In-Situ RDO PRO Process Optical Dissolved Oxygen Sensor Manual - Manual (Page 21)

0% Saturation Calibration Values
These values represent the sensor conditions while the probe is in a 0% saturation calibration
environment. These are not measured values, they are written by the controller during the calibration
process.
Writes to these registers are only accepted if the probe is in the calibration mode. The probe will
return exception 0x85 (invalid device command sequence) if an attempt is made to write these
registers when the calibration mode is off.
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. These registers may
be written independently of the normal internal calibration procedure.
6.5 Entering Calibration Registers
The sensor is calibrated using the following procedure:
1. Optional: Read the Sensor Data Cache Timeout register 9463 and store the value.
2. Write the Sensor Data Cache Timeout register 9463 to a value less than your intended sample
rate and greater than 1000 milliseconds. This will ensure that you get new sensor readings
during the stabilization process.
3. Optional: Read the temperature units register 0049 and saturation units register 0041 and store
their values.
4. Write the temperature units register 0049 to its default value (1) and write the saturation units
register 0041 to its default value (117).
5. Write the Calibration Mode On command (0xE000) to the sensor command register 9305.
6. Update the live salinity and barometric pressure registers if necessary.
7. Place the probe in a 100% saturation environment.
8. Read the DO 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.
9. Prompt the user to place the sensor 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. Note: If you have a version of RDO PRO firmware that is earlier than 1.15, and you
are not doing the zero calibration, you must set the 0% calibration registers to zero.
10. Write the Calibration Update command (0xE001) to the sensor command register. The sensor
will calculate a new slope and offset, 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 if the offset does not
calculate between -0.2 and +0.2 inclusive, then 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.
11. Optional: Read the last user calibration time register, add the next calibration interval, and write
the result to the next user calibration time register.
Page 21 Modbus Registers