User`s manual

Manual revision 016 Appendix I: Functional Description and Circuitry SBE 49
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Appendix I:
Functional Description and Circuitry
Sensors
The SBE 49 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 SeaCAT and
SeaCATplus. The FastCAT uses three independent channels to digitize
temperature, conductivity, and pressure concurrently.
The pressure sensor is a strain-gauge 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 Wien-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.