CTD Observation

Topic outline

• General

  Collapse all Expand all
  • 

    Japanese URL

• 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.
    
    

  • 
    

    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.