Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.8 Temperature error
Analog value processing
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Function Manual, 06/2014, A5E03461439-AC
2.8 Temperature error
Introduction
Analog modules are exposed to operational conditions that have an impact on their precision
and on the measurement results they return. Temperature errors develop, for example, if the
operating temperature of the module deviates from the ambient temperature of 25 °C. These
temperature errors within the approved temperature range are taken into account in the
operational limit.
Definition
The temperature error identifies the maximum drift of measuring/output values caused by
fluctuation of the ambient temperature at the analog module. The maximum drift can occur at
any ambient temperature. The temperature error is specified depending on the module used
in percent per degree Celsius and/or percent per Kelvin, e.g. ±0.005%/K, and relates to the
measuring range end value of the analog module.
Operational limits of compensation
The temperature error of the reference junction temperature compensation function only
occurs with thermocouple measurements. The temperature error of compensation is added
to the actual temperature error if "Internal reference junction" operating mode is selected.
The temperature error of compensation is defined in the technical specifications as a
percentage of the physical rated range of the analog module or as an absolute value in °C.
Note
Calculating the error in thermocouple measurements
Add the error of a thermocouple measurement to the error of the compensation
measurement to determine the total error of a thermocouple measurement. These errors are
defined in the respective manual.
Example
In this example, the total error of the temperature measurement is determined with the help
of a type K thermocouple. The type K thermocouple takes measurements at a temperature
range from -270 °C to +1372 °C.
The technical specifications state an operational limit of >-200 °C ±2.4 K for the type K
thermocouple. The temperature error of compensation amounts to ±6 °C. The total
temperature error is measured as follows at a temperature of >-200 °C:
Operational error (±2.4 K) + temperature error of compensation (±6 °C) = ±8.4 °C.