Rockwell Automation Publication 1756-UM540E-EN-P - December 2017
35
Analog I/O Module Features
Chapter 2
RTD Error
Module error on the 1756-IRT8I or 1756-IR12 module that is used with an
RTD input is defined in ohms. The error is calculated across the entire input
range selected, not the available range of a sensor used with the module. For
example, if the 1…500
Ω
input range is used, the module error is calculated across
510
Ω
(actual range = 0…510
Ω
).
The error in ohms translates to temperature, but that translation varies because
the relationship is non-linear. The most effective way to check the 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 0.05% of
the full range. This 0.05% value is a worst case value. In other words, with the
1…500
Ω
input range that is selected, the worst case module error is 0.255
Ω
.
Finally, you must check an RTD linearization table to determine how the
temperature error of 0.255
Ω
translates.
For example, if the module has a 0.05% (or 0.255
Ω
) error and is at a temperature
of 0 °C (32 °F), the temperature error is ±0.65 °C
(±1.17 °F) when the Platinum
385 sensor type is used. This same error at a temperature of 200 °C (392 °F)
translates to a temperature error of ±0.69 °C (±1.26 °F).
Thermocouple Error
Thermocouple error on the 1756-IRT8I or 1756-IT16 at 25 °C (77 °F) indicates
the module’s accuracy in measuring temperature. This accuracy varies depending
on these factors:
•
Input range = -100…100 mV.
•
Thermocouple sensor type, any of the following:
–
Type B
–
Type C
–
Type D
–
Type E
–
Type J (1756-IT16 default value)
–
Type K (1756-IRT8I default value)
–
Type N
–
Type R
–
Type S
–
Type T
–
Type TXK/XK (L)
