Metadata Report for BODC Series Reference Number 88448

• 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 130 CTD Data Documentation

Introduction

This document covers the CTD data collected on RRS Discovery Cruise 130 (June - August 1982) 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 SeaTech transmissometer.

Sampling Protocols

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.

Stations 10560 to 10574 were shared between the CTD/Multisampler and a 14m long hydrophone employed in listening for moored sound sources. Listening was performed on lowering and both CTD and bottle sample data were obtained on recovery. For stations 10555, 10556 and 10575 to 10599 the hydrophone was removed and CTD data were gathered on lowering and bottle samples obtained on recovery.

On seven stations during the cruise (10575, 10576, 10585, 10587, 10592, 10593 and 10594) repeated lowerings were made between 4000m and the bottom.

Calibrations

Pressure

The pressure sensor was calibrated in a laboratory in September 1980 and repeat calibrations of January and October 1982 showed the same slope. The deck pressure offset was stable and a value of 10db was assumed for all stations.

The table below shows differences between the pressures determined from pairs of reversing thermometers (protected and unprotected) and simultaneous observations of CTD pressure.

For stations where the bottom is flat the depth of the CTD can be estimated from the corrected water depth minus the height of the echosounder above the bottom. This is converted to pressure and compared with the measured CTD pressure. 11 such determinations gave the mean difference 3db (CTD lower) and an rms of 6db very similar to the reversing thermometer values.

Temperature

Laboratory calibrations of January 1982 and October 1982 showed the CTD temperature sensor to be stable. By comparison with 57 reversing thermometer measurements between 2 and 23 °C the corrected CTD temperatures were determined from the equation:

T = 0.047 + 0.0005002 * RAWTEMP.

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

The cell factor required to bring provisional CTD salinities into agreement with this relation was determined for each station.

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

If a constant cell factor had been used the deviations from the POTT-S relation measured with the CTD may be plotted - this indicates that the stability of the CTD is good excluding 3 outliers. These are stations 10556, 10563 and 10567 and may be the result of partial fouling.

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 = temperature
p = pressure

A least squares fit of 46 data sets gave:

C = 1.47E-03
a = -0.035 per °C
b = 1.425E-04 per db

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

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

This equation is a reasonable fit to the behaviour of the electrode from 3000m to 5000m where the water is nearly isothermal (2.5 °C - 3.0 °C).

There are some spikes in the oxygen profiles, particularly during the upcasts. These are probably the result halting recovery of the CTD while collecting a bottle sample. These spikes have not been flagged.

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.

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).

References

Saunders, P.M. (1984).
CTD data from the north east Atlantic Ocean 31° N - 46° N, July 1982. RRS Discovery Cruise 130. Institute of Oceanographic Sciences, Report No. 165.

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: