Siemens SIMATIC ET 200AL, System Manual

Page: 2113 / 2255

Thermocouples
5.3 Compensation for the reference junction temperature
Analog value processing
82
Function Manual, 06/2014, A5E03461439-AC
Compensation options Explanation Use case / special feature
Reference channel of the
module
Properties
With this type of compensation, the reference
junction temperature is determined using an
external thermal resistor (RTD).
Procedure
Connect the thermocouple to the supply lines
directly at the reference junction, or indirectly
using compensating lines. Wire the supply lines to
the corresponding terminals of the module.
Wire the thermal resistor (RTD) to the reference
channel of the module. The thermal resistor (RTD)
must be positioned in the area of the reference
junction; see chapter Compensation with
reference channel of the module (Page 86).
•
They acquire the temperature
directly at the reference junction.
• The measured temperatures of all
channels that you have configured
for this compensation type will be
corrected automatically by the
temperature value of the reference
junction.
•
Advantages:
–
More precise compared to
compensation with internal
reference junction; however, you
need to install and wire an
additional thermal resistor.
Reference channel of
group 0
Properties
With the "TC" (thermocouple...) setting, the
channel acts as receiver for the reference junction
temperature of group 0.
The associated transmitter of group 0 is set up to
for the RTD channel.
Procedure
Connect the thermocouple to the supply lines
directly at the reference junction, or indirectly
using compensating lines. Wire the supply lines to
the corresponding terminals of the module.
Wire the thermal resistor (RTD) to the configured
reference channel for group 0 of the module. The
thermal resistor (RTD) must be positioned in the
area of the reference junction; see chapter
Compensation, reference channel of Group 0
(Page 88).
•
They acquire the temperature
directly at the reference junction.
•
The measured temperatures of all
channels that you have configured
for this compensation type will be
corrected automatically by the
temperature value of the reference
junction (transmitter).
Fixed reference temperature Properties
With this type of compensation, the reference
junction temperature is stored in the module as
fixed value.
Procedure
Connect the thermocouple to the supply lines
directly at the reference junction, or indirectly
using compensating lines. Wire the supply lines to
the corresponding terminals of the module.
Depending on the module, you can enter a fixed
reference junction temperature in the configuration
data (e.g. 20 °C), or use the fixed reference
junction temperature of the module (0 °C); see
chapter Compensation with fixed reference
temperature (Page 91).
• You keep the reference junction
temperature constant and know the
temperature value.
•
To ensure high precision, you must
ensure that the temperature
remains constant (this can be
complicated, depending on the
application).