Metadata Report for BODC Series Reference Number 15582
Near the beginning, in the middle and near the end of its stay on the sea floor, the frame rocked onto its side and back upright. Good data are from 0740h 21 March to 0010h 02 April and 0440h 2 April to 1220h 16 April. For times when the meter was on its side the speed and direction data cycles have been flagged as null.
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Aanderaa Recording Current Meter Model 4/5
Manufacturer's specifications: Meter (recording unit: height 51cm, diameter 12.8cm, vane size 37x100cm; overall: length 137cm, height 75cm) is designed for depths down to 2000m (6000m RCM model 5). It incorporates a spindle which is shackled into the mooring line. The meter is attached to the spindle through a gimbal mounting which permits a maximum 27° deviation of the spindle from the vertical, the meter still remaining horizontal.
Meter comprises :-
Savonius rotor magnetically coupled to an electronic counter - the number of revolutions during the sampling interval giving the average current speed over the interval - starting speed 2cm/s (users find 1.5 to 3cm/s), range 2.5 to 250cm/s, accuracy greater of 1cm/s or 2 per cent.
Vane, which aligns instrument with current flow, has a balance weight ensuring static balance and tail fins to ensure dynamic balance in flows up to 250cm/s.
Magnetic compass (needle is clamped to potentiometer ring at instant of sampling only) - direction recorded with 0.35° resolution, 5° accuracy (1.5° claimed by MAFF, Lowestoft) for speeds 5 to 100cm/s, 7.5° accuracy for remaining speeds within 2.5 to 200cm/s range, maximum compass tilt (i.e. maximum deviation of the meter from the horizontal at which the meter still registers correctly) is 12° in both pitch and roll axes.
Quartz clock, accuracy better than 2sec/day within temperature range 0 to 20°C.
Thermistor (temperature sensor), standard range -2.46 to 21.48°C (max on high range 36.04°C), accuracy 0.15°C, resolution 0.1 per cent of range, 63 per cent response time 12sec.
Inductive cell conductivity sensor (optional), range 0 to 70mmho/cm standard resolution 0.1 per cent of range.
Bourdon tube pressure sensor (optional) driving a potentiometer - range 0 to 100, 200, 500, 1000 or (RCM4 to 3000psi), (RCM5 to 5000, or 8000psi), lowest calibrated pressure 14.24psi, accuracy 1 per cent of range, resolution 0.1 per cent of range.
Self balancing potentiometer which converts the output from each sensor into a 10 bit binary number for storage on magnetic tape.
Sample duration equals nominal interval between data cycles pre-chosen as 0.5, 1, 2, 5, 10, 15, 20, 30, 60 or 180 minutes. Sample recording order: meter reference number, temperature, (conductivity, pressure if installed), current direction, speed.
Manufacturer's calibration formulae:
Meters (manufactured prior to October 1974) with analogue measurement of speed, i.e. the Savonius rotor drives a potentiometer via a magnetically coupled follower and gearbox (6000 : 1 gear ratio):
speed = 1.5 + 246 * (M/T) cm/s (1)
meters with digital measurement of speed i.e. utilizing an electronic reed switch to count the total number of rotor revolutions during the sampling interval:
speed = 1.5 + 42 * B * (M/T) cm/s (2)
direction = 1.5 + 0.349N ° magnetic (3)
B is the number of rotor revolutions per count, M (bits) binary is the count over the sampling interval T (sec) and N (bits) binary is the direction reading.
Data collecting laboratories may calibrate their own meters and so not use the manufacturer's calibration equations.
Aanderaa Current Meter/Pressure Recorder
The instrument comprises a bottom mounted rig which utilises a Digiquartz sensor and an Aanderaa current meter (with the adaption of a small vane replacing the normal vane) both interfaced into a modified Aanderaa current meter logger. Current meter speed and absolute pressure count are integrated over a pre-selected period (usually 10 or 15 minutes) and sampled by the logger at the same pre-selected time interval, together with spot readings of vane direction, temperature, elapsed time and rig orientation. The pressure sensor consists of a convoluted bellows linked to a 40kHz quartz crystal resonator coupled by piezoelectric action to an electronic resonator. A quartz crystal clock is used for controlling the sampling interval.
The data are recorded on magnetic tape as 10 bit binary words in serial form with the frequency count from the pressure sensor stored as most and last significant counts. The pressure sensor pack, containing the Digiquartz type 2-300a quartz crystal pressure transducer but no temperature transducer, is mounted approximately 1m above the rig base, with the current meter 0.7m above the base. The rig is 1.5m in height and the base is formed by a tripod with legs of length approximately 0.7m. The current meter/pressure recorder is usually deployed using a U-shaped mooring (i.e. buoy or pellet float to wire rope to tide gauge to ground line to anchor to wire rope to buoy), but W-shaped moorings are used in conjunction with either current meters or thermistor chains. A W-shaped mooring comprises pellet float to pellet line to sub-surface buoy to current meter or thermistor line to anchor weight to ground line to tide gauge to ground line to anchor weight to wire rope to surface buoy.
BODC Current Meter Screening
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:
- Depths of meter and sea bed.
- Times for mooring deployment and for start/end of data series.
- Length of record or number of data cycles, the cycle interval, the clock error and the period over which accrued.
- Parameters stated as measured 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 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 following types of irregularity, each relying on visual detection in the time series 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, deemed abnormal, then instead of flagging the individual suspect values, a caution may be documented. Likewise documents will highlight irregularities seen in the current vector scatter plots such as incongruous centre holes, evidence of mooring 'knock-down', abnormal asymmetry in tidally dominated records or gaps as when a range of speeds or directions go unregistered due to meter malfunction.
The term 'knock-down' refers to the situation when the 'drag' exerted on a mooring at high current speeds may cause instruments to tilt beyond the angle at which they are intended to operate. At this point the efficiency of the current sensors to accurately record the flow is reduced.
Inconsistencies between the characteristics of the data set and those of its neighbours are sought, and where necessary, documented. This covers inconsistencies in the following:
- Maximum and minimum values of parameters (spikes excluded).
- The orientation and symmetry of the current vector scatter plot.
- The direction of rotation of the current vectors.
- The approximate amplitude and periodicity of the tidal currents.
- The occurrence of meteorological events and, finally, for series for which no time check was possible, the phase.
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.
Data Processing Notes
Magnetic variation assumed 8° W.
Clock gained 10 seconds over 60 days 21 hours 29 minutes 50 seconds; no correction made.
IOS Bidston CMD Calibration, Processing and Validation Procedures
All meters were calibrated before their launch and after their recovery. The thermistors were calibrated over the range -2°C to 20°C in a water bath. Thermistors calibrated before April 1973 were calibrated in an environmental chamber. Prior to August 1973 a straight line was fitted to the results of each calibration. Between August 1973 and December 1977 a cubic polynomial was fitted to the calibration results. Post-1977 calibrations have a cubic polynomial fitted, calibrating for offset and slope only. The compasses were calibrated every 10 degrees from 0° to 360° and every degree through the dead space. The calibrations were performed outside the laboratory with the meters in a special jig. The results were used to create a table which contained the direction to the nearest degree corresponding to each meter reading.
The pressure sensors were calibrated over the range 0 to 13.5 bars above atmospheric pressure using a dead weight tester and a straight line was fitted to the results.
The rotors were not calibrated but the manufacturer's formula was used since experience has shown this to be sufficiently accurate. The meters were balanced in a tank of water to ensure that the meter would be level in the sea.
Data Processing and Validation
Pre-1978 data on magnetic tapes from the instruments were translated onto punched paper tape which was input to the Institute computer. Since Jan 1978 the data have undergone CAMAC translation on to magnetic tape. Errors in the data were discovered by noting either discontinuities in the records or consecutive readings with the same value, the most common errors being:
- A large change in direction between adjacent readings at times of reasonable speed (i.e. greater than 25cm/s).
