Metadata Report for BODC Series Reference Number 382026

• 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 174 CTD Data Documentation

Introduction

Documentation for CTD data collected on RRS Discovery Cruise 174 (May - June 1988) 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 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 SeaTech 1m path transmissometer.

Sampling Protocols

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 sea water samples were analysed using a Guildline Autolab Salinometer.

Calibrations

Pressure

The calibration was based on laboratory measurements and employed the equation for the CTD lowering

P = 0.09966 * PRAW + 5.17E-7 * PRAW**2 - 7.0 - 0.39(Ts-10)

A quadratic expression was found to give an improved fit to the calibration data over the linear one and the last term expresses the measured dependence of output on the sensor temperature. The temperature of the sensor (Ts) lags considerably behind any changes in ambient temperature and a lagged temperature was constructed in the manner described for the oxygen sensor in Saunders (1985). Based on laboratory observations of the response of the sensor to thermal shock, a time constant of 400 seconds was employed. For the raising of the CTD a correction for the hysteresis of the sensor was applied for the first time. The correction was based on laboratory measurements of the difference between the sensor output when the applied pressure is increasing and when the applied pressure is decreasing both for the same external pressure. With the sensor on this CTD, hysteresis effects for a deep cast (> 3000 dbars) reach a maximum value of about 5 dbars at 500 dbar on the upcast and are only 20 per cent smaller for a 1500 dbar lowering.

Temperature

The calibration based on pre-cruise measurements was:

T = 0.0049953 * TRAW + 0.026

This equation has remained virtually unchanged since the acquisition of the platinum resistance thermometer in 1983. However, in February 1989 it appeared to have fallen by approximately 3 milliK. Because the platinum resistance thermometer has a time constant of about 0.25 seconds a lag correction is made as described in Saunders (1985). This procedure is used to reduce spikes in the salinity values.

Comparisons were made on this cruise with a pair of SIS digital reversing thermometers (serial numbers 156 and 180), the results of this are given in the table below. The agreement is satisfactory.

Intercomparison of CTD and Digital Reversing Thermometers

Conductivity/Salinity

Conductivity is not yet calibrated in the laboratory at IOSDL: the conductivity cell factor is carried forward from cruise to cruise. A value of 0.99982 was adopted for use on the ship. For the first time corrections were made for changes in the cell dimensions with pressure and temperature. The equation used was (Crease et al, 1988)

C = 0.99982 * CRAW (1 - 6.5E-6 * (T-15) + 1.5E-8 * P)

Approximately 1580 samples of salinity were analysed on the ship using a Guildline Bench Salinometer, standardised against IAPSO Standard Seawater (batch P107, K=0.99997). These revealed that the nominal CTD salinity needed to be corrected by between 0.003 and 0.005 during the cruise and these corrections were made.

Comparison of CTD and Sample Salinities

Oxygen

A brand new oxygen sensor was available for use on the cruise and some 150 samples were drawn for the Winkler titration method. Plots of oxygen sample values versus potential temperature showed considerable scatter and values approximately 10 per cent lower than Transient Tracers in the Ocean (TTO) values which had been measured (in 1981) at all the three locations on this cruise. After investigation it was found that the sulphuric acid used to fix the sample at an early stage was well below the approved strength and reluctantly the sample measurements were discarded.

Using the TTO data and the observation that, for the Iceland Basin and Charlie Gibbs Fracture Zone sections, near a potential temperature of 3 °C, the dissolved oxygen concentration is 6.30 ml/l (or 274 micromol/kg) and also there exists a slight over-saturation in the near surface values, the various coefficients in the oxygen algorithm for the CTD sensor were calculated. These were:

cell constant = 1.577E-3
alpha = -0.0461 per °C
beta = 0.000136 per dbar
and CTD temperature fraction 0.33 (see Saunders, 1985)

Adjustments were made to yield the canonical value of 6.3 at the potential temperature value of 3 °C. For the data for the Faroes region, the sub- zero temperature water was assumed to have a dissolved oxygen value of 6.9 ml/l (300 micromol/kg).

Transmittance

Nominal transmittance values were calculated for these data, not potential transmittance as on earlier cruises.

Data Processing

Original values were averaged over an interval of one second and calibration coefficients and correction factors applied.

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. Only a limited amount of editing of the data was required. Data were sorted on pressure, averaged at 2 dbars and missing values were interpolated.

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

References

Crease, J. et. al. 1988.
The acquisition, calibration and analysis of CTD data. UNESCO Technical Papers in Marine Science. No. 54, 96pp.

Fofonoff, N.P. and Millard, R.C. 1983.
Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science. No. 44, 53pp.

Saunders, P.M. 1985.
Collection, calibration and processing of CTD data at IOS. International Council for the Exploration of the Sea. C.M. 1985/C:5 12pp.

Saunders, P.M. and Roussopoulos, M. 1989.
CTD data from the RRS Discovery cruise 174, Faeroe Islands to the Charlie-Gibbs Fracture Zone. Institute of Oceanographic Sciences Deacon Laboratory, Report No. 271, 58pp.

Transient Tracers in the Ocean (TTO) 1986.
North Atlantic Study - Shipboard Physical and Chemical Data Report. Scripps Institution of Oceanography, SIO Reference Series 86-15, 720pp.

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: