Single-point calibration, Single-point calibration -16 – Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module User Manual User Manual
Page 36
Publication 1746-UM003A-EN-P
2-16 Installation and Wiring
An autocalibration programming example is provided on page 5-10.
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.
1.
1.
1. Cycle power to the SLC 500 chassis.
2.
2.
2.
2. Select a calibration temperature that is near the control point (±10°C).
3.
3.
3.
3. Determine the exact resistance (±0.01
Ω
) equivalent to the calibration
temperature by using a published temperature vs. resistance chart.
4.
4.
4.
4. Replace the RTD with the fixed-precision resistor. (We recommend you use
a 2 ppm temperature coefficient resistor.)
5.
5.
5.
5. Use the RTD module to determine the temperature equivalent to the fixed
precision resistor and cable combination.
6.
6.
6.
6. Calculate the offset value by subtracting the calculated calibration
temperature from the measured temperature.
7.
7.
7.
7. Reconnect the RTD to the cable.
8.
8.
8.
8. Use ladder logic to apply (subtract) the offset from the measured
temperature to obtain corrected temperature.
TIP
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