- The rotor count going backwards (for early meters only when fitted with potentiometer rotor counter). Most meters are now fitted with electronic rotor counters.
Rotor count errors were corrected and the meter calibrations were then used to calculate the temperature, pressure and the north (true) and east components of velocity. Because the meter integrates the rotor count but records instantaneous directions some further averaging was necessary to derive a simultaneous value of speed and direction. Consider three adjacent readings of rotor count and direction at times t1, t2 and t3. The value for speed and direction at time t2 was derived by associating the speed given by the rotor count at t3 minus that at t1 with the instantaneous measurement of direction at time t2.
After the components of velocity had been calculated errors of type a), which were more common than those of type b), were corrected using a cubic spline routine on each velocity component (pre-1978 data). From Jan 1978 directions have been edited before the components were calculated. Timing errors were determined by comparing the number of samples recorded with the difference between the times of starting and stopping the meter. Since Jan 1978 most meters have been fitted with an elapsed time counter to aid identification of missing scans. The meters were started and stopped on board ship and had pre- and post-cruise checks carried out on them in the laboratory.
The results were displayed in five graphs. The diagrams were:
- A plot of the temperature, pressure and north and east components of velocity against time. The whole data series obtained is used as the input for this graph.
- Histograms of the speed and direction. Plots of the percentage of the data which lie within a certain interval of speed or direction.
- A scatter diagram of the north component of velocity against the east component. This diagram is particularly useful in revealing malfunctions in the meters compass or in the rig itself.
- Two progressive vector diagrams, one using the same data as (1), the other using the data filtered by applying a running average of 24 hours 50 minutes (i.e. 2 tidal cycles) to remove most of the tidal signal and hence show the residual movement more clearly.
- The data processing details apply regardless of the type of meter used.
- For IOS Bidston current meter data, the format of the originator's reference when quoted by BODC will be Station name and/or number / meter number / immersion number.
IOS Bidston CMD Standard Procedures Summary
The manufacturer's calibration formula was used for speed. Calibrations for direction, temperature and pressure were carried out at IOS Bidston before and after use.
U-shaped mooring, i.e. pellet float to pellet line to sub- surface buoy to meter mooring line to anchor weight to ground line to anchor weight to marker buoy wire to surface marker buoy. This is a standard shelf sea rig, which reduces the effects of surface waves on the meter. The mooring is deployed in the order given and recovered in reverse order. Normally lengths of main wires are: surface buoy line 1.5D and ground line 3D, where D is the depth of water. The sub-surface buoy is normally 5m above the top meter. Current meters are attached to a taut line, Plessey meters via an A-frame clamped onto the line and Aanderaa meters by a spindle which is spliced into the mooring wire. Some spindles have been designed at IOS Bidston.
Speed is integrated over the sampling interval. Direction, temperature and, where measured, pressure are taken as spot measurements. Direction is corrected for magnetic variation. Pressure is not corrected for atmospheric variations. and is converted to metres of water assuming a sigma-t of 25.
Parameter values i.e. speed, direction etc. are given at times incremented by the nominal sampling interval. Sea floor depths are calculated from wire length and pressure record, if available, otherwise they are taken directly from Admiralty Charts and are expressed below Chart Datum. The site is positioned by Decca main chain which gives a relative accuracy of 0.1 nautical miles over most of the areas investigated.
No Project Information held for the Series
|Start Date (yyyy-mm-dd)||1977-03-19|
|End Date (yyyy-mm-dd)||1977-04-18|
|Organization Undertaking Activity||Institute of Oceanographic Sciences Bidston Laboratory (now National Oceanography Centre, Liverpool)|
|Country of Organization||United Kingdom|
|Originator's Data Activity Identifier||R125,S12|
|Platform Category||subsurface mooring|
Mooring held 2 meters.
Other Series linked to this Data Activity - 33877
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|