Manual revision 001
Appendix I: Functional Description
SBE 39
plus
-IM
84
Appendix I: Functional Description
Sensors
The SBE 39
plus
-IM includes the same temperature sensor element (pressure-
protected thermistor) previously employed in the modular SBE 3 sensor, in the
SeaCAT family, and in the SBE 39
plus
with RS-232 interface.
The SBE 39plus-IM’s optional strain-gauge pressure sensor is available in the
following ranges: 20, 100, 350, 600, 1000, 2000, 3500, and 7000 meters.
Compensation of the temperature influence on pressure offset and scale is
performed by the SBE 39plus-IM’s CPU.
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 (and optional pressure sensor).
AC excitation and ratiometric comparison using a common processing channel
avoids errors caused by parasitic thermocouples, offset voltages, leakage
currents, and reference errors.
Real-Time Clock
To minimize power drain, a low power
watch
crystal is used as the real-time-
clock frequency source. The sensitivity of the clock to ambient temperature is
accurately measured during calibration. The results are stored in EEPROM
and automatically applied during deployment.
Note:
Pressure ranges are expressed
in meters of deployment
depth capability.
