In-Situ Aqua TROLL 400 Modbus and SDI-12 Reference Guide - Manual (Page 34)

4.16 Calibration Slope and Offset
These values represent the slope and offset that will be applied to the raw concentration reading
from the sensor to generate the final values reported by the sensor parameters. Writes to these
registers are only accepted if the probe is in the calibration mode. The probe will return
exception 0x92 (invalid sensor) if an attempt is made to write these registers when the
calibration mode is off. These registers may be written independently of the normal internal
calibration procedure.
4.17 Calibration Procedure—RDO
The RDO sensor is calibrated using the following procedure.
1. Write the Calibration Mode On command (0xE000) to the Sensor Command
register.
2. Update the live salinity and barometric pressure registers if necessary.
3. Prompt the user to place the probe in a 100% saturation environment.
4. Read the oxygen concentration and temperature parameters. When these values
have reached equilibrium, record them in their respective 100% saturation
calibration registers. Write the current live salinity and barometric pressure
readings to their respective calibration registers.
5. Prompt the user to place the probe in a 0% saturation environment. When these
registers have reached equilibrium, record them in their respective 0% saturation
calibration registers. If a zero calibration is not to be performed, these registers
can be set to zero or left at their previous values.
6. Write the Calibration Update command (0xE001) to the Sensor Command
register. The sensor will calculate a new slope and offset, will write the current
time to the Last User Calibration Time register, and set the Next User Calibration
Time register to zero (disabled). If the concentrations at 100% and 0% saturation
are equal, the probe will return an exception response with code 0x97 (invalid
calibration) and not attempt to compute a new slope and offset due to possible
division by zero. If the slope does not calculate between 0.85 and 1.20 inclusive,
or the offset does not calculate between -0.2 and +0.2 inclusive, the probe will
return an exception response with code 0x97 (invalid calibration). The slope and
offset will be available for read but will not be committed to flash memory.
7. Optionally, read the Last User Calibration Time register, add the next calibration
interval, and write the result to the Next User Calibration Time register.
8. Write the Calibration Mode Off command (0xE002) to the sensor command
register to place the sensor in normal operation. If the calibration mode is turned
off without a calibration update command, or the calibration command returned
an exception, the previous calibration shall be restored.
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