Sea-Bird Electronics SBE 45 MicroTSG, User Manual

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Appendix I: Functional Description SBE 45
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Appendix I: Functional Description
Sensors
The MicroTSG 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, in the SEACAT and
SEACAT
plus family, and in the SBE 37 MicroCAT family.
Sensor Interface
Temperature is acquired by applying an AC excitation to a hermetically-sealed
VISHAY reference resistor and an ultra-stable aged thermistor with a drift rate
of less than 0.002 °C per year. A 24-bit A/D converter digitizes the outputs of
the reference resistor and thermistor. AC excitation and ratiometric
comparison using a common processing channel 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. A high
stability TCXO reference crystal with a drift rate of less than 2 ppm/year is
used to count the frequency from the Wein Bridge oscillator.
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