Sea-Bird Electronics SBE 38 Manual de usuario descargar pdf (Pagina 23)

Appendix I: Functional Description

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.

Maximum power dissipated in the thermistor is 0.5 microwatts, and

contributes less than 200 µK of overheat error.

A raw count (ratio) is related to resistance measurements:

raw counts = 1048576 * (NT) / (NR)

where

NR = output from reference resistor

NT = thermistor output

The SBE 38’s output is computed from the raw count and the calibration

coefficients that are stored in EEPROM.

The number of acquisition cycles (raw counts) averaged per measurement is

user-programmable (NAvg=; see Section 4: Deploying and Operating

RS-232 SBE 38). Increasing the number of cycles per measurement increases

the time to acquire the measurement and the interval between measurements,

while reducing the RMS temperature noise from the sensor. The interval

between measurements is:

interval [seconds] = (0.133 * NAvg) + 0.339

where

NAvg = number of acquisition cycles per measurement

The SBE 38 transmits data after it has completed the previous sample and

before it starts the next sample.

Time to transmit data = (number of characters * 10 bits/character) / baud rate

where

number of characters is dependent on the output format. Include the decimal

point when counting characters; add 2 to the number of characters output, to

account for the carriage return and line feed at the end of each scan.

Settings

Calibration coefficients and setup parameters (Baud=, NAvg=, Interface=,

etc.) are written to EEPROM and are non-volatile. These settings do not

change if power is removed.

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Sea-Bird Electronics SBE 38 Manual de usuario Pagina 23