Measuring instruments

Thermosalinograph

All the ships of the French Oceanographic Fleet are fitted with SeaBird SBE21 sensors on a dedicated seawater system supplied by a centrifugal pump. The sensor is mounted in a water jacket and provides temperature and conductivity data in real time. Another temperature sensor, such as an SBE38 (digital) or an SBE3S (analogue), is mounted near the system’s seawater inlet in order to correct temperature inaccuracies caused by the piping warming the water.

The thermosalinograph comprises a cell fitted with electrodes which determine the conductivity of seawater. Thermistors measure water temperature in the cell and near the hull.

The sensors are calibrated every year.

Conductivity:

Range: 0–70 mmho/cm
Accuracy: ±0.01 mmho/cm
Resolution: ±0.0o1 mmho/cm

Temperature:

Range: -5 to +35 °C
Accuracy: ±0.01 °C
Resolution: ±0.001 °C
Sample interval: 5 seconds or more by increments of 1 second
Recommended flow rate: 1 litre per second

Fluorometers

The fluorometer is connected onto the same sea water circuit as the thermosalinometer and measures the concentration of chlorphyll present in the sea water. The phytoplancton cells are lit by a beam of ultraviolet light. Their chlorophyll content absorbs the light which is then re-emitted in infrared. The phytoplancton density can be determined in this way.

Ferrybox

The Ferrybox is an autonomous and compact system of sensors for measuring surface seawater. The Ferrybox 4H features the following sensors:

SeaBird SBE45 thermosalinograph
Seapoint turbidity meter
Aanderaa 4835 oxygen optode
Meinsberg EGA150 pH sensor

CTD SBE19(plus)

The SBE19 CTD is a measuring instrument for physical oceanography, operated by cable between the surface and the sea floor. It is designed to measure in real time water temperature, conductivity and depth down to 7000 m, at a frequency of 4 Hz.

The CTD is generally mounted on a “rosette” or frame which also carries the water-sampling bottles (triggered by the operator at the desired depth) and perhaps other measuring instruments, such as current meters.

Measuring ranges:

Temperature: -5 to +35 °C
Conductivity: 0–9 S/m
Depth: 0–7000 m

	Temperature (°C)	Conductivity (S/m)	Depth
Measuring range	-5 to +35	0–9	0 to the maximum of the selected range (20, 100, 350, 1000, 2000, 3500, 7000 m)
Accuracy	0.005	0.0005	0.1% of the selected range
Resolution	0.0001	0.00001–0.00007 depending on water salinity	0.002% of the selected range

The CTD is controlled using a SBE36 deck unit installed on the ship.

CUFES

CUFES is a system for counting fish roe contained in water samples pumped in from the subsurface. Water is pumped through a concentrator which isolates the particles contained in the seawater. The particles are then trapped on a 315 micron mesh and, at the end of the 30-minute sequence, recovered for analysis by the biologists.

Gravimeters

Gravimetry comprises a range of techniques used to measure gravity, the latter being the sum of the force of gravitational attraction and the centrifugal force caused by the Earth’s rotation.

Gravimeters of the French Oceanographic Fleet

Gravimeters are fitted to three ships in the French Oceanographic Fleet: Pourquoi pas ?, L’Atalante and Marion Dufresne. A gravimeter can also be installed on the Thalassa, depending on the needs of the scientific cruise.

The following gravimeter models are installed on the ships:

Supplier: BGGS, based in Germany
Model: KSS 31 or KSS 32 (more recent)

The Marion Dufresne also carries a Lacoste MGS6 gravimeter.

Measuring with a gravimeter BGGS KSS 31 or 32

The KSS 31 and KSS 32 gravimeters comprise a gyro-stabilized platform (KT 31 or KT 32) and a gravity meter GSS 30. The platform is fitted with servomotors which maintain it in a horizontal position (using data supplied by two acceleration meters and a gyroscope). The meter measures vertical acceleration by means of a counterweight suspended from a spring which expands in a perfectly linear direction. An external PC is used to configure the system. Data is acquired using the TECHSAS system.

These gravimeters work by interpolation: they produce measurements relative to variations in gravity. In order to calculate absolute gravity, calibrations are carried out whenever the ship is alongside (connection to a reference station using a mobile gravimeter). Calibration enables the operator to estimate the meter’s drift so that measurements taken at sea can then be corrected.

Specifications for the gravimeters are indicated in the table below (straight line acquisition, drift less than 3 mGal/month).

Vertical acceleration	Pre-processing accuracy (RMS)	Post-processing accuracy (RMS)
< 15000 mGal	0.5 mGal	0.2 mGal
15000 – 80000 mGal	1 mGal	0.4 mGal
80000 – 250000 mGal	2 mGal	0.8 mGal

Weather stations

When operating, each ship is fitted with a weather station by Météo France, the French national weather service. This autonomous system sends meteorological data several times a day by satellite link (Inmarsat or Mini-M) to Météo France. The data is also saved for use by the ship’s complement.

The weather station comprises the following sensors:

Vaisala HMP35DE thermometer/hygrometer
Vaisala PTB220 pressure sensor (barometer)
Seawater temperature sensor (PT100 mounted on the hull or data from a temperature sensor on the ship’s seawater system)
Young 70721 pyranometer (sunshine sensor)
Gill Windsonic ultrasonic anemometer (wind speed/direction)

French oceanographic fleet

operated by Ifremer

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