Rockwell Automation 1746-NT4 Series B,D17466.6.1 SLC 500 4-Channel Thermocouple/mV Input Module User Manual - Manual (Page 41)

Publication 1746-UM007C-EN-P - July 2004
Installation and Wiring 3-13
Thermocouple Calibration The thermocouple module is initially calibrated at the factory. The
module also has an auto calibration function. Auto calibration
compensates for offset and gain drift of the A/D converter caused by
temperature change within the module. An internal, high precision,
low drift voltage and system ground reference is used for this
purpose. No external, user supplied device is required for
autocalibration.
When an auto calibration cycle takes place, the module’s multiplexer
is set to system ground potential and an A/D reading is taken. The
A/D converter then sets its internal input to the modules precision
voltage source, and another reading is taken. The A/D converter uses
these numbers to compensate for “system” offset (zero) and gain
(span) error.
Autocalibration of a channel occurs whenever a channel is enabled, or
when a change is made to its input type or filter frequency. You can
also command your module to perform an autocalibration cycle by
disabling a channel, waiting for the status bit to change state (1 to 0)
and then re-enabling that channel. Several channel cycles are required
to perform an autocalibration, and it is important to remember that
during autocalibration the module is not converting input data.
To maintain system accuracy we recommend that you periodically
perform an autocalibration cycle, for example:
•
whenever an event occurs that greatly changes the internal
temperature of the control cabinet, such as opening or closing
its door
•
at a convenient time when the system is not making product,
such as during a shift change
An autocalibration programming example is provided in chapter 6.
Accuracy specifications with and without autocalibration are provided
in Appendix A.