Metadata Report for BODC Series Reference Number 184461

• 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

Guildline 8770 CTD profiling system and 8709 probe

The Guildline 8770 is a portable ocean profiling system comprising a probe (model 8709), control unit and winch. Three versions of the probe are available: standard, Artic and batfish. The differences between the versions lie in numbers and types of connectors, and the existence of a protective cage (not present in the batfish version). The probe has a nominal drop rate of 1 ms^-1 and a pressure rating of 1000 dbar.

The probe includes a resistance thermometer temperature sensor, a strain gauge pressure sensor, a conductivity cell, a temperature-compensated polarographic dissolved oxygen sensor and a combination pH sensor incorporating a glass measuring electrode and a calomel reference electrode. The probe digitises all measurements, and phase encodes and transmits the data to the control unit.

Specifications

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

RV Cirolana Cruise 6/1988 CTD Data Documentation Part 1

Introduction

Documentation for CTD data collected on R.V. Cirolana Cruise 6/88 (June-July 1988) by the Ministry of Agriculture, Fisheries and Food, Fisheries Laboratory, Lowestoft, Suffolk, UK, under the direction of K. Medler.

Instrumentation

The instrument used was a Guildline Model 8770 CTD. The system consisted of a probe and control unit which measured (or calculated) pressure, conductivity ratio, temperature and salinity. The conductivity sensor was an electrode mounted in pyrex glass, the pressure sensor was a strain gauge element in a four arm resistance bridge network and the temperature sensor was a copper resistance element in a stainless steel tube. The probe electronically measured the parameters and sent the data to the control unit after digitising the results.

The control unit converted the data to binary format, displayed the results, had analog output for chart recording and allowed the data to be recorded on audio cassette. This latter facility was usually a precaution against information loss since data were logged (at 1 sample per second) on either a Hewlett Packard mini computer or an Apricot microcomputer. Salinity was calculated by a microprocessor within the system. The algorithm used was that for the Practical Salinity Scale.

The CTD was mounted on a General Oceanics multisampler fitted with Niskin bottles. This enabled the data to be logged whilst the water sample was being collected. Attaching thermometers to the Niskin bottles allowed comparison to be made between reversing thermometers and the CTD temperature sensor. An altimeter had been fitted to the multisampler, which allowed a comparison to be made with the pressure calibration.

Data Processing

Data were logged every one second. Since the lowering rate was usually less than one metre per second, values were often recorded at between 0.2 and 0.4 decibar intervals. These data have not been averaged. However, they have been plotted to identify spurious readings. Estimates of density were also used to help identify suspect values, but in view of the limited accuracy of the instrument and because the water column was well mixed it is inevitable that some density inversions will exist in the data.

RV Cirolana Cruise 6/1988 CTD Data Documentation Part 2

Calibrations and Data Quality

Both the pressure and temperature sensors of the CTD were calibrated using the pre-cruise laboratory calibration. The salinity of water samples collected at thirty-one stations was used to calibrate the conductivity sensor. After rejecting a few samples because of salinity differences >0.006, a total of 162 samples (mostly from duplicates) remained with which to determine the calibration coefficients.

Pressure

In the past, we have tried to corroborate the laboratory calibration by comparing CTD pressure with thermometric pressures from the DSRT. Because of inaccuracies in the latter, this method is no longer used. We have also considered previously how the sample level could be determined from PDR water depth and altimeter height above the seabed. It was impossible to reconcile these sample levels with the CTD pressure when the instrument was used in deep water. Consequently, the pressure sensor was calibrated before and after the cruise with a dead weight tester. Good agreement was obtained and the pre-cruise calibration was used to correct pressure values.

Temperature

Laboratory calibrations conducted before and after the cruise were in fair agreement. They did suggest a small difference (4 to 10 mK) and an attempt was made to detect a change in the sensor response during the cruise by comparing CTD temperature with those from DSRTs. No evidence for a systematic change was found, but the accuracy of our DSRTs would probably prohibit the detection of a response change of the magnitude suggested by the laboratory calibration.

Temperatures were corrected using the pre-cruise calibration.

Conductivity Ratio

The method adopted was to first calculate the conductivity ratio that the water sample would have, if given its measured salinity, at the pressure and temperature at which it was collected. The ratio of this conductivity to that recorded by the CTD was then fitted by least squares to

[CR(WS)/CR(CTD)]= a x T(CTD) + b x P(CTD) + C
where T(CTD) and P(CTD) are the corrected CTD temperature and pressure.

The rms difference between water sample salinity and CTD salinity was 0.011.

Better agreement was obtained if, for each station, an estimated cell coefficient, based on the mean CR(WS) :CR(CTD) ratio, was evaluated before determining the coefficients a, b and c above. This gave an rms difference of 0.005. In view of the tolerances in CTD and bench salinometer measurements this seems as good agreement that could realistically be expected.

A feature of these data was the apparent change in response of the conductivity sensor during the first eight stations (numbers 24 to 71). This is shown in Fig. 2 which displays the ratio CR(CTD) :CR(WS) for all stations (numbers 24 to 71 and 72 to 103) a closer look at the former group, Fig. 3, suggests that at the first two stations (24 and 39) the ratio is approximately 1.0015 and that it appears to change when the four upper samples are taken at the following station (42), the ratio approaching 1.0025. At the next three stations (44, 56 and 68) the ratio remains near 1.0025 (and in this respect is similar to stations 72 through to 103). During stations 70 and 71, however, the ratio decreases to 1.0015 before increasing to 1.0025.

Subsequent examination of theta-S plots from adjacent stations suggested that, in order to obtain consistent data, it was necessary to assume that the CR(CTD):CR(WS) ratio at station 42 was similar (i.e. approx. 1.0025) to that during stations 44, 56 and 68. The theta-S distributions also indicated that the cell factor derived for station 39 was too large and a better alternative was to assign to this station the cell factor calculated for station 42.

Calibration Coefficients for CTD Data Cirolana 6/1988

Pressure

P(COR) = P(CTD) + PA*T(COR) + PB*P(CTD) + PC where T(COR) = corrected CTD temperature
PA = 6.9646 E-1
PB = 3.85187 E-3
PC = -3.74

Temperature

T(COR) = T(CTD) + TA*T(CTD)*T(CTD) + TB*T(CTD) + TC
where TA = -1.70167 E-5
TB = -7.79426 E-3
TC = -7.3795 E-3

Conductivity Ratio

CR(COR) = CR(CTD) x {CF x (CA x T(COR) + CB x P(COR) + CC)}
where CA = 5.579934 E-5
CB = -3.253974 E-8
CC = 0.9998239
CF is the cell factor for each station as follows:

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: