Metadata Report for BODC Series Reference Number 70648

• 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

There are no salinity data for this cast.

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

Plessey 9400 Conductivity, Temperature and Depth

The Plessey 9400 comprises a Plessey 6500 inductive conductivity sensor, a Plessey 4500 platinum resistance temperature sensor and a Plessey 4600 bonded strain pressure sensor.

RV Prince Madog Cruise ISIS 26 CTD Data Documentation

Introduction

Documentation for the CTD data collected on RV Prince Madog Cruise ISIS 26 (August 1981) by the School of Ocean Sciences, University College of North Wales, Menai Bridge, Gwynedd, U.K., under the direction of M. F. Lavin.

Warning

The conductivity calibration bad so no salinity data available from this cruise.

Instrumentation and Protocols

The instrument used was a Plessey model 9400 CTD system together with an 8700 processor unit. Instrument lowering and raising speeds were 1m/s and the instrument sampled every second. Digitised signals from the CTD sea unit were calibrated and checked for erroneous data. A running average was applied to the depth signal to eliminate noise and the temperatures were corrected for the time response of the thermometer. The temperature and conductivity values could, if specified in the control program, be interpolated to 1m intervals. If both the up and down CTD profiles were available, average values of temperature and conductivity at 1m intervals were computed. The data for this cruise are at 1m intervals. Salinity was computed using the algorithm of Cox, Culkin and Riley (1967), and if required, was filtered using a hamming window to reduce the spiking of the computed signal. Sigma-t was calculated using salinity and temperature.

Calibrations and Data Quality

Calibration Procedures

The CTD was not calibrated in the laboratory. The manufacturer's calibration was used and water samples taken to check the calibration and apply corrections where necessary.

Calibration samples using NIO bottles with 2 protected reversing thermometers - fixed 0.5m above the CTD unit - were taken at the bottom and mid-depth. Surface samples (2-3m) were taken using a bottle suspended at the required depth from another winch a few metres from the CTD winch. The samples were collected in regions without gradients. At least five minutes of settling time was allowed for the reversing thermometers. Bottom samples were collected before the up run; surface samples at the end of the cast. Only a few samples were collected at mid-depth. The salinity of the samples was measured on land, at the end of the cruise, using a Bissett Berman 6230 salinometer. The thermometers were left to stabilize for at least 15 minutes. Pairs of temperatures that, after correction, differed by more than 0.05 °C were not used. The Rickard-Miller digital cassette logging system used produced data in the form of periods (microseconds x 100) and the nominal formulae to convert to physical units were as follows.

Pressure

Apart from some sporadic readings in air, the pressure sensor was not calibrated after its factory calibration. Since the correction factor 1.0052 dbar/m was so near to unity, the equation quoted above could be used to provide pressure in decibars (e.g. for salinity calculations), within the accuracy of the instrument, for depths less than about 200m. From the air readings noted above, a shift of -1.5m was applied to the nominal formula.

Temperature

The temperature sensor was calibrated to determine the difference between the CTD temperature values and the corrected reversing thermometer values. A regression was carried out and values of the mean difference and its standard deviation were calculated.

Conductivity

The conductivity sensor was calibrated at the factory to an absolute conductivity value of 47.891 mmho/cm for a sea water of 35ppt at 20 °C and a pressure of 1 atmosphere. The in-situ conductivity at the time of taking the salinity sample was calculated by successive approximations. The salinity value obtained with the Bissett Berman salinometer, the corrected CTD temperature and the CTD depth reading were used as inputs. The difference between the nominal CTD conductivity and the in-situ conductivity of the salinity sample and their standard deviation were calculated, and listed out, so that bad points could be investigated and corrected or deleted. The conductivity was not corrected in an additive way (i.e. by shifting) but by the use of a form factor which is a multiplicative correction. This was done to be consistent with the general use and because the range of values was usually very narrow. The mean form factor was then used to calculate the final accuracy of the standard deviation. Typical values of standard deviation obtained during 1979 and 1980 were about 0.03. During 1981 the conductivity sensor did not work properly, apparently due to an undetected leak in the octopus connector of the sea unit.

Normally a single form factor was applied to each cruise. In 1979 a single form factor was applicable to all the cruises made in that year. During 1981 on some cruises very few or no salinity samples were collected. For those cruises where salinity data were collected for all or most of the casts, an individual form factor was applied to each cast.

The object of the calibration was to obtain small corrections to the nominal formulae and to estimate the standard error associated with the particular sensors.

Fit of CTD Data to NIO Bottle Sample Values

(Second calibrations are with two worst points removed)

References

Cox, R.A., Culkin, F. and Riley, J.P. (1967).
The electrical conductivity / chlorinity relationship in natural sea water. Deep Sea Research, vol. 14, 203 - 220.

Lavin, M.F. (1983).
Report on the CTD calibration for the period 1979 - 1981. University College of North Wales, Oceanography Report 83 - 1.

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: