Metadata Report for BODC Series Reference Number 227524

• 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

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 169 CTD Data Documentation

CTD/Transmissometer Operations (N. Hooker)

The IOS Neil Brown deep CTD system gave some early problems. At the trial Station (11639) the pressure calibration adopted was found to be incorrect, and conductivity was erratic. The former was amended after an exchange of telex messages, and the conductivity behaviour vanished after inspection but was eventually (after Station 11642) found to be due to a poor connection in the sea unit. A second similar fault was corrected after Station 11654.

For the majority of the casts however, the system worked correctly and well. Data were recorded on the Digidata mostly for down-runs only, recording continuing on the level C for the full dips. A small transmissometer drift was observed with the up-runs having consistently higher measured values of light transmittance than the down-runs, but the main variations were clearly reproduced. A pressure-operated pinger was attached to the CTD to enable the system to be profiled to within 10m of the sea bed.

Salinometer, Thermometers and Rosette Multisampler (J. Moorey)

The rosette multisampler was used as a calibration check on every CTD dip. Six water bottles were used and were fired in pairs, the second bottle of each pair having a thermometer frame. After each pair there was a blank position on the multisampler to ensure that thermometer frames could not touch each other when in the horizontal position. (In the past we have had `hang-ups' on thermometer frames which were only two spaces apart. It is thought that the frames may swing violently even past the horizontal as the CTD is hauled and, if the frames are free to swing quickly back to the reversed position, the temperature recording is not lost. However, if the frames are close enough to jam each other beyond the horizontal position, then one or both sets of thermometers drain and the temperature recordings are lost.)

The multisampler generally operated satisfactorily, but on the last few CTDs the deck unit occasionally had to be fired twice before it indicated that a bottle had fired. Another problem was the occasional snagging of thermometer frame lanyards. This problem was almost completely overcome by taping lanyards to minimise loops and to fit spare water bottles (without end caps) in the blank positions to avoid lanyards snagging on the rosette frame.

There were two sets of water bottles with thermometer frames, and these were used on alternate stations. This allowed more time for the incoming thermometers to equilibrate to laboratory temperature before reading. (The forward hydro lab is less well fitted for such work than it once was, presently housing the cooling system for the computer room). Each frame had a pair of protected thermometers and the `deepest' two frames each had an unprotected thermometer. Some thermometers are readable to 0.002 °C (-2 to 6 °C scale) and some to 0.005 °C (-2 to 13 scale). Since each thermometer was read by two people, there were four readings for each `frame'. These were `corrected' for the in-situ values and the four readings averaged and quoted to 0.001 °C. Another advantage of two sets of thermometers was that a possible reading error could be checked if the thermometers were still in the hydro lab.

Two salinity samples were taken from each bottle and measured on the SUDO Guideline Salinometer. The salinometer functioned well. It was run at a bath temperature of 24 °C. It was used and standardised 11 times from 15 to 30 August. The standardised dial (Rs) setting ranged from a maximum of 688 to a minimum of 663, equivalent to a salinity change of 0.002 psu. The salinometer's very good stability is indicated by the consistently good agreement of duplicates, the second of the pair always being measured on a separate day and hence separate standardisation. Also, the salinities from adjacent water bottles showed good agreement indicating that there are no measurable leaks from the water bottles.

Also on board was the IOS Guideline serial No. 42508. This was not used for any of the rosette samples but a set of various salinities was made up by diluting with distilled water. The conductivities were such that the salinity range was from circa 6.0 to 36.6 psu, in the oceanographic range (31 to 36.6 psu). The SUDO read higher than the IOS by only 0.001 psu (at both 31 and 36.6). In the low salinity ranges (down to circa 6.0 psu) the SUDO read higher than the IOS by only 0.004. After 15 hours the two salinometers were compared by using water of `about 35' psu. The SUDO read lower than the IOS by 0.001 psu. This shows that the two salinometers are consistent with each other over a wide range of conductivities although we do not know the absolute values of those conductivities.

CTD Calibrations (S. Boxall and J. Taylor, SUDO)

Pressure

Initial values of pressure as read by both the main shipboard computer and the BBC micro were low by a factor of 10, though correct on the CTD deck unit. This was due to necessary software corrections for both machines to take account of the deep pressure sensor fitted to the Neil Brown CTD. This problem was resolved after Station 11640.

Pressure data from the unprotected reversing thermometers gave no indications of any problems with the pressure transducer. However, a constant offset of approximately 8 db is present in the IOS CTD such that at 10m wire out, a reading of about 2 db was given. This offset drifted by about 3 db during the cruise.

Temperature

The temperature sensors on the CTD performed well, apart from an intermittent fault following Station 11654 which was remedied. This fault did not seem to affect the calibration of that Station.

The trend of the difference between CTD and thermometer values is linear, increasing with increasing temperature, with a spread of values about a line of 0.007 °C. Most points lying outside this limit can be counted as suspect due to samples being taken in a high gradient of temperature or to inconsistent readings on paired reversing thermometers. A linear correction (estimated by eye) of:

T(correct) = 1.0065T - 0.002 °C

where T is the CTD uncorrected temperature is recommended from this data set at the present time.

This was later compared to two other calibrations. The first, of unknown origin, is:

T(correct) = 1.0063T - 0.0015 °C

which is in good agreement with the above, giving no resolvable difference in the deeper water (up to approximately 5 °C), and the difference of 0.002-0.003 in the intermediate water masses at about 10 °C.

The value used on the computer was:

T(correct) = 1.0074T - 0.0081 °C

This does not fit the data set as well, giving errors of 0.005 °C in deeper waters and shallower waters alike.

There were no noticeable trends with time.

Salinity

In all salinity calculations the temperature correction of 1.0065T - 0.002 was used.

Some problems were experienced with the conductivity measurements from the CTD. Readings over 1ppt high were initially given and there were significant jumps in values. The problem was traced to a faulty solder joint on the conductivity interface board on the under-water unit. This meant that any calibration of salinity was impossible prior to Station 11644.

Stations 11644-11655 demonstrated a better performance but also showed a slow calibration drift in time, from approximately 0.24ppt low at Station 11644 up to 0.40ppt low at Station 11654. The drift was neither linear nor consistent, and it is uncertain whether applying an individual correction for each station will produce reliable results. Though this may have to be done as a last resort, a better suggestion would be to correct to a standard water mass within each profile.

After Station 11655 the CTD was stripped down, checked and the conductivity cell thoroughly cleaned. Results from 11661 (the next CTD Station) were good. A plot of sample minus CTD salinity vs station number shows the above problems and the consistency of calibration with time for Station 11661 onwards very clearly. A plot of delta s (Bottle salinity minus CTD salinity) vs salinity showed that with temperature there is a linear trend, though it is not quite so clearly defined as that of temperature.

The recommended calibration for salinity, based on these data, is therefore

S(correct) = 0.989S + 0.590

There are no salinity corrections implemented on the shipborne computer.

Data Quality

This data is also held by ICES, who have chosen to add additional data quality flags to the temperature data. BODC have not added these flags to the BODC data.

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: