Metadata Report for BODC Series Reference Number 1020822
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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.
These specification apply to the MK3C version.
3200 m (optional)
|-3 to 32°C||1 to 6.5 S cm-1|
0.03% FS < 1 msec
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 D233 CTD Instrumentation
The CTD profiles were taken with a WOCE Standard Neil Brown Systems MkIIIc CTD (DEEP04), with a FSI 24 bottle rosette.
|Sensor||Manufacturer/Model||Serial Number||Last calibration date||Comments|
|CTD||Neil Brown MkIIIc||DEEP04||-||Incorporating an oxygen sensor|
|Fluorometer||Chelsea Instruments Aquatracka||88/2360/108||-||-|
|Transmissometer||Sea Tech transmissometer (1 m pathlength)||161/2642/003||-||Cruise reported stated this to be a Chelsea Instruments Transmissometer|
|Altimeter||Simrad||200 m range||-||-|
|LADCP||RDI (150 kHz) - 30 degree beam||-||-||-|
|Reversing thermometers||SIS||T401||-||Failed after stations 13415 and 13416|
|T989||-||Lost at station 13462|
|T714||-||Failed after stations 13415 and 13416|
|Reversing pressure meters||SIS||P6075||-||Sensor failed on station 13440|
|P6132||-||Lost at station 13462|
The Chelsea Instruments Aquatracka is a logarithmic response fluorometer. It uses a pulsed (5.5 Hz) xenon light source discharging between 320 and 800 nm through a blue filter with a peak transmission of 420 nm and a bandwidth at half maximum of 100 nm. A red filter with sharp cut off, 10% transmission at 664 nm and 678 nm, is used to pass chlorophyll-a fluorescence to the sample photodiode.
The instrument may be deployed either in a through-flow tank, on a CTD frame or moored with a data logging package.
Further details can be found in the manufacturer's specification sheet.
RRS Discovery Cruise D233 CTD Processing
CTD profile data are presented from the CHAOS (Chemical and Hydrographic Atlantic Ocean Survey) cruise Discovery 233, as reported by Smythe-Wright (1999).
Originator's Data Processing
A total of 140 CTD stations (13414-13553) were completed during D233. Lowering rates for the CTD package were generally in the range 0.5-1.0 ms-1 but could be up to 1.5 ms-1. Water samples were drawn on casts for the calibration of the instrumentation.
Data Acquisition and Processing
Data were captured by the SOC-DAPS (Southampton Oceanography Centre - Data Acquisition and Processing System) software. The DAPS software checks for pressure jumps and subsequently produces 1 sec averages of the raw 25 Hz data. Two ASCII files are generated for each cast, one for CTD profile data, the other for bottle firing data. Post processing of these output files was conducted in the PSTAR environment using the PEXEC suite of programs.
During D233, the CTD data were also logged simultaneously by the RVS Level A system and used in a backup capacity.
A summary of the CTD calibrations applied to instrumentation are presented below. For further details please refer to the D233 Cruise Report.
Raw temperatures were scaled according to:
Traw = 0.0005 Traw
then calibrated using the following equation (obtained from laboratory calibrations in July 1996):
T = 0.13079 + 0.999314 Traw
Due to a lag between the conductivity and temperature sensor measurements the time rate of change of temperature was used to "speed up" the temperature measurements according to:
T = T + τ δ T / δ t
where the rate of change of temperature is determined over a one second interval. The time constant, τ = 0.25, was used for D233.
Post-cruise calibration coefficients (obtained in October 2008) were subsequently compared with those documented above. No modifications were deemed necessary from these investigations.
Raw pressure measurements were first scaled according to:
Praw = 0.1 Praw
then calibrated using the calibration (obtained from laboratory calibrations in July 1996):
P = -36.685 + 1.07333 Praw
Following laboratory calibration, no further corrections were deemed necessary for temperature dependence or pressure hysteresis.Post-cruise calibration coefficients (obtained in October 2008) were subsequently compared with those documented above. As with temperature, no further changes were required.
Raw conductivities were scaled according to:
Craw = 0.001 Craw
then calibrated using the following equation:
C = -0.015 + 0.96743 Craw
The coefficients above were obtained from comparison of CTD data with bottle samples from all water depths from the first seven casts. Additional small offsets were added to the correction periodically and applied to groups of stations. These offsets were obtained following examination of subsequent deep water (in excess of 2000 dbar) sample data. The table below records these additional corrections:
Station Number Correction 13414-13415 0.0000 13416 0.0014 13417-13420 0.0000 13421-13422 -0.0010 13423-13424 -0.0019 13425-13428 -0.0027 13429-13436 -0.0035 13437-13442 -0.0044 13443-13456 -0.0057 13457-13461 -0.0043 13462-13474 -0.0062 13475-13484 -0.0067 13485 -0.0020 13486 0.0000 13487-13488 0.0030 13489-13494 0.0000 13495-13500 0.0030 13501-13504 0.0013 13505-13516 0.0000 13517-13522 -0.0038 13523-13546 -0.0085 13547-13553 -0.0070
CTD salinity was subsequently generated from the conductivity, C, via a PEXEC program.
Further details regarding the calibration can be found in the Cruise Report(Smythe-Wright, 1999).
Fluorescence was converted to voltages using the CTD's voltage digitiser calibration supplied by Ocean Scientific International (OSI):
fvolts = -5.656 + 1.7267e-4 fraw + -2.244e-12(fraw) 2
Conversion of the fluorescence to chlorophyll concentration was subsequently carried out. Details of this calibration are not held by BODC.
A best fit of downcast CTD oxygen to measured oxygen samples (based on the model of Owens and R.C. Millard, 1985) was obtained and applied to each station. No CTD oxygen data were collected between stations 13415 and 13417.
Post-cruise investigationsA mismatch between the downcast and upcast CTD data from D233 was discovered during further analysis and is an issue that affected other cruises from this period. The problem is not believed to be caused by pressure hysteresis, but is probably a pressure or temperature effect on the temperature sensor or some electronic component of the CTD. With this in mind, 2 dbar binned versions of the upcast CTD (sorted on pressure) were generated as the preferred CTD dataset from D233. These data have been calibrated exactly to the water bottle samples and so have the most accurate salinity.
The pressure-binned upcast PSTAR files were supplied to BODC for archiving. These were accompanied with corresponding downcast PSTAR files, also pressure-binned to 2 dbar. The latter contained the calibrated oxygen data, since the upcast oxygen suffered from hysteresis.
The upcast data (excluding oxygen) were merged with the downcast oxygen data (linked using common pressures) to produce a single BODC QXF file per cast. No further calibrations were applied to the data received by BODC. A number of derived channels were generated by BODC: potential temperature, sigma-theta and oxygen saturation. These were all calculated from the data streams merged into the QXF files, using established algorithms (for further details please refer to the parameter section of this report).
The following table summarises the mapping of originator variables to BODC parameter codes:
Units Description BODC
Units Comments press dbar Pressure exerted by the water body PRESPR01 dbar Calibrated by originator temp °C Temperature of the water body TEMPST01 °C Calibrated by originator salin - Practical salinity of the water body PSALST01 - Calibrated against bottle data by originator oxygen µmol/l Dissolved oxygen DOXYPR01 - Calibrated against bottle data by originator fluor mg m-3 Concentration of chlorophyll-a CPHLPR01 mg m-3 Calibrated against bottle data by originator trans m-1 Transmittance - - Not preserved in BODC files - uncertainty over source data and applied calibrations
The reformatted data were visualised using the in-house EDSERPLO software. Suspect data were marked by adding an appropriate quality control flag, whilst missing data were marked by setting the data to an appropriate absent data value and assigning a quality control flag.
Quality controlled data were ingested into the National Oceanographic Data Base (NODB), where they are supported by comprehensive metadata. During this stage a number of inaccuracies were identified in the CTD Station logsheets that appear in the cruise report. These are noted below in their entirety and corrected for in BODC metadata, where necessary:
Station Change of Time Where Applied 13416 03:21 to 10:57 end time 13422 23:07 to 23:57 bottom of cast 13424 14:05 to 15:05 start time 13425 23:03 to 23:53 bottom of cast 13458 07:11 to 06:11 start time 13552 0.18 to 04:18 start time Station Change of Date Where Applied 13457 09/05 to 08/05 23:05 13511 24/05 to 23/05 all times 13517 25/05 to 24/05 all times 13518 25/05 to 24/05 23:59 13524 26/05 to 25/05 all times 13525 26/05 to 25/05 23:32 13531 27/05 to 26/05 all times 13545 29/05 to 28/05 23:33
Smythe-Wright, D. (1999). RRS Discovery Cruise 233. Southampton Oceanography Centre, Cruise Report No. 24, 86pp.
Owens, W.B., Millard, R.C. (1985). A new algorithm for CTD oxygen calibration. Journal of Physical Oceanography, 15(5), 621-631.
World Ocean Circulation Experiment (WOCE)
The World Ocean Circulation Experiment (WOCE) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.
WOCE had two major goals:
Goal 1. To develop models to predict climate and to collect the data necessary to test them.
Goal 2. To determine the representativeness of the Goal 1 observations and to deduce cost effective means of determining long-term changes in ocean circulation.
The UK made a substantial contribution to the international World Ocean Circulation Experiment (WOCE) project by focusing on two important regions:
- Southern Ocean - links all the worlds oceans, controlling global climate.
- North Atlantic - directly affects the climate of Europe.
A major part of the UK effort was in the Southern Ocean and work included:
- Two surveys, in the South Atlantic as part of the WOCE Hydrographic Programme.
- SWINDEX, a year long study of the Antarctic Circumpolar Current (ACC) where it crosses major topography south of Africa.
- ADOX, a study of deep water flow from the Atlantic to the Indian Ocean.
- ACCLAIM, a study of the ACC by altimetry and island measurements.
In the North Atlantic the UK undertook:
- NATRE, a purposeful tracer experiment to look at cross isopycnic processes.
- CONVEX, a study of the deep ocean circulation and its changes.
- VIVALDI, a seven year programme of seasonally repeated surveys to study the upper ocean.
- Long-term observations of ocean climate in the North West Approaches.
Satellite ocean surface topography, temperature and wind data were merged with in situ observations and models to create a complete description of ocean circulation, eddy motion and the way the ocean is driven by the atmosphere.
The surveys were forerunners to the international Global Ocean Observing System (GOOS). GOOS was later established to monitor annual to decadal changes in ocean circulation and heat storage which are vital in the prediction of climate change.
|Principal Scientist(s)||Denise Smythe-Wright (Southampton Oceanography Centre)|
Complete Cruise Metadata Report is available here
No Fixed Station Information held for the Series
The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:
|<||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.)|
|E||End of CTD Down/Up Cast|
|G||Non-taxonomic biological characteristic uncertainty|
|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|
|O||Improbable value - user quality control|
The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:
|0||no quality control|
|2||probably good value|
|3||probably bad value|
|6||value below detection|
|7||value in excess|
|A||value phenomenon uncertain|
|Q||value below limit of quantification|