Metadata Report for BODC Series Reference Number 671462

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Quality report - D253 Pascal Oxygen sensor data

From visualising the data it is clear that the Pascal Oxygen sensor did not perform well during this cruise. This channel was not calibrated against bottle samples and from the values they appear either far too low or far too high. Therefore all the data from this Oxygen sensor have been flagged 'M' by BODC and consequently the derived Oxygen saturation channel has been flagged too.

CTD casts that included a Pascal Oxygen sensor -

• CTD stations 13965 to 14026 used the Neil Brown Mk3 CTD
• CTD stations 14064 to 14130 used the Neil Brown Mk3 CTD

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

National Marine Facilities Microelectrode Oxygen Sensor

The Microelectrode Oxygen Sensor is a non-membrane dissolved oxygen sensor developed by the Ocean Engineering Division of National Marine Facilities (NMF) at the National Oceanography Centre, and the Electro-Chemistry Group at the University of Southampton Chemistry Department. The sensor is suitable for marine applications. It was first used in 2000 and development is ongoing.

The fundamental parts of the sensor are a platinum microelectrode and a counter electrode (thus far composed of copper). A measurement potential is applied to the microelectrode relative to the counter electrode. This leads to reduction of dissolved oxygen at the microelectrode and produces a current proportional to the number of oxygen molecules at that electrode.

The sensor is suitable for operation on a CTD system as it is free from the pressure effects (e.g. slow response, drift and hysteresis) typically encountered when using oxygen sensors that employ a membrane. Its short response time (approximately 1 second) also makes it suitable for deployment on CTDs and oceanographic undulators. The sensor includes both analogue voltage and serial digital (RS 232) outputs.

The electrode surface is reconditioned by applying a cleaning potential to the microelectrode at which oxidation occurs. This minimises drift and the effects of bio-fouling.

Ongoing development

Early versions of the instrument applied the measurement potential for up to 30 seconds, with sampling starting approximately one second after the potential was applied. The sensor response to oxygen remained almost constant for a given concentration and data could be sampled at frequencies exceeding 1 Hz.

However, the sensor proved to be very sensitive to fluctuations in flow, leading to apparent noise in the data. In an attempt to address this, various changes were made to the functionality:

• Initially, the instrument used a mesoporous layer to increase the surface area of the microelectrode, but this proved unstable.

• The mesoporous layer was replaced by a stop-flow system which pumped water in to a very small chamber, with measurements being taken while the water was still. This significantly improved results but the sensor was still affected by small convection currents.

• The current version of the sensor only applies the measurement potential for a few 10s of milliseconds and data are collected at approximately 1 Hz in an attempt to minimise the effects of convection currents.

• The latest development is a very small sensor head containing an array of microelectrodes (currently an array of five) that work together. The counter electrode consists of silver or silver chloride. This is currently undergoing trials and is yet to be deployed on a CTD profiler.

Instrument Description

CTD Unit and Auxiliary Sensors

During this cruise, two separate CTD rigs with different sensors were deployed for different sections of the line.
CTD stations 13965 to 14026 used the Neil Brown Mk3 CTD and are called subaccession A
CTD stations 14027 to 14062 used the Sea-Bird 911 CTD are called subaccession B
CTD stations 14064 to 14130 used the Neil Brown Mk3 CTD and are called subaccession A

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

Specifications

These specification apply to the MK3C version.

Further details can be found in the specification sheet.

Chelsea Technologies Group Aquatracka MKIII fluorometer

The Chelsea Technologies Group Aquatracka MKIII is a logarithmic response fluorometer. Filters are available to enable the instrument to measure chlorophyll, rhodamine, fluorescein and turbidity.

It uses a pulsed (5.5 Hz) xenon light source discharging along two signal paths to eliminate variations in the flashlamp intensity. The reference path measures the intensity of the light source whilst the signal path measures the intensity of the light emitted from the specimen under test. The reference signal and the emitted light signals are then applied to a ratiometric circuit. In this circuit, the ratio of returned signal to reference signal is computed and scaled logarithmically to achieve a wide dynamic range. The logarithmic conversion accuracy is maintained at better than one percent of the reading over the full output range of the instrument.

