Metadata Report for BODC Series Reference Number 291495

• 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

The DOXYPR01 channel has been flagged for the whole of this series as there were issues with the sensor during the CTD cast.

Data Access Policy

Public domain data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

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.

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 181 CTD Data Documentation

Introduction

This data document covers the CTD data collected during RRS Discovery cruise 181 (1^st April to 1^st May 1989) under the direction of Dr. R.T. Pollard from the Institute of Oceanographic Sciences Deacon Laboratory.

Instrumentation

Two Neil Brown Instrument Systems (NBIS) Mark III CTDs were taken on the cruise, although only one was used. After an initial problem with the deck unit (a loose wire) the system worked well. An old Beckman oxygen sensor was used at the beginning of the cruise but by cast 11820 it had become obvious that this was not giving sensible output. It was replaced by a new sensor for cast 11821 and the data collected prior to this were abandoned. A SeaTech 1m transmissometer was incorporated into the CTD system and worked well throughout the cruise. The multisampler and rosette caused problems by consistently misfiring at depth. A full report of the difficulties encountered is given in the cruise report (Pollard et al., 1989).

Data Capture and Reduction

All CTD, oxygen and transmissometer data were logged on the NBIS deck unit and displayed on a BBC microcomputer in real time. Data were also recorded on digidata tapes as a back up to the main computing system. The data were logged via a level A interface to the level B Plessey computer for transfer to tape and to the level C system: three Sun 3/60 workstations. The level A despiked and averaged the data from 16 Hz to 1 Hz. The data were processed by the procedure outlined by Pollard, Read and Smithers (1987).

Calibration

The initial calibrations applied by CTDCAL were as follows:

Pressure (dbar) P = 0.0 + P(raw) * (0.1 * 0.99286144)
Temperature (deg C) T = 0.020909 + T(raw) * (0.0005 * 0.998965925)
Conductivity C = 0.0 + C(raw) * (0.001 * 1.00109073)

Temperature

The temperature calibration was obtained in the laboratory with the platinum resistance thermometer calibrated in-situ. Deep temperature values (at 3000 m or deeper) were compared with sample measurements taken by a digital reversing thermometer. Over 23 samples, the mean difference between temperature sensor and reversing thermometer was 0.0088 ±0.001 ° C. Very few adequate readings were obtained near the surface because of the failure of the second digital reversing thermometer. On this evidence no change was made to the temperature calibration.

Transmittance

Transmittance was also recorded although not shown in this report. It was calibrated with the equation:

Transmittance (%) T = 20 * V * 1.0032

where:

V (output voltage) = (4.355 / 4.096) * (T(raw) - 0.0) * 0.0001

Absolute Salinity

At the beginning of the cruise six full depth casts were made with twelve water samples each, analysed on the Guildline Autolab salinometer for calibration purposes. These gave a consistent offset over the entire cast which was comparable to the offset obtained by comparing the 0/S profiles to the curve obtained by Saunders (1986) for the deep, stable water of the North East Atlantic. A constant offset was therefore applied to salinity to match the Saunders curve.

After the first few casts the number of salinity samples drawn at each station was reduced to four. These were analysed during the cruise on the Guildline salinometer, but examination of the results after the cruise suggested that the salinometer was not working reliably, standardisations on successive sessions showing large offsets. All the CTD 0/S profiles were compared to the Saunders curve and adjustments were made during the cruise as the conductivity cell drifted slightly.

The bottle values were compared with the CTD data and gave poor statistics, but it is thought that this is most likely to be the result of problems with the salinometer rather than the conductivity sensor.

Oxygen

Calibration of the Beckman oxygen sensor caused more than the usual problems (Read, 1989). Data from the casts prior to station 11821 were discarded as the old sensor did not give sensible output.

Data from the replacement were given an initial calibration of:

oxygen current OXYC = 0.0 + OXYC(raw) * (0.001 * 1.05)
oxygen temperature OXYT = 0.0 + OXYT(raw) * (0.128 * 1.0)

However, OXYT was discarded in favour of CTD temperature (see Pollard, 1985).

The oxygen sensor was then calibrated with the formula:

oxygen O = OXYC * rho * exp (alpha* T + beta * P) * OXYSAT

using a least squares fit to calculate the coefficients rho, alpha and beta from bottle samples. With between ten and twelve samples for each cast, this should have given a good fit. However, it soon became apparent that many of the bottle sample values were imprecise (Read, 1989) and good values had to be carefully selected before the calibration coefficients could be calculated. This still gave widely varying results for rho, alpha and beta for each cast, so a best fit of the deep gradient over several selected casts was used to fix alpha and beta, and rho was then calculated individually for each cast. This correction improved the fit to the sample data but still caused the overall value of oxygen to vary up and down between casts. The problem was particularly pronounced when casts 11824 - 11845 were contoured. To minimise the cast to cast variations, the deepest data were fitted to the oxygen value given by Saunders (5.67 ml/l) and the offset added to the whole cast.

General Data Screening carried out by BODC

BODC screen both the series header qualifying information and the parameter values in the data cycles themselves.

Header information is inspected for:

• Irregularities such as unfeasible values
• Inconsistencies between related information, for example:
  • Times for instrument deployment and for start/end of data series
  • Length of record and the number of data cycles/cycle interval
  • Parameters expected and the parameters actually present in the data cycles
• Originator's comments on meter/mooring performance and data quality

Documents are written by BODC highlighting irregularities which cannot be resolved.

Data cycles are inspected using time or depth series plots of all parameters. Currents are additionally inspected using vector scatter plots and time series plots of North and East velocity components. These presentations undergo intrinsic and extrinsic screening to detect infeasible values within the data cycles themselves and inconsistencies as seen when comparing characteristics of adjacent data sets displaced with respect to depth, position or time. Values suspected of being of non-oceanographic origin may be tagged with the BODC flag denoting suspect value; the data values will not be altered.

The following types of irregularity, each relying on visual detection in the plot, are amongst those which may be flagged as suspect:

• Spurious data at the start or end of the record.
• Obvious spikes occurring in periods free from meteorological disturbance.
• A sequence of constant values in consecutive data cycles.

If a large percentage of the data is affected by irregularities then a Problem Report will be written rather than flagging the individual suspect values. Problem Reports are also used to highlight irregularities seen in the graphical data presentations.

Inconsistencies between the characteristics of the data set and those of its neighbours are sought and, where necessary, documented. This covers inconsistencies such as the following:

• Maximum and minimum values of parameters (spikes excluded).
• The occurrence of meteorological events.

This intrinsic and extrinsic screening of the parameter values seeks to confirm the qualifying information and the source laboratory's comments on the series. In screening and collating information, every care is taken to ensure that errors of BODC making are not introduced.

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: