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Manual revision 006 Appendix I: Functional Description and Circuitry SBE 19plus V2

107

Appendix I: Functional Description

and Circuitry

Sensors

The SBE19plus V2 embodies the same sensor elements (3-electrode,

2-terminal, borosilicate glass cell, and pressure-protected thermistor)

previously employed in Sea-Bird’s modular SBE 3 and SBE 4 sensors and in

the original SEACAT design. The19plus V2 differs from the SBE 19 in that it

uses three independent channels to digitize temperature, conductivity, and

pressure concurrently. Multiplexing is not used for these channels.

The pressure sensor is a strain-gauge or optional Quartz sensor.

Sensor Interface

Temperature is acquired by applying an AC excitation to a bridge circuit

containing an ultra-stable aged thermistor with a drift rate of less than 0.002 ºC

per year. The other elements in the bridge are VISHAY precision resistors.

A 24-bit A/D converter digitizes the output of the bridge. AC excitation and

ratiometric comparison avoids errors caused by parasitic thermocouples, offset

voltages, leakage currents, and reference errors.

Conductivity is acquired using an ultra-precision Wein-Bridge oscillator to

generate a frequency output in response to changes in conductivity.

Strain-gauge pressure is acquired by applying an AC excitation to the pressure

bridge. A 24-bit A/D converter digitizes the output of the bridge. AC

excitation and ratiometric comparison avoids errors caused by parasitic

thermocouples, offset voltages, leakage currents, and reference errors.

A silicon diode embedded in the pressure bridge is used to measure the

temperature of the pressure bridge. This temperature is used to perform offset

and span corrections on the measured pressure signal.

The six external 0 to 5 volt DC voltage channels are processed by differential

amplifiers with an input resistance of 50K ohms and are digitized with a 14-bit

A/D converter.

Real-Time Clock

To minimize power and improve clock accuracy, a temperature-compensated

crystal oscillator (TCXO) is used as the real-time-clock frequency source.

The TCXO is accurate to ± 1 minute per year (0 ºC to 40 ºC).

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Sea-Bird Electronics SBE 38 Manual do Utilizador Página 107