CTD (Conductivity, Temperature, Depth)

CTD (Conductivity, Temperature, Depth)

A CTD – an acronym for conductivity, temperature, and depth – is the primary instrument used to determine the essential physical properties of seawater. It provides scientists with an accurate and comprehensive representation of the distribution and variation of water temperature, salinity, and density to understand how the oceans affect life.

How it works.

The CTD on board consists of a set of small probes attached to a large metal rosette wheel. The rosette is sunk to the seafloor via a cable, and scientists monitor water properties in real time via a data cable that connects the CTD to a computer on the ship. A remote-controlled device allows the water bottles to be selectively closed during the ascent of the instrument. A standard CTD takes between two and five hours to collect a complete data set, depending on water depth. Water samples are often collected at specific depths so scientists can learn about the physical properties of the water column at that particular location and time.

Small, low-power CTD sensors are also used in autonomous instruments:

A moored profiler makes repeated measurements of ocean currents and water properties up and down through almost the entire water column, even in very deep water. The basic instruments it carries are a CTD for temperature and salinity and an ACM (acoustic current meter) to measure currents, but other instruments can be added, including bio-optical and chemical sensors.

Spray Gliders roam the ocean independently, running pre-programmed routes and surfacing occasionally to transmit collected data and accept new commands. As they cruise horizontally through the ocean, internal bladders control their buoyancy, enabling them to navigate up and down through the water column like whales and other marine animals.

Floats are floating robots that take profiles or vertical series of measurements (e.g., temperature and salinity) in the oceans.

Autonomous Underwater Vehicles (AUV’s) are programmable, robotic vehicles that, depending on their design, can drift, drive, or glide through the ocean without real-time control by human operators. Some AUVs communicate with operators periodically or continuously through satellite signals or underwater acoustic beacons to permit some level of control.

What platforms are needed?
A variety of other accessories and instruments may be included with the CTD package. These include Niskin bottles that collect water samples at various depths to measure chemical properties, acoustic Doppler current profilers (ADCP) that measure horizontal velocity, and oxygen sensors that measure dissolved oxygen levels in the water.

Features of the CTD’s sensors

  • Saltwater resistant
  • High accuracy
  • Lightweight
  • Low power consumption
  • Will be used at depths up to several thousand meters

Comments:
The small low power CTD sensors used on autonomous instruments such as water column profilers, spray gliders, floats and AUV’s are more complex to operate. The main limitation is the need to calibrate the individual sensors. This is especially true for autonomous instruments that are deployed for extended periods of time. (Ship CTDs are referenced to water sample data, which is generally not available for autonomous instrument deployments). Therefore, sensors must be stable for the deployment period, or assumptions must be made about seawater properties and referenced to the data. Deep water properties are typically very stable, so autonomous sensor data are matched to historical water properties at depth.

STS provides high precision pressure cells for this specific application.

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Hydrogen effect on piezo transducers (bio fouling)

Hydrogen effect on piezo transducers (bio fouling)

BIOFOULING

Biofouling or biological fouling is the accumulation of microorganisms, plants, algae or animals on wetted surfaces, devices such as water inlets, pipework, grates, ponds and of course on measuring instruments, causing degradation to the primary purpose of those items.

ANTIFOULING

Antifouling is the process of removing or preventing these accumulations from forming. There are different solutions to reduce / prevent fouling processes at the ship hulls and in sea or brackish water tanks.

Special toxic coatings that kill the biofouling organisms; with the new EU Biocide directive many coatings were forbidden due to environment safety reasons.

  • Non-toxic anti-sticking coatings that prevent attachment of microorganisms on the surfaces. These coatings are usually based on organic polymers. They rely on low friction and low surface energies.
  • Ultrasonic antifouling. Ultrasonic transducers may be mounted in or around the hull on small to medium-sized boats. The systems are based on technology proven to control algae blooms.
  • Pulsed laser irradiation. Plasma pulse technology is effective against zebra mussels and works by stunning or killing the organisms with microsecond duration, energizing of the water with high voltage pulses.
  • Antifouling via electrolysis
  • Organisms cannot survive in a copper ions environment.
  • Copper ions occur by electrolysis with a copper anode.
  • In most of the cases, the tank housing or the ship hull serves as cathode.
  • A copper anode installed in the configuration generates an electrolysis between the anode and the cathode.

Electrolysis can appear due to ballast water treatment systems (electrolysis and UV-sytems), corrosion processes or differences of electric potential between different materials.

EFFECT OF ELECTROLYSIS ON THE PIEZO RESISTIVE TRANSDUCER

  • A result of the electrolysis are positive hydrogen ions
  • Because of their polarization, the hydrogen ions move towards the cathode (tank housing or ship hull) where the transducer is installed.
  • In case of direct contact between tank and transducer, the hydrogen ions will permeate through the thinnest component of the anode, which is the diaphragm of the transducer.
  • After permeation of hydrogen ions through the diaphragm, the hydrogen ions grab an electron and transform into molecular hydrogen (H2). The hydrogen accumulates in the fill fluid of the transducer.
  • If this effect lasts for a longer period, the concentration of hydrogen in the fill fluid will increase and the diaphragm will be bloated. As a result, the sensor drifts and issues an incorrect value.

FINDINGS

Stainless steel pressure transmitters used during 2-3 years in ballast tanks of ships were investigated by the Swiss Federal Laboratories for Materials Science and Technology in Zurich.

Findings

The formation of deposits on stainless steel membranes cannot be prevented in practice. As long as corrosion processes can take place on the membrane under anaerobic conditions, the formation of hydrogen and its penetration into the sensor must always be expected.

For this reason, under such conditions, the membrane should be made of a more corrosion-resistant material such as titanium.

Gap corrosion occurs on metal parts in presence of a corrosive medium in narrow, unsealed gaps such as overlaps and in welds that are not through-welded. The driving force is concentration differences between the medium in the gap and the area outside the gap, which are caused by the inhibited diffusion of the reactants in the gap. The potential difference associated with the concentration difference leads to electrochemical corrosion in the gap (hydrogen type) or its immediate surroundings (oxygen type).

For this reason, the membran should be welded to the housing.

RECOMMENDATION

According to this findings, STS Sensor Technik Sirnach AG has been successfully using piezo-resistive elastomer-free sensors with housing and membrane in titanium for applications in marine, brackish water and sea water applications for over 10 years.

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