Two variants of the instrument are available, both manufactured in titanium, capable of operating in depths from shallow water down to 2000 m and 6000 m respectively. The optical characteristics of the instrument in its different detection modes are visible below:

* The wavelengths for the turbidity filters are customer selectable but must be in the range 400 to 700 nm. The same wavelength is used in the excitation path and the emission path.

The instrument measures chlorophyll a, rhodamine and fluorescein with a concentration range of 0.01 µg l^-1 to 100 µg l^-1. The concentration range for turbidity is 0.01 to 100 FTU (other wavelengths are available on request).

The instrument accuracy is ± 0.02 µg l^-1 (or ± 3% of the reading, whichever is greater) for chlorophyll a, rhodamine and fluorescein. The accuracy for turbidity, over a 0 - 10 FTU range, is ± 0.02 FTU (or ± 3% of the reading, whichever is greater).

Further details are available from the Aquatracka MKIII specification sheet.

Chelsea Technologies Group ALPHAtracka and ALPHAtracka II transmissometers

The Chelsea Technologies Group ALPHAtracka (the Mark I) and its successor, the ALPHAtracka II (the Mark II), are both accurate (< 0.3 % fullscale) transmissometers that measure the beam attenuation coefficient at 660 nm. Green (565 nm), yellow (590 nm) and blue (470 nm) wavelength variants are available on special order.

The instrument consists of a Transmitter/Reference Assembly and a Detector Assembly aligned and spaced apart by an open support frame. The housing and frame are both manufactured in titanium and are pressure rated to 6000 m depth.

The Transmitter/Reference housing is sealed by an end cap. Inside the housing an LED light source emits a collimated beam through a sealed window. The Detector housing is also sealed by an end cap. A signal photodiode is placed behind a sealed window to receive the collimated beam from the Transmitter.

The primary difference between the ALPHAtracka and ALPHAtracka II is that the Alphatracka II is implemented with surface-mount technology; this has enabled a much smaller diameter pressure housing to be used while retaining exactly the same optical train as in the Mark I. Data from the Mark II version are thus fully compatible with that already obtained with the Mark I. The performance of the Mark II is further enhanced by two electronic developments from Chelsea Technologies Group - firstly, all items are locked in a signal nulling loop of near infinite gain and, secondly, the signal output linearity is inherently defined by digital circuitry only.

Among other advantages noted above, these features ensure that the optical intensity of the Mark II, indicated by the output voltage, is accurately represented by a straight line interpolation between a reading near full-scale under known conditions and a zero reading when blanked off.

For optimum measurements in a wide range of environmental conditions, the Mark I and Mark II are available in 5 cm, 10 cm and 25 cm path length versions. Output is default factory set to 2.5 volts but can be adjusted to 5 volts on request.

Further details about the Mark II instrument are available from the Chelsea Technologies Group ALPHAtrackaII specification sheet.

BODC Processing

The CTD data were supplied to BODC in 156 PStar files and converted to the BODC internal format, a netCDF subset.

During transfer the originator's variables were mapped to unique BODC parameter codes. The following table shows the parameter mapping. During the cruise, two separate CTD rigs were employed for different sections of the line:- a Neil Brown CTD and a Sea-Bird CTD, which in some cases had different sensors measuring the same quantity. CTD casts recorded by the Neil Brown CTD are identified as subaccession A, those recorded by the Sea-Bird CTD are subaccession B. Where the same originator's variable in the source files was recorded by different sensors on different CTD rigs (possibly resulting in a different BODC parameter mapping) this has been made clear in the table below.

Following transfer the data were screened using BODC in-house visualisation software. A small number of suspect data values were identified during screening and assigned a BODC data quality flag.

Originator's Data Processing

Sampling strategy

The FISHES 2001 (DI253) cruise repeated the occupation of a number of hydrographic lines between Scotland and Iceland and carried out detailed multidisciplinary surveys of the northern ends of the Iceland Basin (including the Iceland Faeroes Front - IFF) and Rockall Trough to resolve both basin- and meso-scale physical, chemical and biological structure. In total, 156 CTD casts were completed.

Data Acquisition and Initial Processing

The data were processed using PEXEC (pstar) routines. Further details on the processing can be found in the cruise report. The data were calibrated post-cruise.

The processed data, together with the raw Sea-Bird, configuration and bottle files, were supplied to BODC for banking.

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: