Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 365)

Publication 1756-UM009C-EN-P - December 2010 365
Additional Specification Information Appendix E
RTD and Thermocouple
Error Calculations When you use the temperature-measuring modules (1756-IR6I, 1756-IT6I,
and 1756-IT6I2), error calculations are achieved in a two-step process.
1. Calculate the module’s error in ohms or volts.
2. Convert the ohm/volt error to temperature for the specific sensor and
at the correct application temperature.
RTD Error
Module error on the 1756-IR6I module is defined in ohms and is calculated
across the entire input range selected, not the available range of a sensor used
with the module. For example, if the 1…487
Ω
input range is used, the module
error is calculated across 507
Ω
(actual range = 0.86…507.86
Ω
).
The error in ohms translates to temperature, but that translation varies because
the relationship is non-linear. The most effective way to check 1756-IR6I
module error is to calculate the error in ohms and use that value in a
linearization table to check the temperature error.
If the module is calibrated at operating temperature and the operating
temperature remains relatively stable, calibration accuracy is better than 0.1%
of the full range for the first year after calibration. This 0.1% value is a worst
case value. In other words, with the 1…487
Ω
input range selected, the worst
case module error is 0.507
Ω
.
Finally, you must check an RTD linearization table to determine the
temperature error to which an error of 0.507
Ω
translates. For example, if the
1756-IR6I has a 0.1% (or 0.507
Ω
) error and is operating at 0 °C (32 °F), the
temperature error is -1.25…1.2 °C (29.75…34.16 °F) when the Platinum 385
sensor type is used. However, this same ohms error calculated in an operating
temperature of 200 °C (392 °F) translates to a temperature error of
-1.4 °C…1.4 °C (29.48…34.52 °F).