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I/O Calibration
AI Calibration
Revision 9 900 Control Station User Guide 161
May 2014
AI Calibration
Overview
Analog inputs are factory calibrated to +/- 0.1% of span unless specifically noted in the range
specifications. A field calibration may be performed on any analog input on a point-by-point basis to
optimize measurement accuracy. The factory calibration parameters are retained in non-volatile memory
and may be re-installed to undo a field calibration using selections from the calibration procedure.
Both Factory and field calibration information is stored on the Analog Input module itself. Therefore, once
a module has been calibrated it can be moved to any slot position or any rack without being recalibrated.
However, the controller will automatically restore a channel’s Factory calibration settings under the
following conditions:
• The channel is reconfigured to use a different gain setting. This may happen if you change the input type
of the channel (for example, change from a TC to a Voltage or RTD input type).
•
The AI module detects an error in its stored field calibration information. In this case, factory
calibration is restored for all of the channels on the affected module.
Note that you must configure a channel’s input type and range before you try to calibrate it. The
calibration will be performed against the gain setting associated with the input type and range. If you
change the input type after calibrating, the module will automatically restore the Factory calibration
settings for that channel. If necessary, you can use the PC Designer’s Monitor Mode feature to determine
whether a given channel is using Factory or field calibration.
Calibration equipment
•
For best results use a calibration source accurate to 1 microvolt.
• You must use a voltage source and copper lead-wire for channels that are configured as thermocouple
and voltage inputs. Do not use a compensated calibrator and TC extension lead-wire to calibrate TC
channels.
• Note that Cold Junction sensor and Analog Input calibrations are performed separately; you do not have
to calibrate the CJ sensors before calibrating your TC input channels.
•
If you are calibrating current-type inputs (0-20 ma or 4-20 ma) and you are using an external shunt
resistor, we recommend that you calibrate the channel using a current-source with the actual shunt
resistor installed on the terminals. This will allow you to calibrate out any inaccuracies in the shunt
resistor. A high-precision shunt resistor should always be used.