Metadata Report for BODC Series Reference Number 290302

• 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.

RRS Discovery Cruise 145 CTD Data Documentation

Introduction

This document covers the CTD data collected during RRS Discovery cruise 145 (25^th February 1984 to 24^th March 1984) under the direction of Dr R.T. Pollard in the NE Atlantic Ocean.

Instrumentation

Both instruments used were Neil Brown Instrument Systems CTDs, which measured pressure, temperature and conductivity. The instruments were referred to as shallow and deep CTDs as they used 1500 and 6000 dbar pressure sensors respectively. The shallow CTD was only used in lowered mode on Station 10986, for calibration purposes, with the deep CTD used for all further ship-lowered CTD work.

Sampling Protocol

Each CTD was used alongside a General Oceanics Rosette Multisampler with 12 Niskin bottles and a 10 Hz pinger. Lowering and retrieval rates of 0.5 to 1.0 m/s were employed and the sensors were flushed with distilled water on recovery.

Bottle samples and reversing thermometer measurements were made on descent and the sea water samples were analysed using a Guildline Autolab Salinometer.

Reversing thermometers were calibrated before and after the cruise.

CTD Casts

• Casts 10985-88 were initial calibration casts, those with the deep CTD being done to full depth to allow calibration by comparison with Saunders' (1985a) deep O/S relation.

• Casts 10992-11002, 11004-7, 11010 and 11012-14 were made around the northern site, before and after a detailed SeaSoar survey (Pollard et al 1986).

• The deep CTD was also used in yoyo mode on casts 11003, 8, 9 and 11 to resolve small space and time scales.

• A short CTD section, casts 11015-29, was made on passage from the north to the south site, spanning a notable change in water properties.

• Two short sections, 11022-27, were made across the current during work at the south site, followed by a short yoyo 11028.

• A final calibration cast 11029 was terminated prematurely due to a cable fault. Calibration of IOS Deep CTD.

Data Acquisition

CTD data were logged onto a PDP11/34 computer (Collins et al, 1983). After sampling (CTDSAMP) and averaging to 1-second raw values (CTDAVE), the data were calibrated (CTDCAL) using approximate calibration constants. Every fifth calibrated value was listed so that final calibration constants could be determined. Downcasts were generally completed without stopping, with calibration samples taken stopping at standard depths during the upcasts. Some bottles carried reversing thermometers. Pressure was generally checked with a single unprotected/protected thermometer pair at the deepest calibration depth.

Deep CTD Calibration

Temperature

Deep CTD temperatures were calibrated at sea by:

T(degC) = Traw * 0.000499156 + 0.034 (JS)

derived from laboratory calibrations.

57 comparisons of CTD values against reversing thermometers were made. Summary statistics given in Table 1a show a trend with depth or temperature.

Table 1a. Temperature Calibrations

Comparison of JS with other calibrations over more than a year (Table 1b.) shows JS to be anomalous.

Table 1b. Time Series of CTD Laboratory Calibrations

* JS, PMS refer to the calibration equations used.

As JS was exceptional the whole instrument being immersed in the calibration bath rather than only the sensor assembly, it was checked whether a different calibration would result. A least squares fit to the data of 16/12/1983, omitting the lowest temperatures measured, yielded:

T(degC) = Traw * 0.00049942 + 0.027 (JS*)

which was no longer anomalous and was very close to the equation used by Saunders and Manning (1984):

T(degC) = Traw * 0.00049943 + 0.027 (PMS)

If the PMS calibration was used in preference to JS, the trend in differences (thermometer - CTD) was removed. Ship calibrated temperatures were linearly adjusted using the equation:

T(PMS) = 1.00056* T(JS) - 0.007

Pressure

There were 35 unprotected reversing thermometer readings, from which the pressure errors (P(thermometer) - P(CTD)) were found to have a mean of 0.5 dbar and a standard deviation of 4.1 dbar.

Salinity

As considerable problems were experienced with salinity calibration, post-cruise corrections were applied to salinity directly, rather than to conductivity. It was inferred that

1. The stability of the (PMS-CTD) errors from several samples on a single cast showed that PMS values were preferable to bottle values
2. 40 per cent of the bottle values were in error by 4ppm or more
3. CTD calibration drifted considerably.

After elimination of faulty values, 93 salinity calibration values remained. It was found that salinities at depth were too low compared to shallow values, by 5ppm at 2000 dbar relative to 0 dbar. A piecewise linear pressure correction was applied as given in Table 2.

Table 2. Final Depth Correction to be Subtracted from CTD Salinities

After completing calibrations, hysteresis was found between down and up O/S plots, with downcasts giving higher salinities by 1-4ppm than upcasts at the same potential temperature. Sudden salinity changes of 1-2ppm were also apparent.

Oxygen

59 samples were available for oxygen calibrations, which gave a mean offset of 0.1 ml/l and standard deviation of 0.2 ml/l (Table 3a.).

Table 3a. Oxygen Errors for Cruise 145

A trend of about 0.1 ml/l/1000 dbar was observed, so the data were piecewise linearly corrected as shown in Table 3b.

Table 3b. Final correction to be Added to Deep-CTD Oxygens

Since the calibration method and equations used here were the same as those used by Pollard (1985), it was concluded that absolute errors may have been as large as 0.3-0.4 ml/l.

New Shallow CTD Calibration

This instrument was used on Station 10986 only.

Temperature

Following comparison of two reversing thermometer measurements, the temperature was left unchanged.

Salinity

An absolute error of no more than 0.005 psu was estimated.

Oxygen

This was omitted due to very poor results.

Editing and Data Quality

The raw data were edited before initial averaging, CTDAVG was modified and errors were checked. The absolute accuracy of T and S was estimated as 3 mK and 0.005 psu, with errors rising to perhaps 20 mK of 0.02 in regions of strong gradients. Even after extensive editing, oxygen data were of relatively poor quality due to the uneven lowering rate caused by surface wave conditions, causing the signal to oscillate peak to peak by about 0.1 ml/l.

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: