Metadata Report for BODC Series Reference Number 55897

• 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 117 CTD Data Documentation

Introduction

This document covers the CTD data collected on RRS Discovery Cruise 117 (Jan - Feb 1981) by the Institute of Oceanographic Sciences (Deacon Laboratory) Godalming, Surrey, UK., under the direction of P. M. Saunders.

Instrumentation

The instrument used was a Neil Brown Instrument Systems CTD which measured pressure, temperature and conductivity and was fitted with a Beckman dissolved oxygen electrode. The CTD was used alongside a General Oceanics Rosette Multisampler with 12 water bottles, a 10 kHz pinger, a bottom echo sounder and a Sea Tech transmissometer.

Sampling Protocol

Lowering and retrieval rates of 0.5 to 1.5m/s were employed and the sensors were flushed with distilled water on recovery. Bottle samples and reversing thermometer measurements were made on ascent and the sea water samples were analysed using a Guildline Autolab Salinometer. Reversing thermometers were calibrated before and after the cruise.

Two CTD units were employed during the cruise, the first for stations 10261 and 10263 and the second for the remainder. After 11 stations with the second unit the conductivity cell was replaced with the hope of reducing the calibration drift.

On four stations during the cruise (10275, 10276, 10279 and 10285) repeated lowerings were made in the bottom 500 m of the water column.

Calibrations

Pressure

The deck pressure offset was stable and a value of 8 db was assumed for all stations. The table below shows differences in pressures determined from pairs of reversing thermometers (protected and unprotected) and simultaneous observations of CTD pressure.

Pressures at the bottom of the cast were converted to depth and added to the CTD height above the bottom to give water depth. The echo sounder depth was corrected using Carter's tables and the 2 depths compared. In the measured depth range 5200-5500 m the corrected echo sounder depth exceeded that calculated from the CTD for 25 observations by a mean of 6 m with a standard deviation of 5 m.

Temperature

Laboratory calibrations were carried out in September 1980. Corrected CTD temperatures were determined from the equation:

T = 0.044 + 0.00050042 * RAWTEMP

A comparison is given in the table below between the CTD temperatures and reversing thermometers separating the deep and shallow measurements.

Salinity

During the cruise bottle salinities and reversing thermometer measurements revealed a linear potential temperature (POTT) - practical salinity (S) relationship for POTT less than 2.6 °C of:

S = 34.698 + 0.098 * POTT

Employing a nominal cell factor of 1.0 * 10 ^-3 provisional salinities were computed and the multiplicative factor required to bring the provisional salinities into agreement with the expected POTT - S relationship was then determined.

For the cell employed on stations 10264 - 10274 this factor varied between 0.99951 and 0.99974 (but not smoothly) corresponding to a salinity change of 0.008.

For the cell used on stations 10275 - 10294 this factor varied between 1.00027 and 1.00039 corresponding to a change of 0.004.

The table below compares salinities derived from the rosette sampler and Guildline salinometer with the CTD values.

Fit of CTD Data to Rosette Sample Values

The depth of the 5 °C isotherm is approximately 1800 db.

Data Processing

Original values were averaged over an interval of one second and calibration coefficients and correction factors applied. A time constant correction algorithm was employed to compensate for the slower response of the platinum resistance thermometer.

Differences between successive values of each parameter were examined; the mean difference and its standard deviation calculated and values greater than several standard deviations from the mean difference were checked. Genuinely suspect data were then replaced by interpolated values.

Derived quantities were computed from algorithms published by Fofonoff and Millard (1983).

At ten of the stations (10268, 10270, 10271, 10272, 10288, 10290, 10291, 10293 and 10294) the data cycles were sorted by pressure to remove the effect of ships heave before all values were averaged at 5 db intervals, centred on 2.5 db, 7.5 db and so on.

References

Saunders, P.M. (1981).
CTD data from the north Madeira Basin: 19 January - 12 February 1981 Discovery Cruise 117. Unpublished manuscript.

Fofonoff, N.P. and Millard Jr., R.C. (1983).
Algorithms for the computation of fundamental properties of sea water. UNESCO Technical Paper on Marine Science 44.

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: