since 1974, the seawater was collected in a rosette of tube samplers combined with the CTD (Voigt et al., 1976 Seehase, 1980). Between 1951 and 1960/1961, currents were measured by means of Ekman-Merz current meters (Helm, 1968). 

Syiplinq, Discrete water column samples were obtained with 5-L Niskin bottles attached to a General Oceanics Rosette fitted with Plessey Environmental Systems model 9040 CTD. A 1-L aliquot of seawater was transferred from the Niskin samplers into glass-stoppered bottles in such a way that air bubbles were not trapped. The bottles were then stored in the dark at... 

Station salinity samples for analysis with a salinometer are taken from water samplers that are closed at desired depths or pressures. In many cases, a rosette sampler is used that carries Niskin or comparable water sampling bottles (see Chapter 1). Ihe bottles are mounted in one or more rings on a frame. Closing the bottles at desired pressure levels is triggered from the deck unit in the ship s laboratory. New s tems allow selection of bottles individually and not just in the order of their position on the frame. Most frequently, and necessary for WOCE standards, the rosette sampler is operated together with a CTD that is mounted on the rosette s frame. The CTD s on-line pressure information is then used for controlled closing of the water samplers. 

CTDs are designed for many different applications and requirements of accuracy. Attainable accuracies are 0.05 % for pressure, 2 mK for temperature and 0.002 in derived salinity on down profiles. Up-profiles have lower data quality since the sensors then are in the turbulent wake of the wire, the underwater unit and possibly a rosette water sampler. 

Rinse both the CTD and the rosette intensively with fresh water to avoid the formation of salt crystals that may falsify the tirst measurements on the next cast or block mechanical parts of the rosette. 

Accuracies achievable for measurements in the ocean are 0.002 mK for T, 0.05 % for P and 0.002 for S. Temperature and pressure sensors are calibrated in the laboratory. In situ comparison of the laboratory calibrations of these sensors only makes sense if additional sensors with digital output can be attached to the CTD. Die accuracy and stability of reversing thermometers and pressure sensors sometimes attached to rosette bottles is not suffi-... 

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