Metadata Report for BODC Series Reference Number 55338
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
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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.
| Pressure | Temperature | Conductivity | |
| Range | 6500 m 3200 m (optional) | -3 to 32°C | 1 to 6.5 S cm-1 |
| Accuracy | 0.0015% FS 0.03% FS < 1 msec | 0.0005°C 0.003°C < 30 msec | 0.0001 S cm-1 0.0003 S cm-1 < 30 msec |
Further details can be found in the specification sheet.
RRS Discovery Cruise 102 CTD Data Documentation
Introduction
This document covers the CTD data collected on RRS Discovery Cruise 102 (May - July 1979) 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. The CTD was used alongside a 10kHz pinger.
Sampling Protocol
Lowering and retrieval rates of 0.5 to 1.0 m/s were employed and the sensors were flushed with distilled water on recovery. Bottle samples and reversing thermometer measurements were made on descent and the sea water samples were analysed using a Guildline Autolab salinometer. Reversing thermometers were calibrated before and after the cruise.
CTD unit 1 (used on Leg 1 of the cruise) was interfaced with an IOS-built vector averaging current meter which prevented use of the multisampler. As a consequence there are relatively few intercomparisons between reversing thermometers and the CTD, and sample salinity comparisons were made indirectly.
On Leg 2 of the cruise a different CTD instrument was used. This model incorporated a Beckman polargraphic dissolved oxygen sensor and a General Oceanics Rosette multisampler with 12 bottles was used.
Calibration and Data Quality (Cruise 102 Leg 1)
Pressure
In situ calibration of the pressure sensor was necessary because of the utilisation of an uncalibrated electronics board. 26 comparisons with unprotected thermometers mounted on NIO bottles clamped to the wire 1m above the CTD unit gave a pressure calibration coefficient of 0.09355 (nominal value 0.1). The table below shows the differences in pressures determined from pairs of reversing thermometers (protected and unprotected) and simultaneous observations of CTD pressure.
Temperature
Comparing temperature difference between reversing thermometers (at both the top and bottom of the cast) and the uncorrected CTD, values revealed a difference of 0.025 °C increasing with temperature. Corrected CTD values were obtained using the equation:
T = 0.025 + 0.0005015 * RAWTEMP.
A comparison is given in the table below between the CTD temperatures and reversing thermometers.
Salinity
Comparisons made between nominal salinities from the CTD and sample salinities at the same potential temperature but made on a separate cast, revealed considerable drift in the CTD conductivity cell calibration. The cell factor varied from 0.0009909 to 0.0009925 during the course of the cruise and large pressure and temperature dependence was found. The drift in the calibration corresponded to a salinity change of nearly 0.1 ppt. The table below compares salinities derived from the NIO bottles with the CTD values.
Fit of CTD to NIO bottle data
| Reversing Thermometers/Sample minus corrected CTD value | ||||
|---|---|---|---|---|
| Variable | Range | Mean Difference | Standard deviation | No. of values |
| Pressure (db) | 700-2000db | 0 | 5 | 26 |
| Temperature (°C) | 2-5 °C | +0.003 | 0.005 | 17 |
| 5-15 °C | -0.008 | 0.010 | 8 | |
| 15-30 °C | +0.002 | 0.015 | 17 | |
| Salinity (ppt) at temperatures | 2-5 °C | +0.0005 | 0.004 | 38 |
| 5-15 °C | -0.003 | 0.012 | 179 | |
| 15-30 °C | -0.010 | 0.021 | 51 | |
Oxygen was not measured with the CTD on this leg.
Approximate conversion of temperature to pressure is as follows:
| 5 °C | 1500±100 db |
| 15 °C | 175±25 db |
Calibration and Data Quality (Cruise 102 Leg 2)
Pressure
No change was made to the laboratory calibration of the sensor (made with a dead weight tester) except for the offset of 29±1db. The table below shows differences in pressures determined from pairs of reversing thermometers (protected and unprotected) and simultaneous observations of CTD pressure.
Temperature
Comparing temperature difference between reversing thermometers and the uncorrected CTD value revealed an offset of 0.065 °C increasing slightly with temperature. The instrument was calibrated in November 1978. Corrected CTD values were obtained using the equation:
T = 0.063 + 0.0005005 * RAWTEMP.
A comparison is given in the table below between the CTD temperatures and reversing thermometers.
Salinity
From on board determinations of salinity using the Guildline Autolab Salinometer sample conductivities at corrected CTD pressure and temperature (Cs) were calculated and compared with in situ conductivities (C). Using:
Cs/C = CCR(1+aT+bp)
a least squares fit of 297 sets of observations gave
CCR = 0.000999737
a = 0.952E-05 per °C
b = 0.809E-07 per db
The table below compares the salinities derived from the rosette sampler with the CTD values - the conductivity cell was very stable on this leg.
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 given by the equation:
ln F = ln I + ln C + aT + bp
where
I is the oxygen sensor raw current
T is a weighted temperature - (0.25 x T(ambient) + 0.75 x T(oxygen))
p is pressure
From the 297 sets of data on leg 2 of this cruise a least squares fit gave:
C = 1.43E-03
a = -0.0323 per °C
b = 1.263E-04 per db
The comparison between sample oxygen concentrations and CTD oxygen values is shown in the table below.
Fit of CTD to Rosette Sample Data
| Reversing Thermometers/Sample minus corrected CTD value | ||||
|---|---|---|---|---|
| Variable | Range | Mean Difference | Standard Deviation | No. of Values |
| Pressure (db) | 300-2000db | 2 | 3 | 63 |
| Temperature (°C) | 2-5 °C | -0.002 | 0.005 | 50 |
| 5-15 °C | -0.004 | 0.007 | 65 | |
| 15-30 °C | +0.001 | 0.012 | 48 | |
| Salinity (ppt) at temperatures | 2-15 °C | 0 | 0.003 | 311 |
| 15-30 °C | 0 | 0.007 | 107 | |
| Oxygen (ml/l) at temperatures | 2-15 °C | -0.05 | 0.22 | 216 |
| 15-30 °C | 0.13 | 0.39 | 81 | |
Approximate conversion of temperature to pressure is as follows:
| 5 °C | 1500±100 db |
| 15 °C | 175±25 db |
Data Processing (Legs 1 and 2)
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 et al (1974). To remove the effect of ships heave data cycles were sorted by pressure before all values were averaged at 2db intervals, centred on 1db, 3db and so forth.
References
Saunders, P.M. (1980).
CTD data from the western equatorial Indian Ocean 10 May - 6 July 1979: Discovery Cruise 102. IOS Data Report No. 23. Unpublished manuscript.
Fofonoff, N.P., Hayes, S.P. and Millard Jr., R.C. (1974).
WHOI/Brown CTD microprofiler: methods of calibration and data handling. Woods Hole Oceanographic Institution WHOI-74-89.
Project Information
No Project Information held for the Series
Data Activity or Cruise Information
Cruise
| Cruise Name | D102_1 |
| Departure Date | 1979-05-10 |
| Arrival Date | 1979-06-04 |
| Principal Scientist(s) | John C Swallow (Institute of Oceanographic Sciences Wormley Laboratory) |
| Ship | RRS Discovery |
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:
| Flag | Description |
|---|---|
| Blank | Unqualified |
| < | Below detection limit |
| > | In excess of quoted value |
| A | Taxonomic flag for affinis (aff.) |
| B | Beginning of CTD Down/Up Cast |
| C | Taxonomic flag for confer (cf.) |
| D | Thermometric depth |
| E | End of CTD Down/Up Cast |
| G | Non-taxonomic biological characteristic uncertainty |
| H | Extrapolated value |
| I | Taxonomic flag for single species (sp.) |
| K | Improbable value - unknown quality control source |
| L | Improbable value - originator's quality control |
| M | Improbable value - BODC quality control |
| N | Null value |
| O | Improbable value - user quality control |
| P | Trace/calm |
| Q | Indeterminate |
| R | Replacement value |
| S | Estimated value |
| T | Interpolated value |
| U | Uncalibrated |
| W | Control value |
| X | Excessive difference |
SeaDataNet Quality Control Flags
The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:
| Flag | Description |
|---|---|
| 0 | no quality control |
| 1 | good value |
| 2 | probably good value |
| 3 | probably bad value |
| 4 | bad value |
| 5 | changed value |
| 6 | value below detection |
| 7 | value in excess |
| 8 | interpolated value |
| 9 | missing value |
| A | value phenomenon uncertain |
| B | nominal value |
| Q | value below limit of quantification |
