CTD | Flanders Marine Institute

Flanders Marine Institute

Platform for marine research


CTD is the acronym for an oceanographic device consisting of sensors that measure the conductivity, temperature and depth. Despite its name, a CTD does not measure the depth but the pressure which is related to the depth. VLIZ added a number of additional sensors that measure the following chemical/biological parameters: photosynthetically active radiation (PAR), dissolved oxygen, turbidity, chlorofyla, acidity and oxidation reduction potential (ORP). Studying these seawater parameters is important because they have a direct influence on the marine life. The CTD is frequently integrated in a carrousel containing Niskin bottles and is deployed from a research vessel.

How does a CTD works?

The CTD is lowered into the water from a research vessel by means of a guiding cable that connects the CTD with an onboard computer. This provides the scientists with an immediate visualization of the data. Scientists can closely monitor the data and can send signals to the carrousel via the cable when they want to close a Niskin bottle (which can be put on a carrousel with the CTD) to obtain a water sample.


Usefulness of a CTD?

  • Relatively easy to operate; possibility to add other sensors; frequently used in combination with the collection of water samples (i.e. carrousel containing Niskin bottles)
  • It gives scientists the possibility to map the distribution and variations of the base parameters of the seawater
  • Very accurate and relatively light instrument
  • If the pressure-depth relation is known, CTD data can give insight into depth, thus allowing for water samples to be taken at specific depths.

Some studies:

  • Influence of the oceanographic characteristics on the behaviour the sand goby Pomatoschistus minutus on the Belgian Continental Shelf:

Metapopulations of marine fishes: a case study of the sand goby Pomatoschistus minutus on the Belgian Continental Shelf

  • Research on toxic algae in combination with oxygen measurements among others

During an algal bloom, algae can disturb the ecosystem by producing toxins that affect other biota. On the other hand, algal blooms can cause a massive reduction in the oxygen concentration causing hypoxia or even anoxia (see photos)

Technical details

VLIZ has two types of CTD at the disposal:

  • SBE 19plus V2 SeaCAT Profiler CTD:
    • Measures conductivity, temperature and pressure at 4 scans/sec (4 Hz)
    • Resolution:
      • Conductivity: 0,00005 S/m
      • Temperature: 0,0001 °C
      • Pressure: strain gauge 0,002% of full scale range; Quartz 0,0025% of full scale range
    • More technical specifications: click here
  • SBE 25plus Sealogger CTD:
    • Measures conductivity, temperature and pressure at 16 Hz
    • Resolution:
      • Conductivity: 0,00004 S/m
      • Temperature: 0,0003°C
      • Pressure: 0,002% of full scale range
    • More technical details: click here

The CTDs can be equipped with sensors that measure the following parameters:

  • Photosynthetically active radiation (PAR)
  • Dissolved oxygen & oxidation reduction potential (ORP)
  • Turbidity
  • Chlorophyla

How to make use of the CTD?

For more information about our rental service, please contact VLIZ.

Use and conditions

Note: if you use the CTD of VLIZ as a scientist, please refer to it in your publications as: 'This work was supported by data & infrastructure provided by the Flanders Marine Institute (VLIZ)'.