Metadata Report for BODC Series Reference Number 1071791

• 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

Data Quality Report

The originator made various data quality comments regarding the oxygen sensor. Users interested in the oxygen data are advised to review the summary of these before proceeding:

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

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.

RRS Discovery Cruise D242 CTD Instrumentation

The CTD profiles were taken using two Neil Brown System MkIIIc CTDs (DEEP03 and DEEP04), with a 24 bottle rosette frame.

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.

RRS Discovery Cruise D242 CTD Processing

Originator's Data Processing

• Sampling Strategy

  Two WOCE-standard CTD packages (DEEP03 and DEEP04) were used during D242, collectively generating 125 profiles. Discrete water samples were also obtained on the casts for calibration of the conductivity and oxygen sensors. Various data quality issues were encountered, which necessitated swapping between the units at various stages of the cruise. These are summarised in the table below. For further details, users should refer to both the cruise and data quality report.

  Stations	Cast numbers	CTD package	Comments
  13630-13648	001-019	DEEP04	Conductivity cell displayed hysteresis on upcast
  13649-13676	020-047	DEEP03	Concerns about the stability of the temperature sensor
  13677-13686	048-057	DEEP04	Replacement conductivity cell still displaying hysteresis on upcast
  13687-13757	058-125	DEEP03	-

  In addition to these issues, the downcast salinity profiles obtained from both packages were also unusually noisy throughout the cruise.

• Data Processing

  Raw CTD data were captured and stored in three ways: 1) to the hard disk of the CTD acquisition PC 2) to the RVS Level A system 3) directly from the CTD deck unit onto the SOC DAPS (Southampton Oceanography Centre - Data Acquisition Processing Software) system. Data from the latter source were used for further processing.

  The DAPS system checks for pressure jumps and subsequently produces 1 sec averages of the raw 25 Hz data. Two ASCII files are generated for each cast, one for CTD profile data, the other for bottle firing data. Post processing of these output files was conducted in the Pstar environment.

• Field Calibrations

  A description of the CTD calibrations applied to instrumentation is presented below:

  Temperature

  Raw temperatures were scaled according to:

  T_raw = 0.0005 T_raw

  then calibrated using the following coefficients:

  T = -2.1443 + 0.991259 T_raw (DEEP03)
  

  T = 0.1309 + 0.999278 T_raw (DEEP04)
  

  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 1 sec interval. The time constants, τ = 0.25 (DEEP03) and τ = 0.20 (DEEP04), were used.

  Pressure

  Raw pressure measurements were first scaled according to:

  P_raw = 0.1 P_raw

  then calibrated using the following coefficients:

  P = -39.3 + 1.07489 P_raw (DEEP03)
  

  P = -37.8 + 1.07378 P_raw (DEEP04)
  

  Following laboratory calibration, no further corrections were deemed necessary for temperature dependence or pressure hysteresis on either instrument.

  Salinity

  Raw conductivities were scaled according to:

  C_raw = 0.001 C_raw

  then calibrated using the following coefficients:

  C = -0.002 + 0.964 C_raw (DEEP03)
  

  C = -0.0238 + 0.9552 C_raw (DEEP04)
  

  This was followed by the cell material deformation correction:

  C = C [1 + α * (T - T_θ) + β * (P - P_θ)]

  where α = -6.5e^-6C^-1, β = 1.5e^-8dbar^-1, T_θ = 15°C and P_θ = 0dbar.

  Further adjustments were made to conductivity following comparisons of CTD data with bottle conductivities. The process differed between the two sensors to account for the hysteresis in DEEP04. DEEP03 corrections were performed directly to the CTD upcast conductivities, whilst casts obtained from DEEP04 had downcast conductivity corrected directly. The cruise report contains full details of offsets and the residuals (bottle salinity-CTD salinity) from the subsequent conversion to salinity.

  Fluorometer and Transmissometer

  The calibration of the CTD's voltage digitiser is given as:

  V = -5.027 + 1.534e^-4 V_raw - 3.704e^-12(V_raw ) ² (DEEP03)
  

  V = -5.656 + 1.727e^-4 V_raw - 2.244e^-12(V_raw ) ² (DEEP04)
  

  Transmissometer clear air and blank voltage readings were also obtained throughout the cruise for further calibration of the sensor, these are documented in the cruise report.

  Oxygen sensor

  The program oxyca3 was used to obtain the best fit of downcast CTD oxygen data to measured oxygen samples, based on the model of Owens and R.C. Millard, 1985. This is used to calibrate the 1 Hz CTD files using program oxygn3. The 2 db files are subsequently recalculated and merged with the original sample files.

  There were various issues with the oxygen sensor throughout the cruise. In some instances the data were unusable and set to null by the originator. The table below summarises these. Separate originator data quality comments relating to preserved oxygen data are noted in the data quality report.

  Stations	Cast numbers	Comment
  13649-13698	020-068	Oxygen data unrealistic
  13704-13705	073-074	Sensor failure
  13711-13714	080-083	High oxygen current values before failure on Station 13714

• Post-cruise investigations

  The persistent problems encountered during D242 prompted a further investigation post cruise. The salinity spiking was attributed possibly to the presence of a horizontal flat bar on the CTD rosette frame, which resulted in the temperature and conductivity sensors measuring slightly different water masses. The source of the salinity hysteresis proved more difficult to identify. Full details of these investigations are documented in the Southampton CTD data quality report.

  Ultimately, the well-mixed, less noisy calibrated upcast data were deemed to be the most suitable final dataset for D242.

BODC Processing

• Reformatting

  The data arrived at BODC as 125 Pstar files at 2 db resolution. These were reformatted to the internal QXF format. The following table shows which variables were mapped to BODC parameter codes and how this was achieved.

  Originator's variable	Units	Description	BODC Parameter code	Units	Comments
  press	dbar	Pressure exerted by the water column	PRESPR01	dbar	Calibrated by originator. Additional comparison with auxiliary pressure meters made by originator
  temp	°C	Temperature of the water column	TEMPST01	°C	Calibrated by originator. Additional comparison with reversing thermometers made by originator
  salin	-	Practical salinity of the water column	PSALST01	-	Calibrated against bottle data, by originator
  oxygen	µmol/l	Dissolved oxygen	DOXYPR01	µmol/l	Calibrated against bottle data, by originator
  fluor	mg m-3	Concentration of chlorophyll-a	CPHLPR01	mg m-3	Conversion from volts to concentration carried out by originator. No further details known.
  chtran	volts	Transmissometer output	TVLTDZ01	volts	-

• Calibration

  Sigma theta and potential temperature were generated by BODC. Oxygen saturation was also calculated for those series containing dissolved oxygen measurements.

• Quality control

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

References

Cunningham, S.A., 1999. Cruise Report No.28. RRS Discovery Cruise 242. 07 Sep - 06 Oct 1999 Atlantic - Norwegian Exchanges.

Holliday, N.P., 2000. Southampton Oceanography Centre Internal Document No.62. CTD Data Quality on Discovery Cruise D242.

Owens, W.B., Millard, R.C., 1985. A new algorithm for CTD oxygen calibration. Journal of Physical Oceanography, 15(5), 621-631.

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: