Metadata Report for BODC Series Reference Number 61830

• 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

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 Discovery 132 CTD Data Documentation

Data Collection

Two Neil Brown CTD profilers were used. These were referred to as `shallow' and `deep' CTDs as they used 1500 and 6000 dbar pressure sensors respectively. The shallow CTD was only used in lowered mode on Station 10620 reported here. The CTD casts (Table 1 and Fig 1) fall into three groups):

• Casts 10621, 24, 25, 48 and 49 were primarily calibration casts, taken before or after SeaSoar runs and used to cross-calibrate the shallow CTD. This was necessary because bottle samples and thermometer readings cannot be taken during SeaSoar towing.
• Casts 10626-38 and 10650 comprise a north-south section along 20±1 ° W from 47.5° N to 40.5° N.
• Casts 10639-47 comprise an east-west section along 41.5 - 42° N from 20° W to 12° W.

Calibration

CTD data were logged in the first instance onto a PDP11/34 computer, After sampling (CTDSAMP) and averaging to 1-second raw values (CTDAVE), the data were calibrated (CTDCAL) using approximate calibration constants (Table 2) and archived to magnetic tape. The values output by CTDCAL are subscripted CAL in Table 2. Every fifth calibrated value was listed so that final calibration constants could be determined as follows.

Down casts were generally completed without stopping. All calibration samples were taken with a General Oceanics Rosette Sampler stopping typically at 200 m intervals during the upcasts. Some bottles carried reversing thermometers. The number of calibration values for salinity, temperature and oxygen on each cast are shown in Table 1. Pressure was generally checked with a single unprotected/protected thermometer pair at the deepest calibration depth.

For the shallow CTD, only one calibration cast (10620) was available.

The changes made to the calibrated values (CAL) to give final corrected (CORR) values are shown in Table 2, and discussed individually below. The statistics of the CORRected values are given in Table 3.

Pressure

The additive constant C2 is determined from the deck offset, or surface value. It was altered from 12 dbar (C1*C2) to 5 dbar after cast 10621.

The default calibration for C1 (0.1 dbar/count, 6553 dbar full scale) was used throughout. If reversing thermometers were used as standard, the 26 sample values obtained would suggest that CTD pressures should have been corrected by -2.6+/-2.5 dbar (Table 3 and Fig 2). No correction was made.

The shallow CTD had a deck offset of just over 1 dbar, and the default calibration (0.025 dbar/count, 1638 dbar full scale) appeared to be correct (Table 3).

Temperature

The laboratory calibration of the deep CTD (TCAL °C=0.035+0.0005005 T(RAW) was used throughout the cruise. The raw CTD values were speeded up with a 0.25 second time constant to match the conductivity time constant.

Analysis of 32 reversing thermometer calibrations suggested that the CTD was reading too low by 16.9±8.4 mK (Table 2, C7; Table 3 and Fig 3). Both CTD and reversing thermometer laboratory calibrations had been referred back to absolute standards, but it was thought at the time that the CTD calibrations might be different if the entire instrument could be immersed in the temperature bath, rather than just the sensor assembly. Accordingly, the CTD temperatures were corrected by adding 17 mK. (The same correction was, somewhat arbitrarily, made to the shallow CTD cast 10620, as three reversing thermometers had given offsets of 15, 20 and 8 mK).

Conductivity/Salinity

The conductivity ratio (C5, Table 2) for the first deep CTD cast (10621) was taken as the best value from the previous cruise (1.0003). Onboard analysis of the 6 rosette samples (with a Guildline bench salinometer) suggested an improved value of 1.0002, which was inadvertently entered as 1.002 for casts 10624, 25 and 26. An improved value of 1.00013 was then calculated, and used for the rest of the cruise, except that the value 1.002 was again inadvertently entered off an old listing for casts 10648 and 49.

The mean salinity offset for casts 10624, 25, 26, 48 and 49 (all with a conductivity ratio of 1.002) was 0.076 psu, the offset for cast 10621 was 0.008 psu, and for all remaining casts was zero (Table 2). Thus the errors in conductivity ratio were corrected by applying constant salinity offsets, after which Table 3 gives a standard deviation of less than 0.003 psu over 129 samples (ignoring 5 obvious outliers).

A final calibration check is provided by comparing the theta/S curves for the two deep casts (10648 and 49) with the line found by Saunders (1984) to fit all deep casts north east of the Azores. By comparison with Saunders' curve, our salinities are 0.002-0.004 psu too high.

For the shallow CTD, a final salinity correction of 0.035 psu was applied (Table 2) based on 7 samples (Table 3).

Oxygen

The first cast with the deep CTD (10621) suggested that a multiplicative adjustment to C6, reducing it by 0.885 from 0.00165 to 0.00146, would be sufficient to correct the oxygen values to the calibrated samples. For the shallow CTD the corrected value was found to be 0.00148. The value 0.00146 should, therefore, have been applied to all casts with the deep CTD. At a late stage of the processing it was found that the value 0.00148 had inadvertently been applied to casts 10639-47. Since the corresponding error in oxygen is less than 0.07 ml/l the data have not been further corrected. From Table 3 it can be seen that the corrected oxygen values are unbiased, with a standard deviation of less than 0.1 ml/l over 111 samples, and nearly all values lie within 0.2 ml/l of the true value.

We note that:

1. Down and up casts differ by up to 0.4 ml/l.
2. Although the CTD was always soaked in the water for five minutes before starting the down cast, there is still a marked hysteresis in the top 400 dbar on cast 10621. This was the first cast of the cruise made with the deep CTD, and shows how long the Beckman oxygen sensor takes to acclimatise.
3. Drift and jumps in the oxygen values can frequently be seen at each calibration level where hauling ceased. (4) The largest persistent differences between down and up casts occur between 1000 and 2000 dbar on the five casts which went deeper than 1200 dbar (10621, 25, 39, 48 and 49).

In summary, the down casts may be absolutely in error by 0.3-0.4 ml/l. Relative errors between casts at similar depths should be smaller.

Editing

Two primary causes of noise were identified:

1. A battery pack had been installed inside the CTD to keep it powered up while the multisampler was fired at calibration points. Without this modification, the oxygen sensor can take five minutes or more to recover on each occasion. Unfortunately, the batteries themselves appear to go open circuit internally when subjected to considerable vibration. This occurred particularly in the top 150 dbar, and at other depths when cable strum built up, which depended on the position of the cable laying guide across the main drum. Irrecoverably noisy data resulted which has had to be deleted, giving rise to the many gaps in the profiles, extending across up to 50 dbar.
2. A subtle computer fault caused the most significant bit of each eight bit byte to be occasionally dropped as it was read in. This affected the conductivity sensor in particular, causing errors of 128 in a single raw value. After averaging over one second, or about 16 samples, the error is reduced to an integer multiple of 8 raw conductivity units, equivalent to about 0.008 psu in salinity. The error could be spotted as quantised spikes in the salinity profiles, most obvious in the deep casts near the end of the cruise.

To remove the computer caused small quantised spikes, a number of the casts were listed in full and scanned, identifying individual data cycles to be deleted.

CTD Station List (Table 1)

Calibration Constants (Table 2)

P(CAL)(dbar) = C1*(P(RAW)-C2)
P(CORR) = P(CAL)
T(CAL)(°C) = C3 + C4*T(RAW)
T(CORR) = T(CAL) + C7
C(CAL) (mmho cm^-3) = C5*C(RAW)
S(CORR) (psu) = S(CAL) + C(8)

0(CAL) (ml/l) = C6* 0(RAW)* EXP(-0.036*T(L)-0.000155 P(CAL)) * 0(Saturated) (T(CAL),S(CAL))

where

T(L) is T(CAL) lagged with a 300 second time constant.
0(CORR) = C9 x 0(CAL)

Calibration Statistics (Table 3)

(Calibration sample - Corrected CTD)

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: