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    CTD Observation

    • KEIRI Imai, KENJI Oguma, and KOUKI Sawada, Oshoro-maru Marine Science Department, School of Fisheries Sciences, Hokkaido University

       

      Water temperature and salinity in the ocean are regarded as the most important observation items. Measuring water temperature and salinity is essential not only to understand the distribution of water masses, their movement and mixing, and the habitats of marine organisms, but also to understand the role of the ocean in the material cycle and climate system.

      The only way to accurately determine water temperature is to measure it on site (in the ocean). In early oceanographic observations, a special mercury thermometer called a tipping thermometer was used to measure water temperature. It was used in conjunction with a water sampler with a tipping mechanism, which could be tipped over at any depth to cut the mercury column and record the water temperature at the site (Fig.1).


      On the other hand, salinity has been determined by quantifying the amount of chlorine contained in seawater collected with a water sampler by chemical analysis and converting it from the amount of chlorine based on the "principle of constant ratio," which states that the relative ratio of the major components of seawater is constant. Later, salinity was measured electrically, utilizing the fact that electrical conductivity changes in accordance with salinity.

    • ナンセン採水器と転倒温度計

      Fig.1. Water temperature measurement with a tipping thermometer

      転倒温度計　Tipping thermometer　ナンゼン採水器　Nanzen water sampler　主温度計　main thermometers

      副温度計　secondary thermometers　防圧型　protective pressure type　被圧型　exposed to pressure type

      転倒　tipping　死菅　dead pipe　切断点　cutting point　環状部　ringside

      a) A weight called a messenger is dropped along a wire and placed against the trigger-type fixture of the water sampler.
      b) When the trigger is actuated, the upper fixture is released.
      c) The thermometer falls over together with the water sampler.
      d) Structure of the tipping thermometer. When measuring, the mercury sphere should be down. When the thermometer is tipped over, the mercury bulb and the mercury column are cut off at the cutting point, and the length of the mercury column at the time of tipping over is preserved. Once recovered on board, the values of the main and secondary thermometers are read. The indicated value of the secondary thermometer, which is an ordinary mercury bar thermometer, is used to compensate for changes in the indicated value of the main thermometer due to differences in ambient temperature. By comparing the indicated values of the pressure-proof type, which is not affected by water pressure, and the pressure-sensitive type, which is affected by water pressure, the water pressure (water depth) at the time of tipping can be determined.

    • The method using a tipping thermometer and a water sampler could only provide information on a sparse layer from the ocean surface to the lowest level of observation. In recent years, with the development of sensor measurement technology and equipment such as winches and cranes necessary for observation, it has become possible to continuously and precisely measure the vertical distribution of water temperature and salinity. As shown in Figure 2, this has made it possible to obtain data on layers that were not available before and to elucidate the dynamics of seawater in greater detail. Today, underwater instruments equipped with conductivity, temperature, and depth sensors are called CTD (Conductivity-Temperature-Depth profiler) and are widely used as major oceanographic instruments.


    • CTD採水システム

      Fig.2. Vertical distribution map of water temperature obtained by tipping thermometer and CTD observation and CTD water sampling system

      水深depth　水温water temperature　転倒水温計tipping water thermometer


    • Currently, the CTD mainly used in CTD observations include the "wired type," which uses a coaxial cable for power supply and signal transmission, the "self-recording type," which operates on battery power and records measurements internally, and the "drop type," in which the sensor is disposed of after each observation. Figure 3 shows the characteristics of each type. The wired type can acquire measurement values in real time and can measure with the highest accuracy. However, because it is larger and heavier than other types, observation is easily affected by weather conditions and requires longer working hours. While the accuracy of the drop-type CTD is less than the others, it can be used in bad weather conditions. These features allow the use of different types of CTD depending on the purpose of the observation and the navigational conditions.




    • Fig. 3. Types and characteristics of CTD

      CTD採水システム　wired type　自己記録式CTD　self-recording type　投下型CTD　drop type

      精度　accuracy　高い　high　低い　low

      荒天の影響　affected by stormy weather　受けやすい　susceptible　受けにくい　less susceptible 

      観測作業時間　observation time　長い　long　短い　short

      観測方法　observation method　停船観測　observation at anchor　係留観測　moored observation

      航走観測　sailing observation　

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    Wired type CTD（CTD water sampling system)

    • A wired CTD consists of a receiver (onboard station) and a sensor (submersible station) connected by a coaxial cable, allowing real-time monitoring of measurements onboard the vessel. Although the sensor is sometimes used alone, it is often used in combination with a water sampling device as a "CTD water sampling system" to accurately collect seawater at the desired depth while checking the CTD's pressure sensor values. In addition, by comparing the measured values obtained by the sensor with the analyzed values of the collected seawater, it is possible to correct the sensor output values and confirm that there are no water sampling malfunctions or analysis errors.

      The size of a CTD water sampling system varies depending on the number and capacity of water samplers. The larger the system, the more personnel are required for observation work, and the more difficult it is to work safely in rough weather.

      Click here for details　→　CTD water sampling system



    • Fig. 4. CTD water sampling system


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    self-recording type CTD

    • The greatest feature of the self-recording CTD is that it becomes an independent device by installing a battery, and the acquired measurement values can be recorded in the main unit. In addition, as shown in Figure 5, the relatively small size and light weight of the self-recording CTD has the advantage that it can be operated by a small number of people even in rough weather to some extent. This feature can also be utilized for long-term observations by incorporating it into a submarine observation system called a mooring system.

      Click here for details　→　self-recording type CTD　

       



    • Fig. 5. self-recording type CTD


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    drop type CTD

    • A drop-type CTD, also called an XCTD (eXpendable CTD), measures water temperature and conductivity by dropping a disposable sensor (probe) into the ocean from a vessel underway. Since there is no need to stop the vessel and no time is required to retrieve the sensor, a wide range of observations can be made in a short period of time. Observations can also be made even in very rough weather.

      Click here for details　→　XCTD

       



    • Fig. 6. Schematic diagram of XCTD observation

      XCTDプローブ　XCTD probe


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