Metadata Report for BODC Series Reference Number 1074948

• 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 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 D245 CTD Instrumentation

The CTD profiles were taken with a Neil Brown Systems MkIIIc CTD (DEEP03), with a FSI 24 bottle rosette.

Aquatracka fluorometer

The Chelsea Instruments Aquatracka is a logarithmic response fluorometer. It uses a pulsed (5.5 Hz) xenon light source discharging between 320 and 800 nm through a blue filter with a peak transmission of 420 nm and a bandwidth at half maximum of 100 nm. A red filter with sharp cut off, 10% transmission at 664 nm and 678 nm, is used to pass chlorophyll-a fluorescence to the sample photodiode.

The instrument may be deployed either in a through-flow tank, on a CTD frame or moored with a data logging package.

Further details can be found in the manufacturer's specification sheet.

SeaTech Transmissometer

Introduction

The transmissometer is designed to accurately measure the the amount of light transmitted by a modulated Light Emitting Diode (LED) through a fixed-length in-situ water column to a synchronous detector.

Specifications

• Water path length: 5 cm (for use in turbid waters) to 1 m (for use in clear ocean waters).
• Beam diameter: 15 mm
• Transmitted beam collimation: <3 milliradians
• Receiver acceptance angle (in water): <18 milliradians
• Light source wavelength: usually (but not exclusively) 660 nm (red light)

Notes

The instrument can be interfaced to Aanderaa RCM7 current meters. This is achieved by fitting the transmissometer in a slot cut into a customized RCM4-type vane.

A red LED (660 nm) is used for general applications looking at water column sediment load. However, green or blue LEDs can be fitted for specilised optics applications. The light source used is identified by the BODC parameter code.

Further details can be found in the manufacturer's Manual.

RRS Discovery Cruise D245 CTD Processing

Originator's Data Processing

• Sampling Strategy

  A total of 44 CTD stations were completed during D245. Salinity and oxygen samples were drawn on all but the first (test) cast for calibrating the CTD.

• 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 are presented 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) in December 1999:

  T = -2.142635 + 0.991186 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. The time constant, τ = 0.25, was used for D245.

  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) in December 1999:

  P = -38.1 + 1.07482 P_raw
  

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

  Salinity

  Raw conductivities were scaled according to:

  C_raw = 0.001 C_raw

  then calibrated using the coefficients provided by Ocean Scientific International (OSI) in December 1999:

  C = -1.721e^-3 + 0.946 C_raw
  

  this was then changed after bottle sample and CTD measurement comparisons were carried out, to:

  C = 1.4372576e^-2 + 0.945609882 C_raw
  

  This was followed by the cell material deformation correction:

  C = C (1-6.5e^-6(T - 15) + 1.5e^-8P)

  Finally, salinity was calculated from conductivity, C, using the PEXEC programme, peos83.

  Further details regarding the calibration with bottle samples can be found in the Cruise Report (Holliday and Griffiths, 2000).

  Fluorometer and Transmissometer

  Fluorescence was converted to voltages using the CTD's voltage digitiser calibration supplied by Ocean Scientific International (OSI):

  fvolts = -5.0326 + 1.5359e^-4 fvolts_raw + 3.383e^-14(fvolts_raw) ²

  Further calibration of the fluorometer was carried out post-cruise using bottle samples collected for chlorophyll analysis. Details of the calibration applied are not held by BODC.

  Transmissometer measurements were converted to voltages; this is a calibration of the voltage digitiser in the ctd, provided by Ocean Scientific International (OSI):

  V = -5.027 + 1.534e^-4 V_raw - 3.704e^-13(V_raw ) ²
  

  Further calibration to give transmittance was finally achieved with clear air and blank voltages measured prior to station 13795:

  Trans = -0.030 + 4.80 (V)
  

  Oxygen sensor

  The program oxyca3 was used to obtain the best fit of downcast CTD oxygen to measured oxygen samples, based on the model of Owens and R.C. Millard, 1985. This fit is subsequently used to calibrate the 1 Hz CTD files using program oxygn3. The 2 db files are finally recalculated and merged with the original sample files. Various problems were identified with the initial data from the oxygen sensor. This necessitated a sensor swap for the final 9 stations, with all previous sensor data being discarded. There were subsequently problems calibrating the sensor data for these final 9 stations - the data in the final dataset being considered unreliable.

BODC Processing

• Reformatting

  The data arrived at BODC as 44 Pstar files, representing all of the CTD casts taken during the cruise. 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 body	PRESPR01	dbar	Calibrated by originator
  temp	°C	Temperature of the water body	TEMPST01	°C	Calibrated by originator
  salin	-	Practical salinity of the water body	PSALST01	-	Calibrated against bottle data by originator
  oxygen	µmol/l	Dissolved oxygen	DOXYPR01	-	Attempted calibration against bottle data by originator
  fluor	mg m-3	Concentration of chlorophyll-a	CPHLPR01	mg m-3	Calibrated against bottle data by originator
  chtran	%	Transmittance	ATTNMR01	m-1	Converted to beam attenuation by BODC

• Calibration

  Percent transmission was converted to beam attenuation:

  Beam attenuation coefficient c = - (1/z) * ln (light transmission [decimal])

  where, z, is the transmissometer path length (1 m), light transmission[decimal] is light transmission [%] divided by 100

• 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

Holliday, N.P. and Griffiths, C.R., 2000.Cruise Report No.29. RRS Discovery Cruise 245. 27 Jan - 20 Feb 2000 A hydrographic section from Scotland to Iceland.

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

Fixed Station Information

DML Muck Deep Station MD2

Station MD2 is one of six fixed CTD stations south west of the Isle of Muck, Scotland. This region has been a focus of studies involving scientists at Dunstaffnage Marine Laboratory (DML).

Related Fixed Station activities are detailed in Appendix 1

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:

Appendix 1: DML Muck Deep Station MD2

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

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