Emerson Rosemount 6888A, Reference Manual

Page: 59 / 136

The electronics determine if the calibration was successful and calculate new calibration
values. New calibration values are not automatically loaded into the electronics after a
successful calibration, however. The technician has the opportunity to accept or reject the
new values. (A significant calibration change may cause a bump in the O
2
readings at the
DCS console, causing operator concern). Record the calibration data on a log (cell slope,
constant, and impedance, as well as the speed of response data). If the electronics is used,
it stores calibration data for the past 10 successful calibrations.
CAUTION!
READING ERRORS
Make sure the calibration gas cap is replaced tightly after calibration is complete. A loose or
missing cap can permit fresh air to bias the O
2
readings high in processes that run at negative
pressure.
3.6.2 Fully automatic calibration
For fully automatic calibration, the Rosemount 6888Xi Electronics must manage the
actuation of solenoids to introduce gases into the probe.
Rosemount 6888 probes with standard electronics housing
In addition to the Rosemount 6888Xi, this arrangement requires a separate single probe
sequencer (SPS), which is a solenoid box for switching calibration gases, or a larger
intelligent multiprobe sequencer (IMPS), which can handle the autocal for up to four
probes in one box.
The automatic calibration can be initiated in several ways:
• Via a calibration recommended diagnostic that is periodically checking cell
impedance
• Via push button on the Rosemount 6888Xi electronics
• Via HART communications from a 475 handheld communicator or AMS
• Via an external contact closure
• Via time since the last successful calibration
If the O
2
measurement is being used for automatic control, always place the O
2
control
loop into manual prior to calibrating. Always inform the operator prior to calibrating. The
Rosemount 6888Xi Electronics provides an
in cal contact closure for this purpose. An
initiate cal contact is also provided.
The Rosemount 6888Xi Electronics sequences the calibration gases in turn into the sensing
cell.A 300 second flow time is the factory default for both gases and also for the purge
cycle, which lets the probe signal come back to the normal flue gas readings. The 4-20 mA
signal representing O
2
can be held during the calibration cycle or permitted to vary with
the bottled gases, in which case a record of the calibration can be trended at the DCS.
Calibration setup is found under the detailed setup menu.
Configuration, startup, and operation
Reference Manual 51