3.1.2 Electrical conductivity, temperature and pressure probe (ECTP)

Temperature. The temperature probe consists of a primary temperature sensor and its associated electronics. The primary temperature sensor is made of copper wires placed between two thin-walled (100 µm) titanium cylinders (Fig. 10a), welded together using a laser welding technique. This construction provides a hermetic seal for protection of the microwire from the seawater. Using pure copper as the material for the primary temperature sensor provides a good linearity of the probe in a wide temperature range (-2°C ± 35°C). The small diameter of microwire and thin-walled cylinders (especially the internal cylinder on which the microwire is wound) provides a response time of the temperature sensor of about 35 ms. For isolation of the microwire from the internal and external cylinders, the internal surfaces of the cylinders are covered by the insulation - nitride of silicon. A filler compound in between cylinders is used in order to decrease the influence of wire strain caused by environmental effects.

a.

b.

FIGURE 10. a. Construction of the primary temperature sensor: 1 are the places of laser welding, 2 is the copper microwire of 20-micrometer diameter, 3, titanium body with 100-micrometer- thick walls; 4, isolation cover by nitride of silicon; 5, compound filling. b. Theoretical observation of the inductive conductivity cell: 1, the epoxy body of the cell; 2, voltage transformer; 3, current transformer; 4, control winding; 5, the `conducting loop'.

Conductivity. The conductivity probe consists of a conductivity cell and its associated electronics. A diagram of the conductivity cell is shown in Fig. 10b. It is of inductive type and consists of two co-axial toroid inductors. In a conductive medium, an inductive conductivity cell has an isolated "conductivity loop" comprising the two inductors. AC voltage of 16 kHz is applied to the primary voltage inductor, which induces current into the "conductivity loop" (seawater). The secondary current inductor converts the current from the "conductivity loop" into output voltage of the conductivity cell. The value of the current in the "conductivity loop" depends on the conductivity of the water. The output AC voltage is amplified and then rectified into the DC signal by the electronic circuits. An important advantage of inductive conductivity cells is that biological growth on its exposed surfaces do not substantially interfere with the conductivity measurement. Also, because no electrodes directly contact seawater, there is no electrical polarization.

Fluctuation temperature and conductivity. There are also fluctuation temperature and fluctuation conductivity channels in the ECTP probe. They filter the amplified signal in the frequency range of 0.012 Hz to 50 Hz, and then additionally provide amplification inside the ECTP probe. The fluctuation channels have a substantially lower digitizing noise level, because they are additionally amplified by factors shown in Table 9. They are intended to detect possible external electrical disturbances on temperature and conductivity channels during the transmission of the analog signals to the computer lab in the ship.

Pressure. For the COARE expedition, the ECTP probes are additionally equipped with strain-gauge pressure sensors (serial type D) produced by the Prompribor Plant (Orel, Russia). The pressure sensor is installed in the tail section of the ECTP probe.

Table 9 gives the ECTP probes' main technical characteristics for the different cruises.

TABLE 9. Main technical characteristics of ECTP probes
Cruise on R/V Moana Wave COARE IOP-3
and EQ-2 		COARE
MW9410 		COARE EQ-3
Parameter \ ECTP # ECTP #A283 ECTP #1 ECTP #1
Temperature
Range (°C)-2 - 35 14-34 14-34
Accuracy (°C) < 0.1 < 0.1 < 0.1
Stability during 8 hours (°C) < 0.01 < 0.01 < 0.01
Digital resolution (°C) 0.003 0.005 0.005
Response time of primary sensor (ms) 35 35 35
Fluctuation temperature
Range (°C) ±2.5 ±1.0 ±1.0 (±2.0 from 11 Apr 94)
Noise level (°C)0.0010.0010.001
Digital resolution (°C) 0.0012 0.0005 0.0005 (0.001 from 11 Apr 94)
Frequency range (Hz) 0.012 - 50 0.012 - 50 0.012 - 50
Amplification factor 25 10 10
Conductivity
Range (S/m) 1.5 - 7.0 3.1 - 6.1 3.1 - 6.1
Accuracy (S/m) < 0.01 < 0.01 < 0.01
Stability during 8 hours (S/m) < 0.001 < 0.001 < 0.001
Digital resolution (S/m) 0.00056 0.0012 0.0012
Space resolution (m) 0.1 0.1 0.1
Fluctuation-conductivity
Range (S/m) ±2.5 ±0.1 ±0.1 (±0.2 from 11 Apr 94)
Digital resolution (S/m) 0.00012 0.00005 0.00005 (0.0001 from 11 Apr 94)
Noise level of output voltage (S/m) 0.0001 0.0001 0.0001
Frequency range (Hz) 0.012 - 50 0.012 - 50 0.012 - 50
Amplification factor 50 25 25
Pressure
Range (dbar) 0 - 25 0 - 25 0 - 25
Accuracy* (dbar) 0.1 0.1 0.1
Digital resolution (dbar)0.010.010.01
* - after the temperature dependence correction.

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