Single-point calibration – Rockwell Automation 1746-NR4 SLC RTD/Resistance Input Module User Manual User Manual
Page 48
Publication 1746-UM008B-EN-P - December 2006
48 Install and Wire the Module
You can command your module to perform an auto-calibration cycle
by disabling a channel, waiting for the channel status bit to change
state (1 to 0) and then re-enabling that channel. Several scan cycles
are required to perform an auto-calibration (refer to page 4-11). It is
important to remember that during auto-calibration the module is not
converting input data.
An auto-calibration programming example is provided in chapter 6.
Single-point Calibration
Single-point calibration is an optional procedure that can be used to
improve the accuracy of the RTD module and cable combination to
greater than +/-0.2
°
C (32.4
°
F) (when the RTD is operating at +/-50
°
C
(122
°
F) of the calibration temperature). The offset, determined by the
single-point calibration, can be used to compensate for inaccuracies in
the RTD module and cable combination.
After single-point calibration is performed, additional calibrations only
need to be performed if the cable is disturbed or degraded. (RTD
replacement should not affect the accuracy of the procedure.)
However, periodic auto-calibrations should be performed. Follow the
steps below to perform a single-point calibration.
1. Cycle power to the SLC 500 chassis.
2. Select a calibration temperature that is near the control point
(+/-10 °C (50
°
F)).
3. Determine the exact resistance (+/-0.01 ohm) equivalent to the
calibration temperature by using a published temperature vs.
resistance chart.
4. Replace the RTD with the fixed–precision resistor. (It is
recommended that you use a 2 ppm temperature coefficient
resistor.)
TIP
To maintain system accuracy it is recommended that
you periodically perform an autocalibration cycle:
• 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