Metadata Report for BODC Series Reference Number 545927

• 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

Falmouth Scientific Integrated CTD (ICTD) Profiler

The FSI ICTD is designed to collect high precision conductivity, temperature and pressure data with self calibrating electronics. This instrument can support five primary sensors (including up to three temperature sensors) and can be coupled with a water bottle sampler. The ICTD is equipped with a titanium housing rated to 7000 m and has a sampling rate of 32 Hz.

Three temperature sensors are available: primary platinum, redundant platinum and exposed thermistor. Any combination of these can be used in the primary channels. The instrument also has multiple RS-232 serial inputs for a variety of sensors including: ADCP, Benthos PSA-916 Altimeter and WetLabs SAFire. There are an additional eight DC input channels that can support virtually any sensor that has a DC output.

Specifications:

*Optional

Further details can be found in the manufacturer's specification sheet.

RV Corystes 1B/1994 CTD Data Documentation

Instrumentation and Protocols

This cruise used the FSI ICTD s/n 1322 at 6 CTD stations. Reversing thermometers were used to measure water temperature and samples were collected for salinity analysis using the portable salinometer.

Reversing thermometers were read on just two occasions and these indicated a difference between thermometer and CTD of 0.00 °C.

Water samples were collected on 10 occasions for analysis with a portable salinometer. Only one sample had duplicate salinity estimates. The instrument was standardised with Standard Seawater of batch P123.

Sensor Calibration for FSI CTD

Pressure

A laboratory calibration during October 1993, at room temperature, suggested that no correction to the recorded CTD pressure was required, but it was noticed that when on deck the CTD displayed a value of 0.1 db and this was taken as an offset to be corrected.

i.e. P(cor) = P(ctd) - 0.1 db

Temperature

The comparison of thermometer and CTD temperatures was in good agreement with the laboratory calibration of 13 October 1993, although thermometer readings were limited to just two.

The sensor was therefore corrected using the laboratory calibration coefficients.

T(cor) = T(ctd) + dT

where dT = -0.004

Salinity

Fig. 1 shows the difference between the water sample salinity as measured with the salinometer and that derived from the CTD before any calibrations have been applied to the latter's sensors.

Fig. 1 suggests that the CTD has a tendency to over-estimate salinity by 0.006 (standard dev. 0.003, 10 values).

Fig. 2 shows the difference between salinometer and CTD salinity after the temperature and pressure sensors have been corrected. The `offset' has decreased to 0.002 (0.003, 10).

Fig. 3 shows the ratio of CTD:Water sample conductivity ratio, also after the CTD pressure and temperature sensors have been corrected using the above coefficients.

A least square fit was used to determine calibration coefficients for the CTD conductivity ratio:

CR(cor) = CR(ctd)*{a*T(cor) + b*P(cor) + c}

where:

T(cor) and P(cor) are the corrected CTD temperature and pressure
a = -2.647128e-4
b = -6.9316356-6
c = 1.001472

RMS salinity difference between water sample and corrected CTD is 0.002 for 10 data values.

Figs. 4 and 5 illustrate how effective the CTD conductivity and derived salinity have been corrected. The tendency of the CTD to over-estimate salinity has been effectively compensated for by the calibration.

The histograms in Fig. 6 show how well the CTD conductivity is corrected since the upper frame has been derived after the CTD temperature and pressure have been corrected, but before the CTD conductivity calibration has been applied.

If it is assumed that the salinometer is accurate to 0.006 and the CTD salinity to 0.01, then differences between ±0.016 are acceptable. All ten values lie well within this range after the calibrations have been applied, although in this instance this applies to the estimates prior to any sensors being calibrated (see Fig. 1).

Transmission/Suspended Load

A 25 cm path length transmissometer (s/n 198) was mounted on the rosette at each CTD station. The instrument was calibrated in terms of suspended load (mg/1) by comparing the logged %transmission with laboratory determinations of suspended load.

suspended load = a*loge(%Trans) + b

where a = -12.84 and b = 55.16

Ken Medler
14 April 1994

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: