Metadata Report for BODC Series Reference Number 703644

• 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

CTD Oxygen Sensor Data

Trials were conducted of a new dissolved oxygen sensor designed by SOC and the University of Southampton Chemistry Department.

The trials showed up various issues that were to be considered ashore. In particular, flow rate, the robustness of manufacture and the up/down effects. Heavily-smoothed signals showed generally realistic variation with known climatological dissolved oxygen content.

Further information on the sensor can be found in the Cruise Report p.52.

Quality Report

Two CTDs were used during the cruise, DEEP03 and DEEP04.
DEEP03 was used for stations 1-18, 21-22 and 33-34.
DEEP04 was used for stations 19-20 and 23-32.

Stations 001-018 were completed using DEEP03 (conductivity cell s/n L53)
Bottle - CTD conductivities had a large systematic station by station drift, such that the CTD was reading higher conductivities on subsequent stations. On stations 016-018 upcast salinities were higher than on downcast and the downcast temperature/salinity (T/S) had a different shape to the upcast T/S.

Stations 019 and 020 were completed using DEEP04.
For both stations the upcast was saltier and had a different T/S shape relative to the downcast.

Stations 021 and 022 were completed using DEEP03 with a new conductivity cell (s/n Q47).
Both stations showed a large drift to fresher salinities throughout the casts, possibly related to the new cell.

Stations 023 to 032 were completed using DEEP04.
Behaviour seemed to have settled so that down and upcasts had the same T/S shape.

Stations 033 and 034 were completed using DEEP03 (with conductivity cells s/n Q47 and s/n G149).
Profiles had large salinity drifts to fresher salinities.

Temperature

Post-cruise calibration revealed that the difference in temperatures measured by DEEP03 and DEEP04 was consistent with a DEEP03 drift. Therefore, DEEP03 temperature data needed to be offset by +0.006°C.

It is not apparent whether the temperature correction has been applied to the data.

Salinity

The salinity was calibrated to bottles and therefore did not require adjustment for the temperature offset

Transmittance

For stations 1, 3-7, 9-17, 19-20, 23-26, 28-34 the transmittance values were very noisy in places, with data spikes of different sizes. Only the large data spikes were flagged.

Full details of the quality issues relating to the CTD can be found in the Cruise Report.

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 Descriptions

CTD Unit and Auxiliary Sensors

^* See the Cruise Report p. 52-55 for more details on the Oxygen Sensor.

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 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 data arrived at BODC as Pstar files, representing all of the CTD casts taken during the cruise. These were reformatted to the internal QXF format. The following table shows how the variables were mapped to the appropriate BODC parameter codes.

The reformatted data were visualised using the in-house EDSERPLO software. Suspect data were marked by adding an appropriate quality control flag.

Originator's Data Processing

Sampling Strategy

A total of 34 full depth CTD stations were completed during JR55. Salinity samples were drawn on all of the stations for calibrating the CTD.

Data Processing

Raw CTD data were captured and stored to the hard disk of the CTD acquisition PC. These data were also recorded directly from the CTD deck unit onto the SOC DAPS (Data Acquisition Processing Software) system. DAPS used routines for one second despiking and averaging of the raw 25Hz data.

DAPS stored two ASCII files for each cast, one for CTD data and another for the bottle firing data.

Nearing the end of JR55, frequent "data time outs" and "frame sync errors" for CTD led to data loss for some of the casts in the DAPS ASCII files.

Field calibrations

A description of the CTD calibrations applied to each instrument are described below.

Temperature

Temperatures were reported in ITS-90.

T₆₈ = 1.00024 x T₉₀

Raw temperatures were scaled according to:

T_raw = 0.0005 T_raw

then calibrated using the coefficients provided by Ocean Scientific International (OSI) for DEEP03 and DEEP04,

DEEP03: T = -1.86750 + 0.992088 T_raw
DEEP04: T = 0.12306 + 0.999249 T_raw

Due to a lag between the conductivity and temperature sensor measurements the time rate of change of temperature was used to "speed up" the temperature measurements according to,

T = T + τ δ T / δ T

where the rate of change of temperature is determined over a one second interval. Estimates of τ from Cunningham (2000) were used.

DEEP03: τ = 0.25
DEEP04: τ = 0.20

Pressure

Raw pressure measurements were first scaled according to:

P_raw = 0.1 P_raw

then calibrated using the coefficients provided by Ocean Scientific International (OSI) for DEEP03 and DEEP04,

DEEP03: P = -39.7 + 1.07439 P_raw
DEEP04: P = -36.9 + 1.07330 P_raw

Following observations of pressure before and after each cast for DEEP03 and DEEP04 it was evident that a correction was required to set pressure readings to zero. The adjustments made were-1.8 dbar and -7.6 dbar for DEEP03 and DEEP04 respectively, changing the above coefficients to

DEEP03: P = -37.9 + 1.07439 P_raw
DEEP04: P = -29.3 + 1.07330 P_raw

The offset was determined by taking the mean pressure values before entering the water and on deck after each cast and calculating the mean pressure. The mean pressure was used to adjust the pressure offset. No relationship between pressure offset and temperature was found.

Salinity

Raw conductivities were scaled according to:

C_raw = 0.001 C_raw

then calibrated using the coefficients provided by Ocean Scientific International (OSI) for DEEP03 and DEEP04,

DEEP03: C = -0.01851 + 0.94717 C_raw
DEEP04: C = -0.07645 + 0.96242 C_raw

This was followed by the cell material deformation correction

C = C x [1 + α x (T - T₀) + β x (P - P₀)]

where the coefficients for the cell material are: α = -6.5E^-6°C^-1 , β = 1.5E^-8dbar^-1 , T₀ = 15°C and P₀ = 0dbar.

Further adjustments to the conductivity offsets were determined using bottle samples. See the Cruise Report (Cunningham, 2001) for further details.

Transmittance and Altimetry

Transmittance was converted to voltages; this is a calibration of the voltage digitiser in the ctd.

DEEP03: V = -5.027 + 1.534 x 10^-4 V_raw - 3.704 x 10^-10 V_raw ²
DEEP04: V = -5.656 + 1.72669 x 10^-4 V_raw - 2.24 x 10^-12 V_raw ²

The altimeter had the following calibration applied

DEEP03: alt = -249.7 + 7.62 x 10^-3 alt_raw - 1.04 x 10^-10 alt_raw ²
DEEP04: alt = -234.5 + 7.16 x 10^-3 alt_raw - 9.48 x 10^-11 alt_raw ²

References

Cunningham, S.A. (2000) RRS Discovery Cruise 242, 07 Sep-06 Oct 1999. Atlantic - Norwegian Exchanges. Southampton, UK, Southampton Oceanography Centre, 128pp.
Cunningham, S.A. (2001) RRS James Clark Ross Cruise JR55, 21 Nov-14 Dec 2000. Drake Passage repeat hydrography: WOCE Southern Section 1b - Burdwood Bank to Elephant Island. Southampton, UK, Southampton Oceanography Centre, 75pp.

Project Information

World Ocean Circulation Experiment (WOCE)

The World Ocean Circulation Experiment (WOCE) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.

WOCE had two major goals:

• Goal 1. To develop models to predict climate and to collect the data necessary to test them.

• Goal 2. To determine the representativeness of the Goal 1 observations and to deduce cost effective means of determining long-term changes in ocean circulation.

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: