Metadata Report for BODC Series Reference Number 55535


Metadata Summary

Data Description

Data Category CTD or STD cast
Instrument Type
NameCategories
Neil Brown MK3 CTD  CTD; water temperature sensor; salinity sensor; dissolved gas sensors
Instrument Mounting research vessel
Originating Country United Kingdom
Originator -
Originating Organization Institute of Oceanographic Sciences Wormley Laboratory (now National Oceanography Centre, Southampton)
Processing Status banked
Project(s) -
 

Data Identifiers

Originator's Identifier WPRDI10048BW
BODC Series Reference 55535
 

Time Co-ordinates(UT)

Start Time (yyyy-mm-dd hh:mm) 1979-06-02 01:02
End Time (yyyy-mm-dd hh:mm) -
Nominal Cycle Interval 2.0 decibars
 

Spatial Co-ordinates

Latitude 4.09000 S ( 4° 5.4' S )
Longitude 42.18000 E ( 42° 10.8' E )
Positional Uncertainty 0.1 to 0.5 n.miles
Minimum Sensor Depth 0.99 m
Maximum Sensor Depth 1970.63 m
Minimum Sensor Height 1329.37 m
Maximum Sensor Height 3299.01 m
Sea Floor Depth 3300.0 m
Sensor Distribution Variable common depth - All sensors are grouped effectively at the same depth, but this depth varies significantly during the series
Sensor Depth Datum Instantaneous - Depth measured below water line or instantaneous water body surface
Sea Floor Depth Datum Instantaneous - Depth measured below water line or instantaneous water body surface
 

Parameters

BODC CODE Rank Units Short Title Title
PRESPR01 1 Decibars Pres_Z Pressure (spatial co-ordinate) exerted by the water body by profiling pressure sensor and corrected to read zero at sea level
SSALPR01 1 Parts per thousand UspSal Salinity of the water body by conductivity cell
TEMPST01 1 Degrees Celsius WC_temp_CTD Temperature of the water body by CTD or STD
 

Definition of Rank

  • Rank 1 is a one-dimensional parameter
  • Rank 2 is a two-dimensional parameter
  • Rank 0 is a one-dimensional parameter describing the second dimension of a two-dimensional parameter (e.g. bin depths for moored ADCP data)

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