Allen-Bradley SLC 500 1746-NR4, User Manual

Page: 51 / 134

Chapter 3
Installation and Wiring
3–14
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.
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 (when the RTD is operating at +/–50
°
C 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
autocalibrations 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).
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. (We recommend you
use a 2 ppm temperature coefficient resistor.)
5. Use the RTD module to determine the temperature equivalent to the fixed
precision resistor and cable combination.
6. Calculate the offset value by subtracting the calculated calibration
temperature from the measured temperature.
7. Reconnect the RTD to the cable.
8. Use ladder logic to apply (subtract) the offset from the measured
temperature to obtain corrected temperature.
Tip
AB Spares