
CTD stands for ''conductivity'', ''temperature'', and ''depth''. A ''CTD'' instrument is an
oceanography
Oceanography (), also known as oceanology, sea science, ocean science, and marine science, is the scientific study of the ocean, including its physics, chemistry, biology, and geology.
It is an Earth science, which covers a wide range of to ...
''sonde'' (French for probe) used to measure the
electrical conductivity
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
,
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
, and
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
of
seawater
Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...
. The ''pressure'' is closely related to ''depth''. ''Conductivity'' is used to determine
salinity
Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
.
The CTD may be incorporated into an array of
Niskin bottles referred to as a carousel or
rosette. The sampling bottles close at predefined depths, triggered either manually or by a computer, and the water samples may subsequently be analyzed further for biological and chemical parameters.
The CTD may also be used for the
calibration
In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...
of sensors.
Measured properties
The instrument is a cluster of sensors which measure conductivity, temperature, and pressure. Sensors commonly scan at a rate of 24 Hz. Depth measurements are derived from measurement of
hydrostatic pressure
Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...
, and salinity is measured from
electrical conductivity
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
. Sensors are arranged inside a metal or resin housing, the material used for the housing determining the depth to which the CTD can be lowered. Titanium housings allow sampling to depths in excess of .
Other sensors may be added to the cluster, including some that measure chemical or biological parameters, such as
dissolved oxygen
Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...
and
chlorophyll fluorescence
Chlorophyll fluorescence is light re-emitted by chlorophyll molecules during return from Excited state, excited to non-excited states. It is used as an indicator of photosynthetic energy conversion in plants, algae and bacteria. Excited chlorop ...
, the latter an indication of the concentration of microscopic photosynthetic organisms (
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
) contained in the water.
Deployment

Deployment of the rosette is from the deck of a research vessel. The instrument is lowered into the water in what is called the downcast to a determined depth or to a few meters above the ocean floor, generally at a rate of about 0.5 m/s. Most of the time a conducting wire cable is attached to the CTD frame connecting the CTD to an onboard computer, and allows instantaneous uploading and real time visualization of the collected data on the computer screen. The
water column
The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...
profile of the downcast is often used to determine the depths at which the rosette will be stopped on its way back to the surface (the upcast) to collect the water samples using the attached bottles.
Technological development
The CTD system was conceived by Neil Brown at the
CSIRO
The Commonwealth Scientific and Industrial Research Organisation (CSIRO) is an Australian Government agency that is responsible for scientific research and its commercial and industrial applications.
CSIRO works with leading organisations arou ...
Division of Marine Research. Lack of interest by the management saw Brown move to Woods Hole Oceanographic Institution. The CTD overcame the limitations of an earlier similar system also developed by Brown, called an STD. The improvement was made possible thanks to increased reliability and reduced cost of computer technology.
[Baker 1981, pp 416-418] Prior to this, the Mechanical Bathythermograph (MBT) was the norm
Advantages and limitations
The advantage of CTD casts is the acquisition of high-resolution data. The limitation of CTD sampling is that only a point in space (the sampling site) can be sampled at one time, and many casts, which are costly and time-consuming, are needed to acquire a broad picture of the marine environment of interest.
From the information gathered during CTD casts, however, scientists can investigate how physical parameters are related, for example, to the observed distribution and variation of organisms that live in the ocean, thus deepening our understanding of the processes that govern ocean life.
Gallery
File:AOML CTD.ogg, A video of a National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with Weather forecasting, forecasting weather, monitoring oceanic and atmospheric conditions, Hydrography, charting the seas, ...
CTD that is used in the Atlantic Ocean.
References
Sources
* Baker D. J. 1981
Ocean instruments and experiment design Chapter 14 pp 416–418
* Pickard George L. and William J. Emery "Descriptive Physical Oceanography, An introduction" 5th ed., Butterworth-Heineman (
Elsevier Science
Elsevier ( ) is a Dutch academic publishing company specializing in scientific, technical, and medical content. Its products include journals such as ''The Lancet'', '' Cell'', the ScienceDirect collection of electronic journals, '' Trends'', t ...
): 1990
Modern oceanographic CTD systemsHistory of the development of CTD systems
Footnotes
{{Commons category, CTD
Oceanographic instrumentation
Water and the environment
Limnology
Hydrography