Metadata Report for BODC Series Reference Number 59099

• 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 138 CTD Data Documentation

Introduction

This document covers the CTD data collected on RRS Discovery Cruise 138 (June - July 1983) 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 10kHz pinger, a bottom echo-sounder and a Sea-Tech transmissometer.

Sampling Protocol

Lowering and retrieval rates of 0.5 to 1.5 m/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.

Calibration

Pressure

The calibration equation based on laboratory measurements was taken to be:

P = 0.1 * PRAW - 10.0

Differences between the pressure measured by the CTD and by pairs of reversing thermometers are shown in the table below.

Temperature

The calibration equation used was:

T = 0.00049943*TRAW+0.027

A comparison between CTD temperatures and reversing thermometers is given in the table below.

On Stations 10797, 10798 a second CTD unit was employed: on Station 10803 both CTDs were lowered together. On the upcast temperature was measured at the same pressure: the differences were only 0.001 for temperatures less than 5° C.

Salinity

In the North East Atlantic the sample measurements show very little scatter about a linear potential temperature (POTT) - Practical Salinity (S) relationship for POTT less than 2.6 °C of:

S = 34.698 + 0.098 * POTT

The cell factor required to bring the CTD salinities into agreement with this equation are calculated for each station. For the first seven stations the conductivity unit was stable but after being fouled by a large (1 square metre) mass of red biological matter on station 10795 (data discarded) the sensor began to drift. It was replaced on Station 10809.

Oxygen

From a sample value of oxygen concentration (determined by the Winkler method) the fractional saturation content was obtained by normalising it by the appropriate saturation value. The Beckman oxygen cell also measures fractional saturation F, given by the equation:

ln F = ln I + ln C + aT + bp

where

I = oxygen probe current
T = ambient temperature (electrode temperature ignored)
p = pressure

The electrode current exhibits a time dependent lag which has been corrected using the expression:

LAG (secs) = 70 e ^{-0.1 * T (ambient) °C}

The comparison between sample oxygen concentrations and CTD oxygen values is shown in the table.

The calibration constants which were used were fitted to sample measurements taken on RRS Discovery Cruise 139 and the resulting oxygen estimates for RRS Discovery 138 are poor. Rescaling of the concentrations to bring the 4000 db value to 5.67 ml/l is suggested.

Transmittance

Potential transmittance, which takes account of the increasing mass of clear water in the 1m path of the instrument with increasing pressure, was calculated. The mean potential transmittance for 16 stations at 4000 db is 69.3 per cent/metre, with a standard deviation of 0.25 per cent.

Fit of CTD data to Rosette Sample Values

Data Procesing

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). To remove the effect of ships heave data cycles were sorted by pressure before all values were averaged at 5db intervals, centred on 2.5db, 7.5db and so on

References

Saunders, P. M. and Dolan, F. 1987.
CTD data from Discovery Gap and the Madeira Abyssal Plain. Institute of Oceanographic Sciences, Report No. 236.

Saunders, P.M. and Manning, A. 1984.
CTD data from the north east Atlantic Ocean 22° N-33° N, 19°W-24°W, July 1983 during RRS Discovery Cruise 138,139. Institute of Oceanographic Sciences, Report No. 188.

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: