Metadata Report for BODC Series Reference Number 470733

• 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

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

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.

Further details can be found in the specification sheet.

RRS James Clark Ross 17 CTD Data Documentation

Introduction

This document covers the CTD data collected on cruise James Clark Ross 17. The information was provided by M.A. Brandon, P. Woodroffe and T. Marwood.

In all CTD stations the 2 dbar averages of the downcast data are reported as the final product. In some cases the 1 db and 3 db levels are missing from the final file. In these cases the shallowest level with data present was copied to these pressure levels.

Instrumentation and Protocols

The CTD unit used for the measurement program was the BAS Neil Brown Mk IIIb (serial number 01 - 3868 - 2086). The most recent calibration had been carried out by Chelsea Instruments on 12 September 1996. The CTD was mounted in a purpose built frame with a General Oceanics 12 position bottle multisampler rosette. On each position on the rosette was a 10 litre General Oceanics sampling bottle. For the near bottom CTD stations the package was fitted with a 10 kHz pinger to enable accurate near bottom approach. On three of the 10 L bottles were SIS Temperature Sensors. These were in two pairs, serial numbers T711 and T713, and serial numbers T715 and T716, with serial number T717 alone.

Deployment of the CTD underwater package was from the midships gantry and A-frame on a single conductor, torque balanced cable. This CTD cable was made by Rochester Cables and was hauled on the 10T traction winch. There were no significant problems deploying the CTD package as close control was maintained with the gib arm and two hand lines whilst the package was suspended above the surface. On one occasion (station 078) the CTD wire came off the roller at the top of the gib arm at the start of a cast. The package was lowered to the deck, the problem cured and the package successfully deployed.

CTD data were logged via a Neil Brown Instrument Systems deck unit, model 1150, to a 386 Viglen PC running E.G. and G. Marine Instruments CTD data acquisition module version 2.02 control software, and also to the RVS ABC system through a dedicated microcomputer. The CTD level A, mainly through historical reasons, averages the data at this point to 1 second values and passes the data through a simple editing procedure. During this editing procedure pressure jumps of greater than 100 raw units (e.g. for the pressure transducer equivalent to 10 db) are removed along with spikes in individual channels through a median sorting routine. The rate of change of temperature change over 1 second is also calculated. These one second data are then passed to the ship's UNIX system and archived. Calibration routines are then applied to the data and are described below.

Bottle Problems

On coming onto the ship in November it was noted that one handle on the CTD package was broken. This handle was changed before the cruise. During the cruise one more handle broke and was changed. Unfortunately, the changing of this handle led to bottle 5 being out of action for three CTD stations as Brandon lost a crucial spring whilst changing the handle. P. Woodroffe inspected the other bottle handles and replaced a further four. Whilst completing this task it was noted that he failed to drop any crucial springs. On a few occasions the bottles did not seal properly and leaked when the package was brought on deck. When this occurred the leaking bottle was noted on the logsheet and the sample treated as suspect. At stations 150, 155 and 157, the reversing thermometers on bottle 5 (711 and 713) did not trigger properly. This problem was cured for subsequent stations.

Bottle Pylon Misfires

The bottle rosette was controlled via a General Oceanics RMS MKVI 1015 - PM controlling unit. There were several misfires indicated on this unit that were indicative of the coming failure of the termination. The full list of bottle misfires is in the following table:

Reterminations

There were four reterminations on the CTD package. These are listed in the table below and are of two types. An electrical retermination was when the electrical part of the connection was remade; the mechanical termination was left intact.

Some concern was expressed at what was felt was a high number of reterminations being required and rotation of the package during deployment was thought to be straining the electrical connection. In a bid to remove this rotation, after station 237 the conducting cable was sent down to 3000 m with a weight attached to try and remove some turns from the wire. On the final electrical retermination a potting compound was used to try and make the electrical termination more robust.

10 kHz Pinger

The 10 KHz pinger was not fitted until station 17ctd157 (MEB 22) as this was the first near bottom station. It worked well throughout the rest of the cruise.

The Calibration of the CTD

As stated, the BAS Neil Brown MK IIIb serial number 01 - 3868 - 2086 was used for all CTD stations. This unit was calibrated on 12 September 1996 by Chelsea Instruments and we use values from this calibration for the pressure and temperature. The conductivity sensor was calibrated against in-situ salinity samples from the GO water bottles. We report three sets of coefficients for the conductivity and this is described in greater detail below.

Temperature Calibration

The temperature calibration was derived by Chelsea instruments using eight temperatures on the ITS-68 scale between 2 °C and 30 °C and was applied to the data through the following equation.

T = 0.0004955T(raw) - 2.101 (1)

To convert from the ITS-68 scale to T90 following Saunders (1990) we multiplied all temperatures by 0.999760057, so

T = T x 0.999760057 (2)

To allow for the mismatch in response times between the temperature sensor and conductivity sensor, following the standard procedure, the temperature was lagged for the salinity calculation. This lag was achieved by adding a fraction delta of the rate of change of temperature that is output from the level A(dT) to the temperature. The temperature is then

T(new) = T + delta dt (3)

From experiment the spiking in the derived salinity was minimised with delta = 0.15.

Pressure Calibration

A pressure calibration derived by Chelsea Instruments from 11 pressures between 0 and 6000 m was applied through the following equation

P = 0.0998569P(raw) - 12.11238 (4)

Following King and Alderson (1994) the pressures were then modified by the addition of a factor deltaP, to take into account the effect of temperature on the pressure sensor so that

P = P + deltaP (5)

And deltaP is calculated from

deltaP = -0.4 x (T(lag) - 20.0)) (6)

Here T(lag) is a lagged temperature in °C and is constructed from the CTD temperatures. We use a time constant for the lagged temperature of 400 seconds and update the temperature following the method put forward in King (1996). If T is the CTD temperature and t(del) the time interval in seconds over which the temperature is being updated, and T(const) our time constant of 400 seconds then the factor W is

W = [exp (- t(del)/T(const))] (7)

and now

T(lag)(t=t(o)+t(del)) = W x T(lag)(t=t(o)) + (1-W) x T(T=T(o) + t(del)) (8)

We finally make an adjustment to the upcast pressure to take into account hysteresis in the sensor. The extent of the hysteresis was calculated using a series of laboratory measurements. The hysteresis after a cast to 5500 m (which we denote by dp5500(p)) is given in the following table:

These values were derived from a laboratory calibration at IOSDL in 1994. Intermediate values are found by linear interpolation. If the pressure of the cast is outside the values in Table 1 then dp5500(p) is set to zero. For a cast in which the maximum pressure reached is P(max) dbar, the correction to the upcast CTD pressure (p(i)) is:

[P(out)/P(max)] = (dp5500(p(i)) - (((P(i)) x dp5500 (p(max))) (9)

Salinity (Conductivity) Calibration

We first describe the principal of our method and then detail the steps. For this cruise we calibrated the conductivity against in-situ samples collected with the GO multisampler rosette. Once the conductivity of the CTD was calibrated we derived salinity. A full data processing route is detailed at the end of this report. In brief, first we applied a nominal calibration of the form

cond = 1 x cond(raw) + 0.0 (10)

From the salinity samples, once successfully matched, we calculated the bottle sample conductivity using in-situ temperature and pressure from the CTD. From this in-situ conductivity we calculated the difference of the bottle conductivity (cond(b)) and CTD conductivity (cond(ctd)) to derive a value deltaC. We now plot bottle conductivity (x variable) against deltaC (y variable). This should give a straight line wherefrom

y = mx + c (11)

We get

deltaC = m cond(b) + c (12)

After rejecting suspect bottles we use the pstar programme plreg2 to derive m and c for deltaC. Now, as

deltaC = cond(b) - cond(ctd) (13)

the calibration coefficients for the CTD conductivity are derived through substituting equation (13) into (12), the CTD conductivities are now

cond(ctd) = a + b cond(raw) (14)

and from the m and c in equation (12)

a = [c/1 - m](15)

and

b = [1/1 - a] (16)

These values for a and b are entered into the calibration files for both the pstar and RVS system. The processing route is then repeated and the new graph of deltaC against cond(b) gives the conductivity residuals; the residuals should now be random with a mean of zero. This calibration procedure does have a feature in that, as we moved south along the section and moved into waters where the entire water column was of lower conductivity than the station used for the initial calibration, the validity of the original m and c are called into question because of extrapolation. Accordingly, we used three sets of coefficients for a and b that are detailed below:

After applying these calibration coefficients to the relevant stations there is still a residual drift within the conductivity signal with time. For each station this drift is

deltaC = residual drift

From substitution into our original equations we can now remove this residual drift.

Salinity Samples

Salinity samples were taken for all of the CTD casts made for the physical oceanographic program. For the 22 stations of the Maurice Ewing Bank section 18 samples were taken from the GO 10 L bottles. This gave one sample for each bottle plus six duplicates. For the core boxes around South Georgia this was reduced to nine samples for each station. This gave a total of 420 samples with 148 duplicates (568 in total). The samples were taken in 300 ml medicine bottles, each bottle being rinsed twice before being filled to just below the neck. The rim of the bottle was then wiped with tissue, a plastic lid inserted and the screw cap replaced. The salinity samples were placed near to a salinometer to allow the sample temperatures to equalise with the salinometer. The samples were then analysed on the BAS Guildline Autosal model 8400 S/N 45363. This salinometer was serviced and electronically aligned by Ocean Scientific International In June 1996. For each CTD station's worth of salinity samples (18 samples) one vial of OSIL standard seawater (batch P130, 1996) was run through the salinometer to enable a calibration offset to be derived. Once analysed, the conductivity ratios were entered by hand into an Apple Macintosh based EXCEL spreadsheet using software written by Dr. Brian King (SOC) before being transferred to the UNIX system as described below. For the 148 duplicate samples the mean difference was 0.000 and the standard deviation 0.001.

The Quality of the Conductivity Calibration Procedure

After applying the calibration coefficients and adjusting for the residual offset deltaC, the salinity of the bottle sample was differenced with the derived CTD salinity. After rejecting, the mean of the remaining samples was 0.0000 with a standard deviation of 0.0018 psu. The drift of the sensor was small.

General Data Screening carried out by BODC

BODC screen both the series header qualifying information and the parameter values in the data cycles themselves.

Header information is inspected for:

• Irregularities such as unfeasible values
• Inconsistencies between related information, for example:
  • Times for instrument deployment and for start/end of data series
  • Length of record and the number of data cycles/cycle interval
  • Parameters expected and the parameters actually present in the data cycles
• Originator's comments on meter/mooring performance and data quality

Documents are written by BODC highlighting irregularities which cannot be resolved.

Data cycles are inspected using time or depth series plots of all parameters. Currents are additionally inspected using vector scatter plots and time series plots of North and East velocity components. These presentations undergo intrinsic and extrinsic screening to detect infeasible values within the data cycles themselves and inconsistencies as seen when comparing characteristics of adjacent data sets displaced with respect to depth, position or time. Values suspected of being of non-oceanographic origin may be tagged with the BODC flag denoting suspect value; the data values will not be altered.

The following types of irregularity, each relying on visual detection in the plot, are amongst those which may be flagged as suspect:

• Spurious data at the start or end of the record.
• Obvious spikes occurring in periods free from meteorological disturbance.
• A sequence of constant values in consecutive data cycles.

If a large percentage of the data is affected by irregularities then a Problem Report will be written rather than flagging the individual suspect values. Problem Reports are also used to highlight irregularities seen in the graphical data presentations.

Inconsistencies between the characteristics of the data set and those of its neighbours are sought and, where necessary, documented. This covers inconsistencies such as the following:

• Maximum and minimum values of parameters (spikes excluded).
• The occurrence of meteorological events.

This intrinsic and extrinsic screening of the parameter values seeks to confirm the qualifying information and the source laboratory's comments on the series. In screening and collating information, every care is taken to ensure that errors of BODC making are not introduced.

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: