4.6
In autocalibration two user defined gases (cal gas 1 and cal gas 2) are provided to the
instrument. These gases may be either for low or high calibration of the transducers. In some
cases the same gas may be used for low calibration of one transducer while being the high
calibration of another. The gases are introduced to the analyser in three phases:
Phase 1 cal gas 1
Phase 2 cal gas 2
Phase 3 cal gas 1 again.
Any of the transducers connected to any sample inlet may be autocalibrated, either
simultaneously or, by the use of calibration groups (see later in this section), completely
independently.
The following parameters must be set up for either autocalibration or autocheck:
• The time and date must be correctly set before using autocalibration.
• Selection of 'LOW' or 'LOW & HIGH' autocalibration (zirconia sensors cannot have a
high autocalibration).
• LOW and HIGH calibration gas concentrations.
• Autocalibration period (i.e. time interval between successive autocalibrations; minimum
one hour, maximum 59 days + 24 hours).
• Date and time of start of cycle (first autocalibration).
• Flush Time - this may be set, to suit the installation, to a value between 0.5 and 16
minutes so that each gas concentration stabilises before being read. After each flush
time the gas will flow for an additional minute to allow the new 'calibrated' level to be
viewed or recorded.
• Selection of autocalibration or autocheck.
• Calibration gas relays (if analyser relays are to be used, the alternative being control by
external monitoring of the RS-232 output).
• It is necessary to specify which calibration gas (1 or 2) is used for the LOW calibration of
each sensor.
Calibration groups
Autocalibration allows up to four independently programmable transducer groups. Groups of
transducers are programmed independently but only one autocalibration can be performed at
any one time. A queuing mechanism is used to ensure that autocalibrations are performed as
soon as possible, if another autocalibration was taking place at the intended start time.
Autocalibrations initiated by the internal timer, the user interface or by Modbus commands may
specify an individual calibration group. Autocalibrations initiated by an external contact closure
will be carried out on all groups in sequence (it effectively causes all groups to be placed in the
queue in sequence). Autocalibration will only be performed for non-empty groups that are
enabled, and have their gas control relays assigned. This input will be ignored if an
autocalibration is already in progress.
NOTE
In the 4900C analyser, internal autocalibration can only be configured to calibrate all
of the transducers on stream 1.
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